学习c最少必要知识

C – Program Structure

A C program basically consists of the following parts −

• Preprocessor Commands
• Functions
• Variables
• Statements & Expressions
• Comments

C – Basic Syntax

A C program consists of various tokens and a token is either a keyword, an identifier, a constant, a string literal, or a symbol.

In a C program, the semicolon is a statement terminator. That is, each individual statement must be ended with a semicolon. It indicates the end of one logical entity.

Comments are like helping text in your C program and they are ignored by the compiler. They start with /* and terminate with the characters */ or start with //.

A C identifier is a name used to identify a variable, function, or any other user-defined item. An identifier starts with a letter A to Z, a to z, or an underscore ‘_’ followed by zero or more letters, underscores, and digits (0 to 9). C is a case-sensitive programming language.

The following list shows the reserved words in C. These reserved words may not be used as constants or variables or any other identifier names.

auto else long switch break enum register typedef case extern return union char float short unsigned const for signed void continue goto sizeof volatile default if static while do int struct _Packed double

A line containing only whitespace, possibly with a comment, is known as a blank line, and a C compiler totally ignores it.

Whitespace is the term used in C to describe blanks, tabs, newline characters and comments. Whitespace separates one part of a statement from another and enables the compiler to identify where one element in a statement, such as int, ends and the next element begins.

C – Data Types

Data types in c refer to an extensive system used for declaring variables or functions of different types. The type of a variable determines how much space it occupies in storage and how the bit pattern stored is interpreted.

The types in C can be classified as follows:

• Basic Types: They are arithmetic types and are further classified into: (a) integer types and (b) floating-point types.
• Enumerated types: They are again arithmetic types and they are used to define variables that can only assign certain discrete integer values throughout the program.
• The type void: The type specifier void indicates that no value is available.
• Derived types: They include (a) Pointer types, (b) Array types, (c) Structure types, (d) Union types and (e) Function types.

Integer Types

Type	Storage size	Value range
char	1 byte	-128 to 127 or 0 to 255
unsigned char	1 byte	0 to 255
signed char	1 byte	-128 to 127
int	2 or 4 bytes	-32,768 to 32,767 or -2,147,483,648 to 2,147,483,647
unsigned int	2 or 4 bytes	0 to 65,535 or 0 to 4,294,967,295
short	2 bytes	-32,768 to 32,767
unsigned short	2 bytes	0 to 65,535
long	8 bytes	-9223372036854775808 to 9223372036854775807
unsigned long	8 bytes	0 to 18446744073709551615

Floating-Point Types

Type	Storage size	Value range	Precision
float	4 byte	1.2E-38 to 3.4E+38	6 decimal places
double	8 byte	2.3E-308 to 1.7E+308	15 decimal places
long double	10 byte	3.4E-4932 to 1.1E+4932	19 decimal places

The void Type

• Function returns as void : There are various functions in C which do not return any value or you can say they return void. A function with no return value has the return type as void. For example, void exit (int status);
• Function arguments as void: There are various functions in C which do not accept any parameter. A function with no parameter can accept a void. For example, int rand(void);
• Pointers to void: A pointer of type void * represents the address of an object, but not its type. For example, a memory allocation function void *malloc( size_t size ); returns a pointer to void which can be casted to any data type.

C – Variables

A variable is nothing but a name given to a storage area that our programs can manipulate. Each variable in C has a specific type, which determines the size and layout of the variable’s memory; the range of values that can be stored within that memory; and the set of operations that can be applied to the variable.

There are the following basic variable types:

• char: Typically a single octet(one byte). This is an integer type.
• int: The most natural size of integer for the machine.
• float: A single-precision floating point value.
• double: A double-precision floating point value.
• void: Represents the absence of type.

A variable definition tells the compiler where and how much storage to create for the variable. A variable definition specifies a data type and contains a list of one or more variables of that type as follows − type variable_list;

Here, type must be a valid C data type including char, w_char, int, float, double, bool, or any user-defined object; and variable_list may consist of one or more identifier names separated by commas. Some valid declarations are shown here −

int    i, j, k;
char   c, ch;
float  f, salary;
double d;

Variables can be initialized (assigned an initial value) in their declaration. The initializer consists of an equal sign followed by a constant expression as follows − type variable_name = value; Some examples are −

extern int d = 3, f = 5;    // declaration of d and f.
int d = 3, f = 5;           // definition and initializing d and f.
byte z = 22;                // definition and initializes z.
char x = 'x';               // the variable x has the value 'x'.

For definition without an initializer: variables with static storage duration are implicitly initialized with NULL (all bytes have the value 0); the initial value of all other variables are undefined.

A variable declaration provides assurance to the compiler that there exists a variable with the given type and name so that the compiler can proceed for further compilation without requiring the complete detail about the variable. A variable definition has its meaning at the time of compilation only, the compiler needs actual variable definition at the time of linking the program.

A variable declaration is useful when you are using multiple files and you define your variable in one of the files which will be available at the time of linking of the program. You will use the keyword extern to declare a variable at any place. Though you can declare a variable multiple times in your C program, it can be defined only once in a file, a function, or a block of code.

There are two kinds of expressions in C −

lvalue − Expressions that refer to a memory location are called “lvalue” expressions. An lvalue may appear as either the left-hand or right-hand side of an assignment.

rvalue − The term rvalue refers to a data value that is stored at some address in memory. An rvalue is an expression that cannot have a value assigned to it which means an rvalue may appear on the right-hand side but not on the left-hand side of an assignment.

Variables are lvalues and so they may appear on the left-hand side of an assignment. Numeric literals are rvalues and so they may not be assigned and cannot appear on the left-hand side. Take a look at the following valid and invalid statements −

int g = 20; // valid statement
10 = 20; // invalid statement; would generate compile-time error

C – Constants and Literals

Constants refer to fixed values that the program may not alter during its execution. These fixed values are also called literals.

Constants can be of any of the basic data types like an integer constant, a floating constant, a character constant, or a string literal. There are enumeration constants as well.

Constants are treated just like regular variables except that their values cannot be modified after their definition.

An integer literal can be a decimal, octal, or hexadecimal constant. A prefix specifies the base or radix: 0x or 0X for hexadecimal, 0 for octal, and nothing for decimal.

An integer literal can also have a suffix that is a combination of U and L, for unsigned and long, respectively. The suffix can be uppercase or lowercase and can be in any order.

Here are some examples of integer literals −

212         /* Legal */
215u        /* Legal */
0xFeeL      /* Legal */
078         /* Illegal: 8 is not an octal digit */
032UU       /* Illegal: cannot repeat a suffix */
85         /* decimal */
0213       /* octal */
0x4b       /* hexadecimal */
30         /* int */
30u        /* unsigned int */
30l        /* long */
30ul       /* unsigned long */

A floating-point literal has an integer part, a decimal point, a fractional part, and an exponent part. You can represent floating point literals either in decimal form or exponential form.

While representing decimal form, you must include the decimal point, the exponent, or both; and while representing exponential form, you must include the integer part, the fractional part, or both. The signed exponent is introduced by e or E.

Here are some examples of floating-point literals −

3.14159       /* Legal */
314159E-5L    /* Legal */
510E          /* Illegal: incomplete exponent */
210f          /* Illegal: no decimal or exponent */
.e55          /* Illegal: missing integer or fraction */

Character literals are enclosed in single quotes, e.g., ‘x’ can be stored in a simple variable of char type.

A character literal can be a plain character (e.g., ‘x’), an escape sequence (e.g., ‘\t’), or a universal character (e.g., ‘\u02C0’).

There are certain characters in C that represent special meaning when preceded by a backslash for example, newline (\n) or tab (\t).

Escape sequence	Meaning
\|\ character
\’	‘ character
\”	” character
\?	? character
\a	Alert or bell
\b	Backspace
\f	Form feed
\n	Newline
\r	Carriage return
\t	Horizontal tab
\v	Vertical tab
\ooo	Octal number of one to three digits
\xhh . . .	Hexadecimal number of one or more digits

String literals or constants are enclosed in double quotes “”. A string contains characters that are similar to character literals: plain characters, escape sequences, and universal characters.

You can break a long line into multiple lines using string literals and separating them using white spaces.

There are two simple ways in C to define constants −

• Using #define preprocessor.
• Using const keyword.

Given below is the form to use #define preprocessor to define a constant − #define identifier value

You can use const prefix to declare constants with a specific type as follows −const type variable = value;

Note that it is a good programming practice to define constants in CAPITALS.

C – Storage Classes

A storage class defines the scope (visibility) and life-time of variables and/or functions within a C Program. They precede the type that they modify. We have four different storage classes in a C program −

• auto
• register
• static
• extern

The auto storage class is the default storage class for all local variables.

The register storage class is used to define local variables that should be stored in a register instead of RAM. This means that the variable has a maximum size equal to the register size (usually one word) and can’t have the unary ‘&’ operator applied to it (as it does not have a memory location).

The register should only be used for variables that require quick access such as counters. It should also be noted that defining ‘register’ does not mean that the variable will be stored in a register. It means that it MIGHT be stored in a register depending on hardware and implementation restrictions.

The static storage class instructs the compiler to keep a local variable in existence during the life-time of the program instead of creating and destroying it each time it comes into and goes out of scope. Therefore, making local variables static allows them to maintain their values between function calls.

The static modifier may also be applied to global variables. When this is done, it causes that variable’s scope to be restricted to the file in which it is declared.

In C programming, when static is used on a global variable, it causes only one copy of that member to be shared by all the objects of its class.

The extern storage class is used to give a reference of a global variable that is visible to ALL the program files. When you use ‘extern’, the variable cannot be initialized however, it points the variable name at a storage location that has been previously defined.

When you have multiple files and you define a global variable or function, which will also be used in other files, then extern will be used in another file to provide the reference of defined variable or function. Just for understanding, extern is used to declare a global variable or function in another file.

The extern modifier is most commonly used when there are two or more files sharing the same global variables or functions as explained below.

First File: main.c

#include <stdio.h>
int count ;
extern void write_extern();
main() {
   count = 5;
   write_extern();
}

Second File: support.c

#include <stdio.h>
extern int count;
void write_extern(void) {
   printf("count is %d\n", count);
}

Here, extern is being used to declare count in the second file, where as it has its definition in the first file, main.c. Now, compile these two files as follows − $gcc main.c support.c

It will produce the executable program a.out. When this program is executed, it produces the following result − count is 5

C – Operators

An operator is a symbol that tells the compiler to perform specific mathematical or logical functions. C language is rich in built-in operators and provides the following types of operators −

• Arithmetic Operators
• Relational Operators
• Logical Operators
• Bitwise Operators
• Assignment Operators
• Misc Operators

Arithmetic Operators

Assume variable A holds 10 and variable B holds 20 then −

Operator	Description	Example
+	Adds two operands.	A + B = 30
−	Subtracts second operand from the first.	A − B = -10
*	Multiplies both operands.	A * B = 200
/	Divides numerator by de-numerator.	B / A = 2
%	Modulus Operator and remainder of after an integer division.	B % A = 0
++	Increment operator increases the integer value by one.	A++ = 11
—	Decrement operator decreases the integer value by one.	A– = 9

Relational Operators

Assume variable A holds 10 and variable B holds 20 then −

Operator	Description	Example
==	Checks if the values of two operands are equal or not. If yes, then the condition becomes true.	(A == B) is not true.
!=	Checks if the values of two operands are equal or not. If the values are not equal, then the condition becomes true.	(A != B) is true.
>	Checks if the value of left operand is greater than the value of right operand. If yes, then the condition becomes true.	(A > B) is not true.
<	Checks if the value of left operand is less than the value of right operand. If yes, then the condition becomes true.	(A < B) is true.
>=	Checks if the value of left operand is greater than or equal to the value of right operand. If yes, then the condition becomes true.	(A >= B) is not true.
<=	Checks if the value of left operand is less than or equal to the value of right operand. If yes, then the condition becomes true.	(A <= B) is true.

Logical Operators

Assume variable A holds 1 and variable B holds 0, then −

Operator	Description	Example
&&	Called Logical AND operator. If both the operands are non-zero, then the condition becomes true.	(A && B) is false.
||	Called Logical OR Operator. If any of the two operands is non-zero, then the condition becomes true.	(A || B) is true.
!	Called Logical NOT Operator. It is used to reverse the logical state of its operand. If a condition is true, then Logical NOT operator will make it false.	!(A && B) is true.

Bitwise Operators

p	q	p & q	p | q	p ^ q
0	0	0	0	0
0	1	0	1	1
1	1	1	1	0
1	0	0	1	1

Assume A = 60 and B = 13 in binary format, they will be as follows −

A = 0011 1100
B = 0000 1101
A&B = 0000 1100
A|B = 0011 1101
A^B = 0011 0001
~A = 1100 0011

The following table lists the bitwise operators supported by C. Assume variable ‘A’ holds 60 and variable ‘B’ holds 13, then −

Operator	Description	Example
&	Binary AND Operator copies a bit to the result if it exists in both operands.	(A & B) = 12, i.e., 0000 1100
|	Binary OR Operator copies a bit if it exists in either operand.	(A | B) = 61, i.e., 0011 1101
^	Binary XOR Operator copies the bit if it is set in one operand but not both.	(A ^ B) = 49, i.e., 0011 0001
~	Binary One’s Complement Operator is unary and has the effect of ‘flipping’ bits.	(~A ) = ~(60), i.e,. -0111101
<<	Binary Left Shift Operator. The left operands value is moved left by the number of bits specified by the right operand.	A << 2 = 240 i.e., 1111 0000
>>	Binary Right Shift Operator. The left operands value is moved right by the number of bits specified by the right operand.	A >> 2 = 15 i.e., 0000 1111

Assignment Operators

Operator	Description	Example
=	Simple assignment operator. Assigns values from right side operands to left side operand	C = A + B will assign the value of A + B to C
+=	Add AND assignment operator. It adds the right operand to the left operand and assign the result to the left operand.	C += A is equivalent to C = C + A
-=	Subtract AND assignment operator. It subtracts the right operand from the left operand and assigns the result to the left operand.	C -= A is equivalent to C = C – A
*=	Multiply AND assignment operator. It multiplies the right operand with the left operand and assigns the result to the left operand.	C *= A is equivalent to C = C * A
/=	Divide AND assignment operator. It divides the left operand with the right operand and assigns the result to the left operand.	C /= A is equivalent to C = C / A
%=	Modulus AND assignment operator. It takes modulus using two operands and assigns the result to the left operand.	C %= A is equivalent to C = C % A
<<=	Left shift AND assignment operator.	C <<= 2 is same as C = C << 2
>>=	Right shift AND assignment operator.	C >>= 2 is same as C = C >> 2
&=	Bitwise AND assignment operator.	C &= 2 is same as C = C & 2
^=	Bitwise exclusive OR and assignment operator.	C ^= 2 is same as C = C ^ 2
|=	Bitwise inclusive OR and assignment operator.	C |= 2 is same as C = C | 2

Operators Precedence in C

Operator precedence determines the grouping of terms in an expression and decides how an expression is evaluated. Certain operators have higher precedence than others; for example, the multiplication operator has a higher precedence than the addition operator.

Here, operators with the highest precedence appear at the top of the table, those with the lowest appear at the bottom. Within an expression, higher precedence operators will be evaluated first.

Category	Operator	Associativity
Postfix	() [] -> . ++ —	Left to right
Unary	+ – ! ~ ++ — (type)* & sizeof	Right to left
Multiplicative	* / %	Left to right
Additive	+ –	Left to right
Shift	<< >>	Left to right
Relational	< <= > >=	Left to right
Equality	== !=	Left to right
Bitwise AND	&	Left to right
Bitwise XOR	^	Left to right
Bitwise OR	|	Left to right
Logical AND	&&	Left to right
Logical OR	||	Left to right
Conditional	?:	Right to left
Assignment	= += -= *= /= %=>>= <<= &= ^= |=	Right to left
Comma	,	Left to right

C – Decision Making

Decision making structures require that the programmer specifies one or more conditions to be evaluated or tested by the program, along with a statement or statements to be executed if the condition is determined to be true, and optionally, other statements to be executed if the condition is determined to be false.

C programming language assumes any non-zero and non-null values as true, and if it is either zero or null, then it is assumed as false value.

C programming language provides the following types of decision making statements.

Sr.No.	Statement & Description
1	if statement
An if statement consists of a boolean expression followed by one or more statements.
2	if…else statement
An if statement can be followed by an optional else statement, which executes when the Boolean expression is false.
3	nested if statements
You can use one if or else if statement inside another if or else if statement(s).
4	switch statement
A switch statement allows a variable to be tested for equality against a list of values.
5	nested switch statements
You can use one switch statement inside another switch statement(s).

We have covered conditional operator ? : in the previous chapter which can be used to replace if…else statements. It has the following general form −

Exp1 ? Exp2 : Exp3;

Where Exp1, Exp2, and Exp3 are expressions. Notice the use and placement of the colon.

The value of a ? expression is determined like this −

• Exp1 is evaluated. If it is true, then Exp2 is evaluated and becomes the value of the entire ? expression.
• If Exp1 is false, then Exp3 is evaluated and its value becomes the value of the expression.

C – Loops

A loop statement allows us to execute a statement or group of statements multiple times.

C programming language provides the following types of loops to handle looping requirements.

Sr.No.	Loop Type & Description
1	while loop
Repeats a statement or group of statements while a given condition is true. It tests the condition before executing the loop body.
2	for loop
Executes a sequence of statements multiple times and abbreviates the code that manages the loop variable.
3	do…while loop
It is more like a while statement, except that it tests the condition at the end of the loop body.
4	nested loops
You can use one or more loops inside any other while, for, or do..while loop.

Loop control statements change execution from its normal sequence. When execution leaves a scope, all automatic objects that were created in that scope are destroyed.

C supports the following control statements.

Sr.No.	Control Statement & Description
1	break statement
Terminates the loop or switch statement and transfers execution to the statement immediately following the loop or switch.
2	continue statement
Causes the loop to skip the remainder of its body and immediately retest its condition prior to reiterating.
3	goto statement
Transfers control to the labeled statement.

A loop becomes an infinite loop if a condition never becomes false. The for loop is traditionally used for this purpose. Since none of the three expressions that form the ‘for’ loop are required, you can make an endless loop by leaving the conditional expression empty.

C – Functions

A function is a group of statements that together perform a task. Every C program has at least one function, which is main(), and all the most trivial programs can define additional functions.

A function declaration tells the compiler about a function’s name, return type, and parameters. A function definition provides the actual body of the function.

The C standard library provides numerous built-in functions that your program can call. For example, strcat() to concatenate two strings, memcpy() to copy one memory location to another location, and many more functions.

A function can also be referred as a method or a sub-routine or a procedure, etc.

The general form of a function definition in C programming language is as follows −

return_type function_name( parameter list ) {
   body of the function
}

A function definition in C programming consists of a function header and a function body. Here are all the parts of a function −

• Return Type − A function may return a value. The return_type is the data type of the value the function returns. Some functions perform the desired operations without returning a value. In this case, the return_type is the keyword void.
• Function Name − This is the actual name of the function. The function name and the parameter list together constitute the function signature.
• Parameters − A parameter is like a placeholder. When a function is invoked, you pass a value to the parameter. This value is referred to as actual parameter or argument. The parameter list refers to the type, order, and number of the parameters of a function. Parameters are optional; that is, a function may contain no parameters.
• Function Body − The function body contains a collection of statements that define what the function does.

A function declaration tells the compiler about a function name and how to call the function. The actual body of the function can be defined separately.

A function declaration has the following parts − return_type function_name( parameter list );

Parameter names are not important in function declaration only their type is required.

While creating a C function, you give a definition of what the function has to do. To use a function, you will have to call that function to perform the defined task.

When a program calls a function, the program control is transferred to the called function. A called function performs a defined task and when its return statement is executed or when its function-ending closing brace is reached, it returns the program control back to the main program.

To call a function, you simply need to pass the required parameters along with the function name, and if the function returns a value, then you can store the returned value.

Like:

#include <stdio.h>
int max(int a, int b) { return (a > b) ? a : b; }
int main() {
    int a = 100, b = 200;
    printf("Max value is : %d\n", max(a, b));
    return 0;
}

If a function is to use arguments, it must declare variables that accept the values of the arguments. These variables are called the formal parameters of the function.

Formal parameters behave like other local variables inside the function and are created upon entry into the function and destroyed upon exit.

While calling a function, there are two ways in which arguments can be passed to a function −

• Call by value: This method copies the actual value of an argument into the formal parameter of the function. In this case, changes made to the parameter inside the function have no effect on the argument.
• Call by reference: This method copies the address of an argument into the formal parameter. Inside the function, the address is used to access the actual argument used in the call. This means that changes made to the parameter affect the argument.

By default, C uses call by value to pass arguments. In general, it means the code within a function cannot alter the arguments used to call the function.

C – Scope Rules

A scope in any programming is a region of the program where a defined variable can have its existence and beyond that variable it cannot be accessed. There are three places where variables can be declared in C programming language −

• Inside a function or a block which is called local variables.
• Outside of all functions which is called global variables.
• In the definition of function parameters which are called formal parameters.

Variables that are declared inside a function or block are called local variables. They can be used only by statements that are inside that function or block of code. Local variables are not known to functions outside their own.

Global variables are defined outside a function, usually on top of the program. Global variables hold their values throughout the lifetime of your program and they can be accessed inside any of the functions defined for the program. A global variable can be accessed by any function. That is, a global variable is available for use throughout your entire program after its declaration.

A program can have same name for local and global variables but the value of local variable inside a function will take preference.

Formal parameters, are treated as local variables with-in a function and they take precedence over global variables.

When a local variable is defined, it is not initialized by the system, you must initialize it yourself. Global variables are initialized automatically by the system when you define them as follows −

Data Type	Initial Default Value
int	0
char	‘\0’
float	0
double	0
pointer	NULL

C – Arrays

Arrays a kind of data structure that can store a fixed-size sequential collection of elements of the same type. An array is used to store a collection of data, but it is often more useful to think of an array as a collection of variables of the same type.

Instead of declaring individual variables, such as number0, number1, …, and number99, you declare one array variable such as numbers and use numbers[0], numbers[1], and …, numbers[99] to represent individual variables. A specific element in an array is accessed by an index.

All arrays consist of contiguous memory locations. The lowest address corresponds to the first element and the highest address to the last element.

To declare an array in C, a programmer specifies the type of the elements and the number of elements required by an array as follows −
type arrayName [ arraySize ];

This is called a single-dimensional array. The arraySize must be an integer constant greater than zero and type can be any valid C data type, such as – double balance[10];

You can initialize an array in C either one by one or using a single statement as follows −
double balance[5] = {1000.0, 2.0, 3.4, 7.0, 50.0};

The number of values between braces { } cannot be larger than the number of elements that we declare for the array between square brackets [ ].

If you omit the size of the array, an array just big enough to hold the initialization is created. Therefore, if you write −
double balance[] = {1000.0, 2.0, 3.4, 7.0, 50.0};

You will create exactly the same array as you did in the previous example.

The above statement assigns the 5th element in the array with a value of 50.0. All arrays have 0 as the index of their first element which is also called the base index and the last index of an array will be total size of the array minus 1.

An element is accessed by indexing the array name. This is done by placing the index of the element within square brackets after the name of the array. For example −
double salary = balance[9];

The above statement will take the 10th element from the array and assign the value to salary variable.

The following important concepts related to array should be clear to a C programmer −

1. Multi-dimensional arrays: C supports multidimensional arrays. The simplest form of the multidimensional array is the two-dimensional array.
2. Passing arrays to functions: You can pass to the function a pointer to an array by specifying the array’s name without an index.
3. Return array from a function: C allows a function to return an array.
4. Pointer to an array: You can generate a pointer to the first element of an array by simply specifying the array name, without any index.

C – Pointers

A pointer is a variable whose value is the address of another variable, i.e., direct address of the memory location. Like any variable or constant, you must declare a pointer before using it to store any variable address. The general form of a pointer variable declaration is −
type *var-name;

Here, type is the pointer’s base type; it must be a valid C data type and var-name is the name of the pointer variable. The asterisk * used to declare a pointer is the same asterisk used for multiplication. However, in this statement the asterisk is being used to designate a variable as a pointer. Take a look at some of the valid pointer declarations −

int    *ip;    /* pointer to an integer */
double *dp;    /* pointer to a double */
float  *fp;    /* pointer to a float */
char   *ch     /* pointer to a character */

The actual data type of the value of all pointers, whether integer, float, character, or otherwise, is the same, a long hexadecimal number that represents a memory address. The only difference between pointers of different data types is the data type of the variable or constant that the pointer points to.

There are a few important operations, which we will do with the help of pointers very frequently. (a) We define a pointer variable, (b) assign the address of a variable to a pointer and (c) finally access the value at the address available in the pointer variable. This is done by using unary operator * that returns the value of the variable located at the address specified by its operand.

#include <stdio.h>
int main() {
    int var = 20; /* actual variable declaration */
    int *ip;      /* pointer variable declaration */
    ip = &var; /* store address of var in pointer variable*/
    printf("Address of var variable: %p\n", &var);
    /* address stored in pointer variable */
    printf("Address stored in ip variable: %p\n", ip);
    /* access the value using the pointer */
    printf("Value of *ip variable: %d\n", *ip);
    return 0;
}

It is always a good practice to assign a NULL value to a pointer variable in case you do not have an exact address to be assigned. This is done at the time of variable declaration. A pointer that is assigned NULL is called a null pointer.

The NULL pointer is a constant with a value of zero defined in several standard libraries.

Pointers have many but easy concepts and they are very important to C programming. The following important pointer concepts should be clear to any C programmer −

1. Pointer arithmetic: There are four arithmetic operators that can be used in pointers: ++, –, +, –
2. Array of pointers: You can define arrays to hold a number of pointers.
3. Pointer to pointer: C allows you to have pointer on a pointer and so on.
4. Passing pointers to functions in C: Passing an argument by reference or by address enable the passed argument to be changed in the calling function by the called function.
5. Return pointer from functions in C: C allows a function to return a pointer to the local variable, static variable, and dynamically allocated memory as well.

C – Strings

Strings are actually one-dimensional array of characters terminated by a null character ‘\0’. Thus a null-terminated string contains the characters that comprise the string followed by a null.

The following declaration and initialization create a string consisting of the word “Hello”. To hold the null character at the end of the array, the size of the character array containing the string is one more than the number of characters in the word “Hello.”

char greeting[6] = {'H', 'e', 'l', 'l', 'o', '\0'};

Notice that char s[100]="hello"; printf("%d", strlen(s); will output 5 instead of 100;

If you follow the rule of array initialization then you can write the above statement as follows −
char greeting[] = "Hello";

The C compiler automatically places the ‘\0’ at the end of the string when it initializes the array.

C supports a wide range of functions that manipulate null-terminated strings −

1. strcpy(s1, s2); Copies string s2 into string s1.
2. strcat(s1, s2); Concatenates string s2 onto the end of string s1.
3. strlen(s1); Returns the length of string s1.
4. strcmp(s1, s2); Returns 0 if s1 and s2 are the same; less than 0 if s1s2.
5. strchr(s1, ch); Returns a pointer to the first occurrence of character ch in string s1.
6. strstr(s1, s2); Returns a pointer to the first occurrence of string s2 in string s1.

C – Structures

Arrays allow to define type of variables that can hold several data items of the same kind. Similarly structure is another user defined data type available in C that allows to combine data items of different kinds. Structures are used to represent a record.

To define a structure, you must use the struct statement. The struct statement defines a new data type, with more than one member. The format of the struct statement is as follows −

struct [structure tag] {
   member definition;
   member definition;
   ...
   member definition;
} [one or more structure variables];

struct without a tag has an “anonymous structure type” and you won’t be able to declare another variable of the same type.

To access any member of a structure, we use the member access operator (.). The member access operator is coded as a period between the structure variable name and the structure member that we wish to access. You would use the keyword struct to define variables of structure type.

#include <stdio.h>
#include <string.h>
struct Books {
    char title[50];
    char author[50];
    char subject[100];
    int book_id;
};
int main() {
    struct Books book; /* Declare book of type Book */
    /* book 1 specification */
    // book.title = "C Programming"; // will cause error: array type 'char [50]' is not assignable
    strcpy(book.title, "C Programming");
    strcpy(book.author, "Nuha Ali");
    strcpy(book.subject, "C Programming Tutorial");
    book.book_id = 6495407;
    /* print book info */
    printf("Book title : %s\n", book.title);
    printf("Book author : %s\n", book.author);
    printf("Book subject : %s\n", book.subject);
    printf("Book book_id : %d\n", book.book_id);
    return 0;
}

You can pass a structure as a function argument in the same way as you pass any other variable or pointer.

You can define pointers to structures in the same way as you define pointer to any other variable −
struct Books *pb = &book;

To find the address of a structure variable, place the ‘&’; operator before the structure’s name.

To access the members of a structure using a pointer to that structure, you must use the → operator as follows − pb->id

#include <stdio.h>
#include <string.h>
struct Books {
    char title[50];
    char author[50];
    char subject[100];
    int book_id;
};
void printBook(struct Books);
void updateBookId(struct Books *, int);
int main() {
    struct Books book, *pb = &book;
    strcpy(book.title, "C Programming");
    strcpy(book.author, "Nuha Ali");
    strcpy(book.subject, "C Programming Tutorial");
    book.book_id = 6495407;
    printBook(book);
    updateBookId(pb, 100);
    strcpy(pb->author, "Alan");
    printBook(book);
    return 0;
}
void printBook(struct Books book) {
    printf("Book title : %s\n", book.title);
    printf("Book author : %s\n", book.author);
    printf("Book subject : %s\n", book.subject);
    printf("Book book_id : %d\n", book.book_id);
}
void updateBookId(struct Books *book, int id) { book->book_id = id; }

Bit Fields

Bit Fields allow the packing of data in a structure. This is especially useful when memory or data storage is at a premium. Typical examples include −

Packing several objects into a machine word. e.g. 1 bit flags can be compacted.

Reading external file formats — non-standard file formats could be read in, e.g., 9-bit integers.

C allows us to do this in a structure definition by putting :bit length after the variable. For example −

struct packed_struct {
   unsigned int f1:1;
   unsigned int f2:1;
   unsigned int f3:1;
   unsigned int f4:1;
   unsigned int type:4;
   unsigned int my_int:9;
} pack;

Here, the packed_struct contains 6 members: Four 1 bit flags f1..f3, a 4-bit type and a 9-bit my_int.

C automatically packs the above bit fields as compactly as possible, provided that the maximum length of the field is less than or equal to the integer word length of the computer. If this is not the case, then some compilers may allow memory overlap for the fields while others would store the next field in the next word.

C – Unions

A union is a special data type available in C that allows to store different data types in the same memory location. You can define a union with many members, but only one member can contain a value at any given time. Unions provide an efficient way of using the same memory location for multiple-purpose.

To define a union, you must use the union statement in the same way as you did while defining a structure. The union statement defines a new data type with more than one member for your program. The format of the union statement is as follows −

union [union tag] {
   member definition;
   member definition;
   ...
   member definition;
} [one or more union variables];

The union tag is optional and each member definition is a normal variable definition, such as int i; or float f; or any other valid variable definition. At the end of the union’s definition, before the final semicolon, you can specify one or more union variables but it is optional. Here is the way you would define a union type named Data having three members i, f, and str −

union Data {
   int i;
   float f;
   char str[20];
} data;

Now, a variable of Data type can store an integer, a floating-point number, or a string of characters. It means a single variable, i.e., same memory location, can be used to store multiple types of data. You can use any built-in or user defined data types inside a union based on your requirement.

The memory occupied by a union will be large enough to hold the largest member of the union. For example, in the above example, Data type will occupy 20 bytes of memory space because this is the maximum space which can be occupied by a character string. The following example displays the total memory size occupied by the above union −

#include <stdio.h>
#include <string.h>
union Data {
    int i;
    float f;
    char str[20];
};
int main() {
    union Data data;
    printf("Memory size occupied by data : %lu\n", sizeof(data)); //20
    return 0;
}

To access any member of a union, we use the member access operator (.). The member access operator is coded as a period between the union variable name and the union member that we wish to access. You would use the keyword union to define variables of union type. The following example shows how to use unions in a program −

#include <stdio.h>
#include <string.h>
union Data {
    int i;
    float f;
    char str[20];
};
int main() {
    union Data data;
    data.i = 10;
    data.f = 220.5;
    strcpy(data.str, "C Programming");
    printf("data.i : %d\n", data.i);
    printf("data.f : %f\n", data.f);
    printf("data.str : %s\n", data.str);
    return 0;
}

output:

data.i : 1917853763
data.f : 4122360580327794860452759994368.000000
data.str : C Programming

Here, we can see that the values of i and f members of union got corrupted because the final value assigned to the variable has occupied the memory location and this is the reason that the value of str member is getting printed very well.

Now let’s look into the same example once again where we will use one variable at a time which is the main purpose of having unions −

#include <stdio.h>
#include <string.h>
union Data {
    int i;
    float f;
    char str[20];
};
int main() {
    union Data data;
    data.i = 10;
    printf("data.i : %d\n", data.i);
    data.f = 220.5;
    printf("data.f : %f\n", data.f);
    strcpy(data.str, "C Programming");
    printf("data.str : %s\n", data.str);
    return 0;
}

output:

data.i : 10
data.f : 220.500000
data.str : C Programming

C – Bit Fields

struct {
   unsigned int widthValidated;
   unsigned int heightValidated;
} status;

can be rewritten as:

struct {
   unsigned int widthValidated : 1;
   unsigned int heightValidated : 1;
} status;

The above structure requires 4 bytes of memory space for status variable, but only 2 bits will be used to store the values.

If you will use up to 32 variables each one with a width of 1 bit, then also the status structure will use 4 bytes. However as soon as you have 33 variables, it will allocate the next slot of the memory and it will start using 8 bytes. Let us check the following example to understand the concept −

The declaration of a bit-field has the following form inside a structure −

struct {
   type [member_name] : width ;
};

type: An integer type that determines how a bit-field’s value is interpreted. The type may be int, signed int, or unsigned int.

member_name: The name of the bit-field.

width: The number of bits in the bit-field. The width must be less than or equal to the bit width of the specified type.

The variables defined with a predefined width are called bit fields. A bit field can hold more than a single bit; for example, if you need a variable to store a value from 0 to 7, then you can define a bit field with a width of 3 bits as follows −

struct {
   unsigned int age : 3;
} Age;

The above structure definition instructs the C compiler that the age variable is going to use only 3 bits to store the value. If you try to use more than 3 bits, then it will not allow you to do so. Let us try the following example −

#include <stdio.h>
#include <string.h>
struct {
    unsigned int age : 3;
} Age;
int main() {
    Age.age = 4;
    printf("Sizeof( Age ) : %lu\n", sizeof(Age));
    printf("Age.age : %d\n", Age.age);
    Age.age = 7;
    printf("Age.age : %d\n", Age.age);
    Age.age = 8;
    printf("Age.age : %d\n", Age.age); // output 0 with warning
    return 0;
}

C – typedef

The C programming language provides a keyword called typedef, which you can use to give a type a new name. Following is an example to define a term BYTE for one-byte numbers −

typedef unsigned char BYTE;

After this type definition, the identifier BYTE can be used as an abbreviation for the type unsigned char, for example..

BYTE b1, b2;

By convention, uppercase letters are used for these definitions to remind the user that the type name is really a symbolic abbreviation, but you can use lowercase, as follows −

typedef unsigned char byte;

You can use typedef to give a name to your user defined data types as well. For example,

typedef struct Books {
   char title[50];
   char author[50];
   char subject[100];
   int book_id;
} Book;

define is a C-directive which is also used to define the aliases for various data types similar to typedef but with the following differences −

typedef is limited to giving symbolic names to types only where as #define can be used to define alias for values as well, you can define 1 as ONE etc.

typedef interpretation is performed by the compiler whereas #define statements are processed by the pre-processor.

C – Input and Output

When we say Input, it means to feed some data into a program. An input can be given in the form of a file or from the command line. C programming provides a set of built-in functions to read the given input and feed it to the program as per requirement.

When we say Output, it means to display some data on screen, printer, or in any file. C programming provides a set of built-in functions to output the data on the computer screen as well as to save it in text or binary files.

C programming treats all the devices as files. So devices such as the display are addressed in the same way as files and the following three files are automatically opened when a program executes to provide access to the keyboard and screen.

Standard File	File Pointer	Device
Standard input	stdin	Keyboard
Standard output	stdout	Screen
Standard error	stderr	Your screen

The file pointers are the means to access the file for reading and writing purpose. This section explains how to read values from the screen and how to print the result on the screen.

The int getchar(void) function reads the next available character from the screen and returns it as an integer. This function reads only single character at a time. You can use this method in the loop in case you want to read more than one character from the screen.

The int putchar(int c) function puts the passed character on the screen and returns the same character. This function puts only single character at a time. You can use this method in the loop in case you want to display more than one character on the screen. Check the following example −

#include <stdio.h>
int main( ) {
   int c;
   printf( "Enter a value :");
   c = getchar( );
   printf( "\nYou entered: ");
   putchar( c );
   return 0;
}

The char *gets(char *s) function reads a line from stdin into the buffer pointed to by s until either a terminating newline or EOF (End of File).

The int puts(const char *s) function writes the string ‘s’ and ‘a’ trailing newline to stdout.

NOTE: Though it has been deprecated to use gets() function, Instead of using gets, you want to use fgets().

#include <stdio.h>
int main() {
    char str[100];
    printf("Enter a value :");
    // gets(str); warning: this program uses gets(), which is unsafe.
    fgets(str, 100, stdin);
    printf("You entered: ");
    puts(str);
    return 0;
}

The int scanf(const char *format, …) function reads the input from the standard input stream stdin and scans that input according to the format provided.

The int printf(const char *format, …) function writes the output to the standard output stream stdout and produces the output according to the format provided.

The format can be a simple constant string, but you can specify %s, %d, %c, %f, etc., to print or read strings, integer, character or float respectively. There are many other formatting options available which can be used based on requirements. Let us now proceed with a simple example to understand the concepts better −

Here, it should be noted that scanf() expects input in the same format as you provided %s and %d, which means you have to provide valid inputs like “string integer”. If you provide “string string” or “integer integer”, then it will be assumed as wrong input. Secondly, while reading a string, scanf() stops reading as soon as it encounters a space, so “this is test” are three strings for scanf().

C – File I/O

A file represents a sequence of bytes, regardless of it being a text file or a binary file. C programming language provides access on high level functions as well as low level (OS level) calls to handle file on your storage devices.

You can use the fopen( ) function to create a new file or to open an existing file. This call will initialize an object of the type FILE, which contains all the information necessary to control the stream. The prototype of this function call is as follows −

FILE *fopen( const char * filename, const char * mode );

Here, filename is a string literal, which you will use to name your file, and access mode can have one of the following values −

1. r: Opens an existing text file for reading purpose.
2. w: Opens a text file for writing. If it does not exist, then a new file is created. Here your program will start writing content from the beginning of the file.
3. a: Opens a text file for writing in appending mode. If it does not exist, then a new file is created. Here your program will start appending content in the existing file content.
4. r+: Opens a text file for both reading and writing.
5. w+: Opens a text file for both reading and writing. It first truncates the file to zero length if it exists, otherwise creates a file if it does not exist.
6. a+: Opens a text file for both reading and writing. It creates the file if it does not exist. The reading will start from the beginning but writing can only be appended.

If you are going to handle binary files, then you will use following access modes instead of the above mentioned ones −

"rb", "wb", "ab", "rb+", "r+b", "wb+", "w+b", "ab+", "a+b"

To close a file, use the fclose( ) function. The prototype of this function is −

int fclose( FILE *fp );

The fclose(-) function returns zero on success, or EOF if there is an error in closing the file. This function actually flushes any data still pending in the buffer to the file, closes the file, and releases any memory used for the file. The EOF is a constant defined in the header file stdio.h.

Following is the simplest function to write individual characters to a stream −

int fputc( int c, FILE *fp );

The function fputc() writes the character value of the argument c to the output stream referenced by fp. It returns the written character written on success otherwise EOF if there is an error. You can use the following functions to write a null-terminated string to a stream −

int fputs( const char *s, FILE *fp );

The function fputs() writes the string s to the output stream referenced by fp. It returns a non-negative value on success, otherwise EOF is returned in case of any error. You can use int fprintf(FILE *fp,const char *format, …) function as well to write a string into a file.

#include <stdio.h>
int main() {
   FILE *fp;
   fp = fopen("test.log", "w+");
   fprintf(fp, "This is testing for fprintf...\n");
   fputs("This is testing for fputs...\n", fp);
   fclose(fp);
}

Given below is the simplest function to read a single character from a file −

int fgetc( FILE * fp );

The fgetc() function reads a character from the input file referenced by fp. The return value is the character read, or in case of any error, it returns EOF. The following function allows to read a string from a stream −

char *fgets( char *buf, int n, FILE *fp );

The functions fgets() reads up to n-1 characters from the input stream referenced by fp. It copies the read string into the buffer buf, appending a null character to terminate the string.

If this function encounters a newline character ‘\n’ or the end of the file EOF before they have read the maximum number of characters, then it returns only the characters read up to that point including the new line character. You can also use int fscanf(FILE *fp, const char *format, ...) function to read strings from a file, but it stops reading after encountering the first space character.

There are two functions, that can be used for binary input and output −

size_t fread(void *ptr, size_t size_of_elements, size_t number_of_elements, FILE *a_file);

size_t fwrite(const void *ptr, size_t size_of_elements, size_t number_of_elements, FILE *a_file);

Both of these functions should be used to read or write blocks of memories – usually arrays or structures.

C – Preprocessors

The C Preprocessor is not a part of the compiler, but is a separate step in the compilation process. In simple terms, a C Preprocessor is just a text substitution tool and it instructs the compiler to do required pre-processing before the actual compilation. We’ll refer to the C Preprocessor as CPP.

All preprocessor commands begin with a hash symbol (#). It must be the first nonblank character, and for readability, a preprocessor directive should begin in the first column. The following section lists down all the important preprocessor directives −

1. #define: Substitutes a preprocessor macro.
2. #include: Inserts a particular header from another file.
3. #undef: Undefines a preprocessor macro.
4. #ifdef: Returns true if this macro is defined.
5. #ifndef: Returns true if this macro is not defined.
6. #if: Tests if a compile time condition is true.
7. #else: The alternative for #if.
8. #elif: #else and #if in one statement.
9. #endif: Ends preprocessor conditional.
10. #error: Prints error message on stderr.
11. #pragma: Issues special commands to the compiler, using a standardized method.

Examples:

#define MAX_ARRAY_LENGTH 20

This directive tells the CPP to replace instances of MAX_ARRAY_LENGTH with 20. Use #define for constants to increase readability.

#include <stdio.h>
#include "myheader.h"

These directives tell the CPP to get stdio.h from System Libraries and add the text to the current source file. The next line tells CPP to get myheader.h from the local directory and add the content to the current source file.

#undef  FILE_SIZE
#define FILE_SIZE 42

It tells the CPP to undefine existing FILE_SIZE and define it as 42.

#ifndef MESSAGE
   #define MESSAGE "You wish!"
#endif

It tells the CPP to define MESSAGE only if MESSAGE isn’t already defined.

#ifdef DEBUG
   /* Your debugging statements here */
#endif

It tells the CPP to process the statements enclosed if DEBUG is defined. This is useful if you pass the -DDEBUG flag to the gcc compiler at the time of compilation. This will define DEBUG, so you can turn debugging on and off on the fly during compilation.

ANSI C defines a number of macros. Although each one is available for use in programming, the predefined macros should not be directly modified.

1. DATE The current date as a character literal in “MMM DD YYYY” format.
2. TIME The current time as a character literal in “HH:MM:SS” format.
3. FILE This contains the current filename as a string literal.
4. LINE This contains the current line number as a decimal constant.
5. STDC Defined as 1 when the compiler complies with the ANSI standard.

The C preprocessor offers the following operators to help create macros −

The Macro Continuation () Operator

A macro is normally confined to a single line. The macro continuation operator () is used to continue a macro that is too long for a single line. For example −

#define  message_for(a, b)  \
   printf(#a " and " #b ": We love you!\n")

The Stringize (#) Operator

The stringize or number-sign operator ( ‘#’ ), when used within a macro definition, converts a macro parameter into a string constant. This operator may be used only in a macro having a specified argument or parameter list. For example −

#include <stdio.h>
#define  message_for(a, b)  \
       printf(#a " and " #b ": We love you!\n")
int main(void) {
   message_for(Carole, Debra);
   return 0;
}

The Token Pasting (##) Operator

The token-pasting operator (##) within a macro definition combines two arguments. It permits two separate tokens in the macro definition to be joined into a single token. For example −

#include <stdio.h>
#define tokenpaster(n) printf("token" #n " = %d\n", token##n)
int main(void) {
    int token34 = 40;
    int token35 = 50;
    tokenpaster(34); // = printf ("token34 = %d", token34);
    tokenpaster(35);
    return 0;
}

The Defined() Operator

The preprocessor defined operator is used in constant expressions to determine if an identifier is defined using #define. If the specified identifier is defined, the value is true (non-zero). If the symbol is not defined, the value is false (zero). The defined operator is specified as follows −

#include <stdio.h>
#if !defined(MESSAGE)
    #define MESSAGE "You wish!"
#endif
int main(void) {
    printf("Here is the message: %s\n", MESSAGE);
    return 0;
}

Parameterized Macros

One of the powerful functions of the CPP is the ability to simulate functions using parameterized macros. For example, we might have some code to square a number as follows −

int square(int x) {return x * x;}

We can rewrite above the code using a macro as follows −

#define square(x) ((x) * (x))

Macros with arguments must be defined using the #define directive before they can be used. The argument list is enclosed in parentheses and must immediately follow the macro name. Spaces are not allowed between the macro name and open parenthesis. For example −

#include <stdio.h>
#define MAX(x, y) ((x) > (y) ? (x) : (y))
int main(void) {
    printf("Max between 20 and 10 is %d\n", MAX(10, 20));
    return 0;
}

C – Header Files

A header file is a file with extension .h which contains C function declarations and macro definitions to be shared between several source files. There are two types of header files: the files that the programmer writes and the files that comes with your compiler.

You request to use a header file in your program by including it with the C preprocessing directive #include, like you have seen inclusion of stdio.h header file, which comes along with your compiler.

Including a header file is equal to copying the content of the header file but we do not do it because it will be error-prone and it is not a good idea to copy the content of a header file in the source files, especially if we have multiple source files in a program.

A simple practice in C or C++ programs is that we keep all the constants, macros, system wide global variables, and function prototypes in the header files and include that header file wherever it is required.

Both the user and the system header files are included using the preprocessing directive #include. It has the following two forms −

#include <file>

This form is used for system header files. It searches for a file named ‘file’ in a standard list of system directories. You can prepend directories to this list with the -I option while compiling your source code.

#include "file"

This form is used for header files of your own program. It searches for a file named ‘file’ in the directory containing the current file. You can prepend directories to this list with the -I option while compiling your source code.

The #include directive works by directing the C preprocessor to scan the specified file as input before continuing with the rest of the current source file. The output from the preprocessor contains the output already generated, followed by the output resulting from the included file, followed by the output that comes from the text after the #include directive. For example, if you have a header file header.h as follows −

char *test (void);

and a main program called program.c that uses the header file, like this −

int x;
#include "header.h"
int main (void) {
   puts (test ());
}

the compiler will see the same token stream as it would if program.c read.

int x;
char *test (void);
int main (void) {
   puts (test ());
}

If a header file happens to be included twice, the compiler will process its contents twice and it will result in an error. The standard way to prevent this is to enclose the entire real contents of the file in a conditional, like this −

#ifndef HEADER_FILE
#define HEADER_FILE
the entire header file file
#endif

This construct is commonly known as a wrapper #ifndef. When the header is included again, the conditional will be false, because HEADER_FILE is defined. The preprocessor will skip over the entire contents of the file, and the compiler will not see it twice.

Sometimes it is necessary to select one of the several different header files to be included into your program. For instance, they might specify configuration parameters to be used on different sorts of operating systems. You could do this with a series of conditionals as follows −

#if SYSTEM_1
   # include "system_1.h"
#elif SYSTEM_2
   # include "system_2.h"
#elif SYSTEM_3
   ...
#endif

But as it grows, it becomes tedious, instead the preprocessor offers the ability to use a macro for the header name. This is called a computed include. Instead of writing a header name as the direct argument of #include, you simply put a macro name there −

#define SYSTEM_H "system_1.h"
...
#include SYSTEM_H

SYSTEM_H will be expanded, and the preprocessor will look for system_1.h as if the #include had been written that way originally. SYSTEM_H could be defined by your Makefile with a -D option.

C – Type Casting

Type casting is a way to convert a variable from one data type to another data type. For example, if you want to store a ‘long’ value into a simple integer then you can type cast ‘long’ to ‘int’. You can convert the values from one type to another explicitly using the cast operator as follows −

(type_name) expression

Consider the following example where the cast operator causes the division of one integer variable by another to be performed as a floating-point operation −

#include <stdio.h>
main() {
   int sum = 17, count = 5;
   double mean;
   mean = (double) sum / count;
   printf("Value of mean : %f\n", mean ); // Value of mean : 3.400000
}

It should be noted here that the cast operator has precedence over division, so the value of sum is first converted to type double and finally it gets divided by count yielding a double value.

Type conversions can be implicit which is performed by the compiler automatically, or it can be specified explicitly through the use of the cast operator. It is considered good programming practice to use the cast operator whenever type conversions are necessary.

Integer promotion is the process by which values of integer type “smaller” than int or unsigned int are converted either to int or unsigned int. Consider an example of adding a character with an integer −

#include <stdio.h>
int main() {
    int i = 17;
    char c = 'c'; /* ascii value is 99 */
    int sum = i + c;
    printf("Value of sum : %d\n", sum); //Value of sum : 116
}

The usual arithmetic conversions are implicitly performed to cast their values to a common type. The compiler first performs integer promotion; if the operands still have different types, then they are converted to the type that appears highest in the following hierarchy −

int->unsigned int->long->unsigned long->long long->unsigned long long->float->double->long double

The usual arithmetic conversions are not performed for the assignment operators, nor for the logical operators && and ||. Let us take the following example to understand the concept −

#include <stdio.h>
int main() {
    int i = 17;
    char c = 'c'; /* ascii value is 99 */
    float sum = i + c;
    printf("Value of sum : %f\n", sum); // Value of sum : 116.000000
}

Here, it is simple to understand that first c gets converted to integer, but as the final value is double, usual arithmetic conversion applies and the compiler converts i and c into ‘float’ and adds them yielding a ‘float’ result.

C – Error Handling

As such, C programming does not provide direct support for error handling but being a system programming language, it provides you access at lower level in the form of return values. Most of the C or even Unix function calls return -1 or NULL in case of any error and set an error code errno. It is set as a global variable and indicates an error occurred during any function call. You can find various error codes defined in header file.

So a C programmer can check the returned values and can take appropriate action depending on the return value. It is a good practice, to set errno to 0 at the time of initializing a program. A value of 0 indicates that there is no error in the program.

The C programming language provides perror() and strerror() functions which can be used to display the text message associated with errno.

The perror() function displays the string you pass to it, followed by a colon, a space, and then the textual representation of the current errno value.

The strerror() function, which returns a pointer to the textual representation of the current errno value.

Let’s try to simulate an error condition and try to open a file which does not exist. Here I’m using both the functions to show the usage, but you can use one or more ways of printing your errors. Second important point to note is that you should use stderr file stream to output all the errors.

#include <errno.h>
#include <stdio.h>
#include <string.h>
extern int errno;
int main() {
    FILE *pf;
    int errnum;
    pf = fopen("unexist.txt", "rb");
    if (pf == NULL) {
        errnum = errno;
        fprintf(stderr, "Value of errno: %d\n", errno);
        perror("Error printed by perror");
        fprintf(stderr, "Error opening file: %s\n", strerror(errnum));
    } else {
        fclose(pf);
    }
    return 0;
}

output:

Value of errno: 2
Error printed by perror: No such file or directory
Error opening file: No such file or directory

It is a common problem that at the time of dividing any number, programmers do not check if a divisor is zero and finally it creates a runtime error.

The code below fixes this by checking if the divisor is zero before dividing −

#include <stdio.h>
#include <stdlib.h>
int main() {
    int dividend = 20;
    int divisor = 0;
    int quotient;
    if (divisor == 0) {
        fprintf(stderr, "Division by zero! Exiting...\n");
        exit(-1);
    }
    quotient = dividend / divisor;
    fprintf(stderr, "Value of quotient : %d\n", quotient);
    exit(0);
}

It is a common practice to exit with a value of EXIT_SUCCESS in case of program coming out after a successful operation. Here, EXIT_SUCCESS is a macro and it is defined as 0.

If you have an error condition in your program and you are coming out then you should exit with a status EXIT_FAILURE which is defined as -1. So let’s write above program as follows −

#include <stdio.h>
#include <stdlib.h>
int main() {
    int dividend = 20;
    int divisor = 5;
    int quotient;
    if (divisor == 0) {
        fprintf(stderr, "Division by zero! Exiting...\n");
        exit(EXIT_FAILURE);
    }
    quotient = dividend / divisor;
    fprintf(stderr, "Value of quotient : %d\n", quotient);
    exit(EXIT_SUCCESS);
}

C – Recursion

Recursion is the process of repeating items in a self-similar way. In programming languages, if a program allows you to call a function inside the same function, then it is called a recursive call of the function.

void recursion() {
   recursion(); /* function calls itself */
}

The C programming language supports recursion, i.e., a function to call itself. But while using recursion, programmers need to be careful to define an exit condition from the function, otherwise it will go into an infinite loop.

Recursive functions are very useful to solve many mathematical problems, such as calculating the factorial of a number, generating Fibonacci series, etc.

The following example calculates the factorial of a given number using a recursive function −

#include <stdio.h>
#define N 200
long double fibonacci(unsigned int i) {
    static long double f[N];
    if (i <= 1) {
        f[i] = i;
    } else {
        if (f[i] == 0) {
            f[i]= fibonacci(i - 1) + fibonacci(i - 2);
        }
    }
    return f[i];
}
int main() {
    int i;
    for (i = 0; i < N; i++) {
        printf("%Lf\t\n", fibonacci(i));
    }
    return 0;
}

C – Variable Arguments

Sometimes, you may come across a situation, when you want to have a function, which can take variable number of arguments, i.e., parameters, instead of predefined number of parameters. The C programming language provides a solution for this situation and you are allowed to define a function which can accept variable number of parameters based on your requirement. The following example shows the definition of such a function.

int func(int, ... ) {
   .
}
int main() {
   func(1, 2, 3);
   func(1, 2, 3, 4);
}

It should be noted that the function func() has its last argument as ellipses, i.e. three dotes (…) and the one just before the ellipses is always an int which will represent the total number variable arguments passed. To use such functionality, you need to make use of stdarg.h header file which provides the functions and macros to implement the functionality of variable arguments and follow the given steps −

• Define a function with its last parameter as ellipses and the one just before the ellipses is always an int which will represent the number of arguments.
• Create a va_list type variable in the function definition. This type is defined in stdarg.h header file.
• Use int parameter and va_start macro to initialize the va_list variable to an argument list. The macro va_start is defined in stdarg.h header file.
• Use va_arg macro and va_list variable to access each item in argument list.
• Use a macro va_end to clean up the memory assigned to va_list variable.

Now let us follow the above steps and write down a simple function which can take the variable number of parameters and return their average −

#include <stdarg.h>
#include <stdio.h>
double average(int num, ...) {
    va_list valist;
    double sum = 0.0;
    int i;
    /* initialize valist for num number of arguments */
    va_start(valist, num);
    /* access all the arguments assigned to valist */
    for (i = 0; i < num; i++) {
        sum += va_arg(valist, int);
    }
    /* clean memory reserved for valist */
    va_end(valist);
    return sum / num;
}
int main() {
    printf("Average of 2, 3, 4, 5 = %f\n", average(4, 2, 3, 4, 5));
    printf("Average of 5, 10, 15 = %f\n", average(3, 5, 10, 15));
}

C – Memory Management

The C programming language provides several functions for memory allocation and management. These functions can be found in the header file.

1. void *calloc(int num, int size); This function allocates an array of num elements each of which size in bytes will be size.
2. void free(void *address); This function releases a block of memory block specified by address.
3. void *malloc(int num); This function allocates an array of num bytes and leave them uninitialized.
4. void *realloc(void *address, int newsize); This function re-allocates memory extending it upto newsize.

While programming, if you are aware of the size of an array, then it is easy and you can define it as an array. For example, to store a name of any person, it can go up to a maximum of 100 characters, so you can define something as follows −

char name[100];

But now let us consider a situation where you have no idea about the length of the text you need to store, for example, you want to store a detailed description about a topic. Here we need to define a pointer to character without defining how much memory is required and later, based on requirement, we can allocate memory as shown in the below example −

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int main() {
    char name[100];
    char *description;
    strcpy(name, "Zara Ali");
    /* allocate memory dynamically */
    // description = malloc(200 * sizeof(char));
    description = calloc(200, sizeof(char));
    if (description == NULL) {
        fprintf(stderr, "Error - unable to allocate required memory\n");
    } else {
        strcpy(description, "Zara ali a DPS student in class 10th");
    }
    printf("Name = %s\n", name);
    printf("Description: %s\n", description);
}

When your program comes out, operating system automatically release all the memory allocated by your program but as a good practice when you are not in need of memory anymore then you should release that memory by calling the function free().

Alternatively, you can increase or decrease the size of an allocated memory block by calling the function realloc(). Let us check the above program once again and make use of realloc() and free() functions −

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int main() {
    char name[100];
    char *description;
    strcpy(name, "Zara Ali");
    /* allocate memory dynamically */
    description = malloc(30 * sizeof(char));
    if (description == NULL) {
        fprintf(stderr, "Error - unable to allocate required memory\n");
    } else {
        strcpy(description, "Zara ali a DPS student.");
    }
    /* suppose you want to store bigger description */
    description = realloc(description, 100 * sizeof(char));
    if (description == NULL) {
        fprintf(stderr, "Error - unable to allocate required memory\n");
    } else {
        strcat(description, "She is in class 10th");
    }
    printf("Name = %s\n", name);
    printf("Description: %s\n", description);
    /* release memory using free() function */
    free(description);
    return 0;
}

C – Command Line Arguments

It is possible to pass some values from the command line to your C programs when they are executed. These values are called command line arguments and many times they are important for your program especially when you want to control your program from outside instead of hard coding those values inside the code.

The command line arguments are handled using main() function arguments where argc refers to the number of arguments passed, and argv[] is a pointer array which points to each argument passed to the program. Following is a simple example which checks if there is any argument supplied from the command line and take action accordingly −

#include <stdio.h>
int main(int argc, char *argv[]) {
    printf("Program name %s\n", argv[0]);
    if (argc == 2) {
        printf("The argument supplied is %s\n", argv[1]);
    } else if (argc > 2) {
        printf("Too many arguments supplied.\n");
    } else {
        printf("One argument expected.\n");
    }
}

It should be noted that argv[0] holds the name of the program itself and argv[1] is a pointer to the first command line argument supplied, and *argv[n] is the last argument. If no arguments are supplied, argc will be one, and if you pass one argument then argc is set at 2.

You pass all the command line arguments separated by a space, but if argument itself has a space then you can pass such arguments by putting them inside double quotes “” or single quotes ”.

The C Programming Language Notes

ch1 A Tutorial Introduction p22-p46

1. Getting Started
   printf(“hello, world\n”);
2. Variables and Arithmetic Expressions
   celsius = 5 * (fahr-32) / 9;
3. The for statement
   for (fahr = 0; fahr <= 300; fahr = fahr + 20) printf(“%3d %6.1f\n”, fahr, (5.0/9.0)*(fahr-32));
4. Symbolic Constants
   #define UPPER 300
5. Character Input and Output c = getchar(); putchar(c);
   1. File Copying
   2. Character Counting
   3. Line Counting
   4. Word Counting
6. Arrays
   int ndigit[10];
7. Functions
   A function provides a convenient way to encapsulate some computation, which can then be used without worrying about its implementation.
8. Arguments – Call by Value
   In C, all function arguments are passed “by value.” This means that the called function is given the values of its arguments in temporary variables rather than the originals.
9. Character Arrays
   Character array is stored as an array of characters containing the characters in the string and terminated with a ‘\0’ to mark the end.
10. External Variables and Scope
    An external variable must be defined, exactly once, outside of any function; this sets aside storage for it.

ch2 Types, Operators and Expressions p47-p63

1. Variable Names
   Names are made up of letters and digits; the first character must be a letter. The underscore “_” counts as a letter; it is sometimes useful for improving the readability of long variable names. Don’t begin variable names with underscore, however, since library routines often use such names. Upper and lower case letters are distinct, so x and X are two different names.
2. Data Types and Sizes
   There are only a few basic data types in C: char, int, float, double
3. Constants
   A long constant is written with a terminal l (ell) or L, as in 123456789L; an integer constant too big to fit into an int will also be taken as a long. Unsigned constants are written with a terminal u or U, and the suffix ul or UL indicates unsigned long.
   The first name in an enum has value 0, the next 1, and so on, unless explicit values are specified.
4. Declarations
   All variables must be declared before use, although certain declarations can be made implicitly by content.
5. Arithmetic Operators
   The binary arithmetic operators are +, -, *, /, and the modulus operator %.
6. Relational and Logical Operators
   The relational operators are > >= < <=. Expressions connected by && or || are evaluated left to right, and evaluation stops as soon as the truth or falsehood of the result is known.
7. Type Conversions
   When an operator has operands of different types, they are converted to a common type according to a small number of rules.
8. Increment and Decrement Operators
   The increment operator ++ adds 1 to its operand, while the decrement operator — subtracts 1.
9. Bitwise Operators
   The bitwise exclusive OR operator ^ sets a one in each bit position where its operands have different bits, and zero where they are the same.
10. Assignment Operators and Expressions
    i += 2 is equivalent to i = i + 2
11. Conditional Expressions
    z = (a > b) ? a : b;
12. Precedence and Order of Evaluation
    x = f() + g();
    f may be evaluated before g or vice versa; thus if either f or g alters a variable on which the other depends, x can depend on the order of evaluation.

ch3 Control Flow p64-p74

1. Statements and Blocks
   x = 0; Braces { and } are used to group declarations and statements together into a compound statement, or block, so that they are syntactically equivalent to a single statement.
2. If-Else
   if (a > b) z = a; else z = b;
3. Else-If
   if (expression)
   statement
   else if (expression)
   statement
   else
   statement
4. Switch
   switch (expression) {
   case const-expr: statements
   case const-expr: statements
   default: statements
   }
5. Loops – While and For
   while (expression)
   statement
   for (expr1; expr2; expr3)
   statement
6. Loops – Do-While
   do
   statement
   while (expression);
7. Break and Continue
   The break statement provides an early exit from for, while, and do, just as from switch. The continue statement causes the next iteration of the enclosing for, while, or do loop to begin.
8. Goto and labels
   C provides the infinitely-abusable goto statement, and labels to branch to.

ch4 Functions and Program Structure p75-p95

1. Basics of Functions
   cc main.c getline.c strindex.c -o grep.exe
2. Functions Returning Non-integers
   double atof(char s[])
3. External Variables
   A C program consists of a set of external objects, which are either variables or functions.
4. Scope Rules
   If an external variable is to be referred to before it is defined, or if it is defined in a different source file from the one where it is being used, then an extern declaration is mandatory.
   in file1: extern int sp;
   in file2: int sp = 0;
5. Header Files
   #include “calc.h”
6. Static Variables
   The static declaration, applied to an external variable or function, limits the scope of that object to the rest of the source file being compiled.
7. Register Variables
   A register declaration advises the compiler that the variable in question will be heavily used.
8. Block Structure
   Declarations of variables (including initializations) may follow the left brace that introduces any compound statement, not just the one that begins a function.
9. Initialization
   In the absence of explicit initialization, external and static variables are guaranteed to be initialized to zero; automatic and register variables have undefined (i.e., garbage) initial values.
   char pattern[] = { ‘o’, ‘u’, ‘l’, ‘d’, ‘\0’ };// char pattern = “ould”; wouldn’t work.
10. Recursion
    C functions may be used recursively; that is, a function may call itself either directly or indirectly.
11. The C Preprocessor
    1. File Inclusion
       If the filename is quoted, searching for the file typically begins where the source program was found; if it is not found there, or if the name is enclosed in < and >, searching follows an implementation-defined rule to find the file.
    2. Macro Substitution
       The scope of a name defined with #define is from its point of definition to the end of the source file being compiled.
       #define max(A, B) ((A) > (B) ? (A) : (B))
       The expressions are evaluated twice; this is bad if they involve side effects like increment operators or input and output.
       #define dprint(expr) printf(#expr ” = %g\n”, expr)
       If a parameter name is preceded by a # in the replacement text, the combination will be expanded into a quoted string with the parameter replaced by the actual argument.
    3. Conditional Inclusion
       It is possible to control preprocessing itself with conditional statements that are evaluated during preprocessing.

ch5 pointer p96-125

1. Pointers and Addresses
   int x=1, y;
   int *p; p=&x; y = *p;
2. Pointers and Function Arguments
   void swap(int *px, int *py)
   swap(&a, &b);
3. Pointers and Arrays
   int a[10], *pa, x;
   pa = &a[i]; // pa = a+i;
   x = *pa; // x= a[0];
   x = *(pa+1); // x = a[1]; x= *(a+1)
   f(&a[2]); // f(a+2)
4. Address Arithmetic
   int *ptr; ptr = (int *)malloc(1024 * sizeof(ptr));
5. Character Pointers and Functions
   char amessage[] = “now is the time”; /* an array */
   char *pmessage = “now is the time”; /* a pointer */
   void strcpy(char *s, char t){ while (s++ = *t++);
   }
6. Pointer Arrays; Pointers to Pointers
   char lineptr[MAXLINES]; / pointers to text lines */
   lineptr[i] is a character pointer, *lineptr[i] is the character it points to.
7. Multi-dimensional Arrays
   static char daytab[2][13] = {
   {0, 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31},
   {0, 31, 29, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31}
   };
   int *daytab[13] is an array of 13 pointers to integers.
   daytab[i] is a integer pointer, *daytab[i] is the interger it points to.
8. Initialization of Pointer Arrays
   char *month_name(int n)
   {
   static char *name[] = {
   “Illegal month”,
   “January”, “February”, “March”,
   “April”, “May”, “June”,
   “July”, “August”, “September”,
   “October”, “November”, “December”
   };
   return (n < 1 || n > 12) ? name[0] : name[n];
   }
9. Pointers vs. Multi-dimensional Arrays
   int a[10][20]; //fixed size
   int *b[10]; // initialization must be done explicitly, either statically or with code.
10. Command-line Arguments
    int main(int argc, char *argv[])
    For “echo hello, world” command, argc is 3, and argv[0], argv[1], and argv[2] are “echo”, “hello,”, and “world” respectively.
11. Pointers to Functions
    void qsort(void lineptr[], int left, int right, int (comp)(void *, void *));
    int (*comp)(void *, void *) says that comp is a pointer to a function that has two void * arguments and returns an int.
    call pointer to function with
    (*comp)(v[i], v[left]) // comp(v[i],v[left]) also works, because a function’s name can also be used to get functions’ address.
12. Complicated Declarations
    void comp() // comp: function returning pointer to void void (comp)() // comp: pointer to function returning void

ch6 struct p126-p146

1. Basics of Structures
   struct point {int x; int y;} x, y, z; // similar to int x, y, z;
2. Structures and Functions
   legal operations to a struct: copy, assign it as a unit, take address by &, accessing its member.
   struct point makepoint(int x, int y)
   {
   struct point temp;
   temp.x = x;
   temp.y = y;
   return temp;
   }
   struct point pp; access x by（pp).x or pp->x;
3. Arrays of Structures
   struct key {
   char *word;
   int count;
   } keytab[NKEYS];
   //equals
   struct key {
   char *word;
   int count;
   };
   struct key keytab[NKEYS];
4. Pointers to Structures
   struct key *binsearch(char *word, struct key *tab, int n)
   pointer arithmetic that involves the first element beyond the end of an array (that is, &tab[n]) will work correctly.
5. Self-referential Structures
   struct tnode{
   char *word;
   int count;
   struct tnode *left;
   struct tnode *right;
   }
6. Table Lookup
   struct nlist *lookup(char *s)
   for (ptr = head; ptr != NULL; ptr = ptr->next)
7. Typedef
   typedef int Length;
   typedef char *String;
   String p, lineptr[MAXLINES], alloc(int);
   int strcmp(String, String);
   p = (String) malloc(100);
8. Unions
   union u_tag {
   int ival;
   float fval;
   char *sval;
   } u;
   struct {
   char *name;
   int flags;
   int utype;
   union {
   int ival;
   float fval;
   char *sval;
   } u;
   } symtab[NSYM];
9. Bit-fields
   struct {
   unsigned int is_keyword : 1;
   unsigned int is_extern : 1;
   unsigned int is_static : 1;
   } flags;
   flags that contains three 1-bit fields

ch7 Input and Output p147-p161

1. Standard Input and Output
   while ((c = getchar()) != EOF) putchar(tolower(c));
2. Formatted Output – printf
   int printf(char *format, arg1, arg2, …);
3. Variable-length Argument Lists
   void minprintf(char *fmt, …)
4. Formatted Input – Scanf
   int scanf(char *format, …)
5. File Access
   FILE *fp;p
6. Error Handling – Stderr and Exit
   fprintf(stderr, “%s: can’t open %s\n”, prog, *argv);
7. Line Input and Output
   char *fgets(char *line, int maxline, FILE *fp)
   int fputs(char *line, FILE *fp)
8. Miscellaneous Functions
   system(“date”);
   #define frand() ((double) rand() / (RAND_MAX+1.0))

ch8 The UNIX System Interface p162-178

1. File Descriptors
   prog outfile
2. Low Level I/O – Read and Write
   int n_read = read(int fd, char *buf, int n);
   int n_written = write(int fd, char *buf, int n);
3. Open, Creat, Close, Unlink
   int open(char *name, int flags, int perms);
   int creat(char *name, int perms);
4. Random Access – Lseek
   long lseek(int fd, long offset, int origin);
5. Example – An implementation of Fopen and Getc
   FILE *fopen(char *name, char *mode)
6. Example – Listing Directories
   void fsize(char *name)
   void dirwalk(char *dir, void (*fcn)(char *))
7. Example – A Storage Allocator
   void *malloc(unsigned nbytes)

Appendix A: Reference Manual p179-p233

1. Introduction
2. Lexical Conventions
   There are six classes of tokens: identifiers, keywords, constants, string literals, operators, and other separators.
   Upper and lower case letters are different.
   Keywords auto double int struct break else long switch case enum register typedef char extern return union const float short unsigned continue for signed void default goto sizeof volatile do if static while
3. Syntax Notation
   { expression\sub(opt) } means an optional expression, enclosed in braces.
4. Meaning of Identifiers
   Identifiers, or names, refer to a variety of things: functions; tags of structures, unions, and enumerations; members of structures or unions; enumeration constants; typedef names; and objects.
   There are two storage classes: automatic and static.
5. Objects and Lvalues
   An Object is a named region of storage; an lvalue is an expression referring to an object.
6. Conversions
   Integral Promotion, Integral Conversions, Integer and Floating, Floating Types, Arithmetic Conversions, Pointers and Integers
7. Expressions
   A pair of expressions separated by a comma is evaluated left-to-right, and the value of the left expression is discarded.
8. Declarations
   In a declaration T D:
   When D is an unadored identifier, the type of the identifier is T.
   When D has the form ( D1 ) then the type of the identifier in D1 is the same as that of D. The parentheses do not alter the type, but may change the binding of complex declarators.
   int i, *pi, *const cpi = &i;
   const int ci = 3, *pci;
   declare an integer i and a pointer to an integer pi. The value of the constant pointer cpi may not be changed; it will always point to the same location, although the value to which it refers may be altered. The integer ci is constant, and may not be changed (though it may be initialized, as here.) The type of pci is “pointer to const int,” and pci itself may be changed to point to another place, but the value to which it points may not be altered by assigning through pci.
9. Statements
   Except as described, statements are executed in sequence.
10. External Declarations
    The unit of input provided to the C compiler is called a translation unit; it consists of a sequence of external declarations, which are either declarations or function definitions.
11. Scope and Linkage
    A program need not all be compiled at one time: the source text may be kept in several files containing translation units, and precompiled routines may be loaded from libraries.
12. Preprocessor
    A preprocessor performs macro substitution, conditional compilation, and inclusion of named files.
13. Grammar
    A recapitulation of the grammar that was given throughout the earlier part of this appendix.

Appendix B – Standard Library p234-p296

1. Input and Output: The input and output functions, types, and macros defined in represent nearly one third of the library.
   1. File Operations
   2. Formatted Output
   3. Formatted Input
   4. Character Input and Output Functions
   5. Direct Input and Output Functions
   6. File Positioning Functions
   7. Error Functions
2. Character Class Tests:
   The header declares functions for testing characters.
3. String Functions:
   There are two groups of string functions defined in the header . The first have names beginning with str; the second have names beginning with mem.
4. Mathematical Functions:
   The header declares mathematical functions and macros.
5. Utility Functions:
   The header declares functions for number conversion, storage allocation, and similar tasks.
6. Diagnostics:
   The assert macro is used to add diagnostics to programs.
7. Variable Argument Lists:
   The header provides facilities for stepping through a list of function arguments of unknown number and type.
8. Non-local Jumps:
   The declarations in provide a way to avoid the normal function call and return sequence, typically to permit an immediate return from a deeply nested function call.
9. Signals:
   The header provides facilities for handling exceptional conditions that arise during execution, such as an interrupt signal from an external source or an error in execution.
10. Date and Time Functions:
    The header declares types and functions for manipulating date and time.
11. Implementation-defined Limits: and
    The header defines constants for the sizes of integral types. The values below are acceptable minimum magnitudes; larger values may be used.

今天在twitter上面看到一个好玩的视频，我就想下载下来，结果折腾了几个小时。

首先看网页代码，视频那里是一个blob：

<video preload="none" playsinline="" aria-label="Embedded video" poster="https://pbs.twimg.com/ext_tw_video_thumb/1200926756321951745/pu/img/l-V1sNMeAZL5tcGN.jpg" src="blob:https://twitter.com/dd866381-2ada-4c2b-80af-9cec2a580039" style="width: 100%; height: 100%; position: absolute; background-color: black; top: 0%; left: 0%; transform: rotate(0deg) scale(1.005);"></video>

我google了一下，这是流媒体的格式，要下载的话需要找到对应的m3u8文件，就是在chrom浏览器的Network里面过滤m3u8. 看到一个https://video.twimg.com/ext_tw_video/1200926756321951745/pu/pl/460×258/Kuir9CTQnfaw-EWA.m3u8

Google说可以用youtube-dl下载，然后我先去下载这个玩意，因为还有同学说可以直接用ffmpeg转换格式。

等youtube-dl下载完了之后，我运行下面的命令：

./youtube-dl -f mp4 -o baby.mp4 https://video.twimg.com/ext_tw_video/1200926756321951745/pu/pl/460x258/Kuir9CTQnfaw-EWA.m3u8

结果提示需要ffmpeg或者avconv，尼玛还是得安装ffmpeg。

然后我就老实的brew install ffmpeg。完事之后。

运行：

./youtube-dl -f mp4 -o baby.mp4 https://video.twimg.com/ext_tw_video/1200926756321951745/pu/pl/460x258/Kuir9CTQnfaw-EWA.m3u8

结果卡在那儿不动弹。

我就试一下直接用ffmpeg转换:

ffmpeg -i 'https://video.twimg.com/ext_tw_video/1200926756321951745/pu/pl/460x258/Kuir9CTQnfaw-EWA.m3u8' -vcodec copy -acodec copy -absf aac_adtstoasc o.mp4

也是一样的，下到一半卡在那儿不动弹。

我就换了个思路，想先看看m3u8里面的内容,

wget https://video.twimg.com/ext_tw_video/1200926756321951745/pu/pl/460x258/Kuir9CTQnfaw-EWA.m3u8
cat Kuir9CTQnfaw-EWA.m3u8

结果是这样:

#EXTM3U
#EXT-X-VERSION:6
#EXT-X-MEDIA-SEQUENCE:0
#EXT-X-TARGETDURATION:4
#EXT-X-PLAYLIST-TYPE:VOD
#EXT-X-ALLOW-CACHE:YES
#EXTINF:3.000,
/ext_tw_video/1200926756321951745/pu/vid/0/3000/460x258/G5KaOzbFFR97qdzl.ts
#EXTINF:3.000,
/ext_tw_video/1200926756321951745/pu/vid/3000/6000/460x258/4lEqbRtnJ2YtXHRw.ts
#EXTINF:3.000,
/ext_tw_video/1200926756321951745/pu/vid/6000/9000/460x258/uSvCKT4MckCDGKSf.ts
#EXTINF:3.000,
/ext_tw_video/1200926756321951745/pu/vid/9000/12000/460x258/QSnzs6dHUar2OXfY.ts
#EXTINF:3.000,
/ext_tw_video/1200926756321951745/pu/vid/12000/15000/460x258/zdytmI8intABhyFW.ts
#EXTINF:3.000,
/ext_tw_video/1200926756321951745/pu/vid/15000/18000/460x258/Gh8owrBVf_s8SwDs.ts
#EXTINF:3.000,
/ext_tw_video/1200926756321951745/pu/vid/18000/21000/460x258/GWnvSye6J8mkjr35.ts
#EXTINF:3.000,
/ext_tw_video/1200926756321951745/pu/vid/21000/24000/460x258/Bmd-gN7sTAtj_bme.ts
#EXTINF:3.000,
/ext_tw_video/1200926756321951745/pu/vid/24000/27000/460x258/lkTRSPSKrGpFB_h6.ts
#EXTINF:3.000,
/ext_tw_video/1200926756321951745/pu/vid/27000/30000/460x258/DcZQHhRTJOJ-HNyJ.ts
#EXTINF:3.000,
/ext_tw_video/1200926756321951745/pu/vid/30000/33000/460x258/-3GD9_NdfoDUVF9q.ts
#EXTINF:3.000,
/ext_tw_video/1200926756321951745/pu/vid/33000/36000/460x258/WuuI_REDOMnYjzSU.ts
#EXTINF:3.958,
/ext_tw_video/1200926756321951745/pu/vid/36000/39958/460x258/AZrLcuvgp5XsaOcU.ts
#EXT-X-ENDLIST

我就写了个脚本wget.sh先把这个视频下载下来再转换:

wget https://video.twimg.com/ext_tw_video/1200926756321951745/pu/vid/0/3000/460x258/G5KaOzbFFR97qdzl.ts
wget https://video.twimg.com/ext_tw_video/1200926756321951745/pu/vid/3000/6000/460x258/4lEqbRtnJ2YtXHRw.ts
wget https://video.twimg.com/ext_tw_video/1200926756321951745/pu/vid/6000/9000/460x258/uSvCKT4MckCDGKSf.ts
wget https://video.twimg.com/ext_tw_video/1200926756321951745/pu/vid/9000/12000/460x258/QSnzs6dHUar2OXfY.ts
wget https://video.twimg.com/ext_tw_video/1200926756321951745/pu/vid/12000/15000/460x258/zdytmI8intABhyFW.ts
wget https://video.twimg.com/ext_tw_video/1200926756321951745/pu/vid/15000/18000/460x258/Gh8owrBVf_s8SwDs.ts
wget https://video.twimg.com/ext_tw_video/1200926756321951745/pu/vid/18000/21000/460x258/GWnvSye6J8mkjr35.ts
wget https://video.twimg.com/ext_tw_video/1200926756321951745/pu/vid/21000/24000/460x258/Bmd-gN7sTAtj_bme.ts
wget https://video.twimg.com/ext_tw_video/1200926756321951745/pu/vid/24000/27000/460x258/lkTRSPSKrGpFB_h6.ts
wget https://video.twimg.com/ext_tw_video/1200926756321951745/pu/vid/27000/30000/460x258/DcZQHhRTJOJ-HNyJ.ts
wget https://video.twimg.com/ext_tw_video/1200926756321951745/pu/vid/30000/33000/460x258/-3GD9_NdfoDUVF9q.ts
wget https://video.twimg.com/ext_tw_video/1200926756321951745/pu/vid/33000/36000/460x258/WuuI_REDOMnYjzSU.ts
wget https://video.twimg.com/ext_tw_video/1200926756321951745/pu/vid/36000/39958/460x258/AZrLcuvgp5XsaOcU.ts

下载下来之后，因为m3u8里面的路径也要改一下:
把前面的“/ext_tw_video/1200926756321951745/pu/vid/33000/36000/460×258/”这一串也去掉。变成k.m3u8。

#EXTM3U
#EXT-X-VERSION:6
#EXT-X-MEDIA-SEQUENCE:0
#EXT-X-TARGETDURATION:4
#EXT-X-PLAYLIST-TYPE:VOD
#EXT-X-ALLOW-CACHE:YES
#EXTINF:3.000,
G5KaOzbFFR97qdzl.ts
#EXTINF:3.000,
4lEqbRtnJ2YtXHRw.ts
#EXTINF:3.000,
uSvCKT4MckCDGKSf.ts
#EXTINF:3.000,
QSnzs6dHUar2OXfY.ts
#EXTINF:3.000,
zdytmI8intABhyFW.ts
#EXTINF:3.000,
Gh8owrBVf_s8SwDs.ts
#EXTINF:3.000,
GWnvSye6J8mkjr35.ts
#EXTINF:3.000,
Bmd-gN7sTAtj_bme.ts
#EXTINF:3.000,
lkTRSPSKrGpFB_h6.ts
#EXTINF:3.000,
DcZQHhRTJOJ-HNyJ.ts
#EXTINF:3.000,
-3GD9_NdfoDUVF9q.ts
#EXTINF:3.000,
WuuI_REDOMnYjzSU.ts
#EXTINF:3.958,
AZrLcuvgp5XsaOcU.ts
#EXT-X-ENDLIST

然后再来用ffmpeg -i k.m3u8 k.mp4，就搞定了。

总结一下， ffmpeg这个肯定是需要安装的，youtube-dl是调用这个的。如果网络给力的话，youtube-dl应该可以直接完成转换，但是我试了两次就没成功，所以用wget先把ts文件下载下来再转换。