Cprogramming 简明教程

$ gcc -v

Using built-in specs.
Target: i386-redhat-linux
Configured with: ../configure --prefix=/usr .......
Thread model: posix
gcc version 4.1.2 20080704 (Red Hat 4.1.2-46)

#include <stdio.h>

int main() {
   /* my first program in C */
   printf("Hello, World! \n");

   return 0;
}

$ gcc hello.c
$ ./a.out
Hello, World!

printf("Hello, World! \n");

printf
(
   "Hello, World! \n"
)
;

printf("Hello, World! \n");
return 0;

/* my first program in C */

mohd       zara    abc   move_name  a_123
myname50   _temp   j     a23b9      retVal

int age;

fruit = apples + oranges;   // get the total fruit

#include <stdio.h>
#include <stdlib.h>
#include <limits.h>
#include <float.h>

int main(int argc, char** argv) {

    printf("CHAR_BIT    :   %d\n", CHAR_BIT);
    printf("CHAR_MAX    :   %d\n", CHAR_MAX);
    printf("CHAR_MIN    :   %d\n", CHAR_MIN);
    printf("INT_MAX     :   %d\n", INT_MAX);
    printf("INT_MIN     :   %d\n", INT_MIN);
    printf("LONG_MAX    :   %ld\n", (long) LONG_MAX);
    printf("LONG_MIN    :   %ld\n", (long) LONG_MIN);
    printf("SCHAR_MAX   :   %d\n", SCHAR_MAX);
    printf("SCHAR_MIN   :   %d\n", SCHAR_MIN);
    printf("SHRT_MAX    :   %d\n", SHRT_MAX);
    printf("SHRT_MIN    :   %d\n", SHRT_MIN);
    printf("UCHAR_MAX   :   %d\n", UCHAR_MAX);
    printf("UINT_MAX    :   %u\n", (unsigned int) UINT_MAX);
    printf("ULONG_MAX   :   %lu\n", (unsigned long) ULONG_MAX);
    printf("USHRT_MAX   :   %d\n", (unsigned short) USHRT_MAX);

    return 0;
}

CHAR_BIT    :   8
CHAR_MAX    :   127
CHAR_MIN    :   -128
INT_MAX     :   2147483647
INT_MIN     :   -2147483648
LONG_MAX    :   9223372036854775807
LONG_MIN    :   -9223372036854775808
SCHAR_MAX   :   127
SCHAR_MIN   :   -128
SHRT_MAX    :   32767
SHRT_MIN    :   -32768
UCHAR_MAX   :   255
UINT_MAX    :   4294967295
ULONG_MAX   :   18446744073709551615
USHRT_MAX   :   65535

#include <stdio.h>
#include <stdlib.h>
#include <limits.h>
#include <float.h>

int main(int argc, char** argv) {

    printf("Storage size for float : %d \n", sizeof(float));
    printf("FLT_MAX     :   %g\n", (float) FLT_MAX);
    printf("FLT_MIN     :   %g\n", (float) FLT_MIN);
    printf("-FLT_MAX    :   %g\n", (float) -FLT_MAX);
    printf("-FLT_MIN    :   %g\n", (float) -FLT_MIN);
    printf("DBL_MAX     :   %g\n", (double) DBL_MAX);
    printf("DBL_MIN     :   %g\n", (double) DBL_MIN);
    printf("-DBL_MAX     :  %g\n", (double) -DBL_MAX);
    printf("Precision value: %d\n", FLT_DIG );

    return 0;
}

Storage size for float : 4
FLT_MAX      :   3.40282e+38
FLT_MIN      :   1.17549e-38
-FLT_MAX     :   -3.40282e+38
-FLT_MIN     :   -1.17549e-38
DBL_MAX      :   1.79769e+308
DBL_MIN      :   2.22507e-308
-DBL_MAX     :  -1.79769e+308
Precision value: 6

type variable_list;

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

type variable_name = value;

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'.

int g = 20; // valid statement

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

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 */

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

#include <stdio.h>

int main() {
   printf("Hello\tWorld\n\n");

   return 0;
}

Hello World

"hello, dear"

"hello, \

dear"

"hello, " "d" "ear"

#define identifier value

#include <stdio.h>

#define LENGTH 10
#define WIDTH  5
#define NEWLINE '\n'

int main() {
   int area;

   area = LENGTH * WIDTH;
   printf("value of area : %d", area);
   printf("%c", NEWLINE);

   return 0;
}

value of area : 50

const type variable = value;

#include <stdio.h>

int main() {
   const int  LENGTH = 10;
   const int  WIDTH = 5;
   const char NEWLINE = '\n';
   int area;

   area = LENGTH * WIDTH;
   printf("value of area : %d", area);
   printf("%c", NEWLINE);

   return 0;
}

value of area : 50

{
   int mount;
   auto int month;
}

{
   register int  miles;
}

#include <stdio.h>

/* function declaration */
void func(void);

static int count = 5; /* global variable */

main() {

   while(count--) {
      func();
   }

   return 0;
}

/* function definition */
void func( void ) {

   static int i = 5; /* local static variable */
   i++;

   printf("i is %d and count is %d\n", i, count);
}

i is 6 and count is 4
i is 7 and count is 3
i is 8 and count is 2
i is 9 and count is 1
i is 10 and count is 0

#include <stdio.h>

int count ;
extern void write_extern();

main() {
   count = 5;
   write_extern();
}

#include <stdio.h>

extern int count;

void write_extern(void) {
   printf("count is %d\n", count);
}

$gcc main.c support.c

count is 5

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 −

link:../cprogramming/c_bitwise_operators.html[Show Examples]
[%autowidth]
|===

|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

The following table lists the assignment operators supported by the C language −

link:../cprogramming/c_assignment_operators.html[Show Examples]
[%autowidth]
|===

|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
|===


=== Misc Operators ↦ sizeof & ternary

Besides the operators discussed above, there are a few other important operators including *sizeof* and *? :* supported by the C Language.

link:../cprogramming/c_sizeof_operator.html[Show Examples]
[%autowidth]
|===

|Operator|Description|Example
|sizeof()|Returns the size of a variable.|sizeof(a), where a is integer, will return 4.
|&|Returns the address of a variable.|&a; returns the actual address of the variable.
|*|Pointer to a variable.|*a;
|? :|Conditional Expression.|If Condition is true ? then value X : otherwise value Y
|===


=== 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.

For example, x = 7 + 3 * 2; here, x is assigned 13, not 20 because operator * has a higher precedence than +, so it first gets multiplied with 3*2 and then adds into 7.

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.

link:../cprogramming/c_operators_precedence.html[Show Examples]
[%autowidth]
|===

|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.

Show below is the general form of a typical decision making structure found in most of the programming languages −

image::https://www.iokays.com/tutorialspoint/cprogramming/_images/decision_making.jpg[Decision making statements in C]

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.
[%autowidth]
|===

|Sr.No.|Statement & Description
|1|link:../cprogramming/if_statement_in_c.html[if statement]An *if statement* consists of a boolean expression followed by one or more statements.

|2|link:../cprogramming/if_else_statement_in_c.html[if...else statement]An *if statement* can be followed by an optional *else statement*, which executes when the Boolean expression is false.

|3|link:../cprogramming/nested_if_statements_in_c.html[nested if statements]You can use one *if* or *else if* statement inside another *if* or *else if* statement(s).

|4|link:../cprogramming/switch_statement_in_c.html[switch statement]A *switch* statement allows a variable to be tested for equality against a list of values.

|5|link:../cprogramming/nested_switch_statements_in_c.html[nested switch statements]You can use one *switch* statement inside another *switch* statement(s).

|===


=== The ? : Operator

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

[source]

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

You may encounter situations, when a block of code needs to be executed several number of times. In general, statements are executed sequentially: The first statement in a function is executed first, followed by the second, and so on.

Programming languages provide various control structures that allow for more complicated execution paths.

A loop statement allows us to execute a statement or group of statements multiple times. Given below is the general form of a loop statement in most of the programming languages −

image::https://www.iokays.com/tutorialspoint/cprogramming/_images/loop_architecture.jpg[Loop Architecture]

C programming language provides the following types of loops to handle looping requirements.
[%autowidth]
|===

|Sr.No.|Loop Type & Description
|1|link:../cprogramming/c_while_loop.html[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|link:../cprogramming/c_for_loop.html[for loop]Executes a sequence of statements multiple times and abbreviates the code that manages the loop variable.

|3|link:../cprogramming/c_do_while_loop.html[do...while loop]It is more like a while statement, except that it tests the condition at the end of the loop body.

|4|link:../cprogramming/c_nested_loops.html[nested loops]You can use one or more loops inside any other while, for, or do..while loop.

|===


=== Loop Control Statements

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.
[%autowidth]
|===

|Sr.No.|Control Statement & Description
|1|link:../cprogramming/c_break_statement.html[break statement]Terminates the *loop* or *switch* statement and transfers execution to the statement immediately following the loop or switch.

|2|link:../cprogramming/c_continue_statement.html[continue statement]Causes the loop to skip the remainder of its body and immediately retest its condition prior to reiterating.

|3|link:../cprogramming/c_goto_statement.html[goto statement]Transfers control to the labeled statement.

|===


=== The Infinite Loop

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.

[source]

When the conditional expression is absent, it is assumed to be true. You may have an initialization and increment expression, but C programmers more commonly use the for(;;) construct to signify an infinite loop.

*NOTE* − You can terminate an infinite loop by pressing Ctrl + C keys.


== 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.

You can divide up your code into separate functions. How you divide up your code among different functions is up to you, but logically the division is such that each function performs a specific task.

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.


=== Defining a Function

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

[source]

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.



=== Example

Given below is the source code for a function called *max()*. This function takes two parameters num1 and num2 and returns the maximum value between the two −

[source]

=== Function Declarations

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 −

[source]

For the above defined function max(), the function declaration is as follows −

[source]

Parameter names are not important in function declaration only their type is required, so the following is also a valid declaration −

[source]

Function declaration is required when you define a function in one source file and you call that function in another file. In such case, you should declare the function at the top of the file calling the function.


=== Calling a Function

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. For example −

[source]

We have kept max() along with main() and compiled the source code. While running the final executable, it would produce the following result −

[source]

=== Function Arguments

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 −
[%autowidth]
|===

|Sr.No.|Call Type & Description
|1|link:../cprogramming/c_function_call_by_value.html[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.

|2|link:../cprogramming/c_function_call_by_reference.html[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.


Let us understand what are *local* and *global* variables, and *formal* parameters.


=== Local Variables

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. The following example shows how local variables are used. Here all the variables a, b, and c are local to main() function.

[source]

=== Global Variables

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. The following program show how global variables are used in a program.

[source]

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

[source]

When the above code is compiled and executed, it produces the following result −

[source]

=== Formal Parameters

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

[source]

When the above code is compiled and executed, it produces the following result −

[source]

=== Initializing Local and 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 −
[%autowidth]
|===

|Data Type|Initial Default Value
|int|0
|char|'\0'
|float|0
|double|0
|pointer|NULL
|===

It is a good programming practice to initialize variables properly, otherwise your program may produce unexpected results, because uninitialized variables will take some garbage value already available at their memory location.


== 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.

image::https://www.iokays.com/tutorialspoint/cprogramming/_images/arrays.jpg[Arrays in C]


=== Declaring Arrays

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 −

[source]

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. For example, to declare a 10-element array called *balance* of type double, use this statement −

[source]

Here balance is a variable array which is sufficient to hold up to 10 double numbers.


=== Initializing Arrays

You can initialize an array in C either one by one or using a single statement as follows −

[source]

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 −

[source]

You will create exactly the same array as you did in the previous example. Following is an example to assign a single element of the array −

[source]

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. Shown below is the pictorial representation of the array we discussed above −

image::https://www.iokays.com/tutorialspoint/cprogramming/_images/array_presentation.jpg[Array Presentation]


=== Accessing Array Elements

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 −

[source]

The above statement will take the 10th element from the array and assign the value to salary variable. The following example Shows how to use all the three above mentioned concepts viz. declaration, assignment, and accessing arrays −

[source]

When the above code is compiled and executed, it produces the following result −

[source]

=== Arrays in Detail

Arrays are important to C and should need a lot more attention. The following important concepts related to array should be clear to a C programmer −
[%autowidth]
|===

|Sr.No.|Concept & Description
|1|link:../cprogramming/c_multi_dimensional_arrays.html[Multi-dimensional arrays]C supports multidimensional arrays. The simplest form of the multidimensional array is the two-dimensional array.

|2|link:../cprogramming/c_passing_arrays_to_functions.html[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|link:../cprogramming/c_return_arrays_from_function.html[Return array from a function]C allows a function to return an array.

|4|link:../cprogramming/c_pointer_to_an_array.html[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

Pointers in C are easy and fun to learn. Some C programming tasks are performed more easily with pointers, and other tasks, such as dynamic memory allocation, cannot be performed without using pointers. So it becomes necessary to learn pointers to become a perfect C programmer. Let's start learning them in simple and easy steps.

As you know, every variable is a memory location and every memory location has its address defined which can be accessed using ampersand (&) operator, which denotes an address in memory. Consider the following example, which prints the address of the variables defined −

[source]

When the above code is compiled and executed, it produces the following result −

[source]

=== What are 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 −

[source]

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 −

[source]

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.


=== How to Use Pointers?

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. The following example makes use of these operations −

[source]

When the above code is compiled and executed, it produces the following result −

[source]

=== NULL Pointers

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. Consider the following program −

[source]

When the above code is compiled and executed, it produces the following result −

[source]

In most of the operating systems, programs are not permitted to access memory at address 0 because that memory is reserved by the operating system. However, the memory address 0 has special significance; it signals that the pointer is not intended to point to an accessible memory location. But by convention, if a pointer contains the null (zero) value, it is assumed to point to nothing.

To check for a null pointer, you can use an 'if' statement as follows −

[source]

=== Pointers in Detail

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 −
[%autowidth]
|===

|Sr.No.|Concept & Description
|1|link:../cprogramming/c_pointer_arithmetic.html[Pointer arithmetic]There are four arithmetic operators that can be used in pointers: ++, --, +, -

|2|link:../cprogramming/c_array_of_pointers.html[Array of pointers]You can define arrays to hold a number of pointers.

|3|link:../cprogramming/c_pointer_to_pointer.html[Pointer to pointer]C allows you to have pointer on a pointer and so on.

|4|link:../cprogramming/c_passing_pointers_to_functions.html[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|link:../cprogramming/c_return_pointer_from_functions.html[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."

[source]

If you follow the rule of array initialization then you can write the above statement as follows −

[source]

Following is the memory presentation of the above defined string in C/C++ −

image::https://www.iokays.com/tutorialspoint/cprogramming/_images/string_representation.jpg[String Presentation in C/C++]

Actually, you do not place the null character at the end of a string constant. The C compiler automatically places the '\0' at the end of the string when it initializes the array. Let us try to print the above mentioned string −

[source]

When the above code is compiled and executed, it produces the following result −

[source]

C supports a wide range of functions that manipulate null-terminated strings −
[%autowidth]
|===

|Sr.No.|Function & Purpose
|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 s1<s2; greater than 0 if s1>s2.

|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.

|===

The following example uses some of the above-mentioned functions −

[source]

When the above code is compiled and executed, it produces the following result −

[source]

== 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. Suppose you want to keep track of your books in a library. You might want to track the following attributes about each book −

. Title

. Author

. Subject

. Book ID


=== Defining a Structure

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 −

[source]

The *structure 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 structure's definition, before the final semicolon, you can specify one or more structure variables but it is optional. Here is the way you would declare the Book structure −

[source]

=== Accessing Structure Members

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. The following example shows how to use a structure in a program −

[source]

When the above code is compiled and executed, it produces the following result −

[source]

=== Structures as Function Arguments

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

[source]

When the above code is compiled and executed, it produces the following result −

[source]

=== Pointers to Structures

You can define pointers to structures in the same way as you define pointer to any other variable −

[source]

Now, you can store the address of a structure variable in the above defined pointer variable. To find the address of a structure variable, place the '&'; operator before the structure's name as follows −

[source]

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

[source]

Let us re-write the above example using structure pointer.

[source]

When the above code is compiled and executed, it produces the following result −

[source]

=== 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 −

[source]

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.


=== Defining a Union

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 −

[source]

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 −

[source]

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 −

[source]

When the above code is compiled and executed, it produces the following result −

[source]

=== Accessing Union Members

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 −

[source]

When the above code is compiled and executed, it produces the following result −

[source]

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 −

[source]

When the above code is compiled and executed, it produces the following result −

[source]

Here, all the members are getting printed very well because one member is being used at a time.


== C - Bit Fields

Suppose your C program contains a number of TRUE/FALSE variables grouped in a structure called status, as follows −

[source]

This structure requires 8 bytes of memory space but in actual, we are going to store either 0 or 1 in each of the variables. The C programming language offers a better way to utilize the memory space in such situations.

If you are using such variables inside a structure then you can define the width of a variable which tells the C compiler that you are going to use only those number of bytes. For example, the above structure can be re-written as follows −

[source]

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 −

[source]

When the above code is compiled and executed, it produces the following result −

[source]

=== Bit Field Declaration

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

[source]

The following table describes the variable elements of a bit field −
[%autowidth]
|===

|Sr.No.|Element & Description
|1|*type*
An integer type that determines how a bit-field's value is interpreted. The type may be int, signed int, or unsigned int.

|2|*member_name*
The name of the bit-field.

|3|*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 −

[source]

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 −

[source]

When the above code is compiled it will compile with a warning and when executed, it produces the following result −

[source]

== 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 −

[source]

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

[source]

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 −

[source]

You can use *typedef* to give a name to your user defined data types as well. For example, you can use typedef with structure to define a new data type and then use that data type to define structure variables directly as follows −

[source]

When the above code is compiled and executed, it produces the following result −

[source]

=== typedef vs

*#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, q., you can define 1 as ONE etc.


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


The following example shows how to use #define in a program −

[source]

When the above code is compiled and executed, it produces the following result −

[source]

== 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.


=== The Standard 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.
[%autowidth]
|===

|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 getchar() and putchar() Functions

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 −

[source]

When the above code is compiled and executed, it waits for you to input some text. When you enter a text and press enter, then the program proceeds and reads only a single character and displays it as follows −

[source]

=== The gets() and puts() Functions

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 link:../c_standard_library/c_function_fgets.html[fgets()].

[source]

When the above code is compiled and executed, it waits for you to input some text. When you enter a text and press enter, then the program proceeds and reads the complete line till end, and displays it as follows −

[source]

=== The scanf() and printf() Functions

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 −

[source]

When the above code is compiled and executed, it waits for you to input some text. When you enter a text and press enter, then program proceeds and reads the input and displays it as follows −

[source]

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

The last chapter explained the standard input and output devices handled by C programming language. This chapter cover how C programmers can create, open, close text or binary files for their data storage.

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. This chapter will take you through the important calls for file management.


=== Opening Files

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 −

[source]

Here, *filename* is a string literal, which you will use to name your file, and access *mode* can have one of the following values −
[%autowidth]
|===

|Sr.No.|Mode & Description
|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 −

[source]

=== Closing a File

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

[source]

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*.

There are various functions provided by C standard library to read and write a file, character by character, or in the form of a fixed length string.


=== Writing a File

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

[source]

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 −

[source]

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. Try the following example.

Make sure you have */tmp* directory available. If it is not, then before proceeding, you must create this directory on your machine.

[source]

When the above code is compiled and executed, it creates a new file *test.txt* in /tmp directory and writes two lines using two different functions. Let us read this file in the next section.


=== Reading a File

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

[source]

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 −

[source]

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.

[source]

When the above code is compiled and executed, it reads the file created in the previous section and produces the following result −

[source]

Let's see a little more in detail about what happened here. First, *fscanf()* read just *This* because after that, it encountered a space, second call is for *fgets()* which reads the remaining line till it encountered end of line. Finally, the last call *fgets()* reads the second line completely.


=== Binary I/O Functions

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

[source]

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 −
[%autowidth]
|===

|Sr.No.|Directive & Description
|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.

|===


=== Preprocessors Examples

Analyze the following examples to understand various directives.

[source]

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

[source]

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.

[source]

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

[source]

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

[source]

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.


=== Predefined Macros

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

|Sr.No.|Macro & Description
|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.

|===

Let's try the following example −

[source]

When the above code in a file *test.c* is compiled and executed, it produces the following result −

[source]

=== Preprocessor Operators

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 −

[source]

==== The Stringize (

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 −

[source]

When the above code is compiled and executed, it produces the following result −

[source]

==== The Token Pasting (

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 −

[source]

When the above code is compiled and executed, it produces the following result −

[source]

It happened so because this example results in the following actual output from the preprocessor −

[source]

This example shows the concatenation of token##n into token34 and here we have used both *stringize* and *token-pasting*.


==== 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 −

[source]

When the above code is compiled and executed, it produces the following result −

[source]

=== 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 −

[source]

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

[source]

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 −

[source]

When the above code is compiled and executed, it produces the following result −

[source]

== 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.


=== Include Syntax

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

[source]

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.

[source]

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.


=== Include Operation

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 −

[source]

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

[source]

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

[source]

=== Once-Only Headers

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 −

[source]

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.


=== Computed Includes

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 −

[source]

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 −

[source]

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 −

[source]

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 −

[source]

When the above code is compiled and executed, it produces the following result −

[source]

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

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 −

[source]

When the above code is compiled and executed, it produces the following result −

[source]

Here, the value of sum is 116 because the compiler is doing integer promotion and converting the value of 'c' to ASCII before performing the actual addition operation.


=== Usual Arithmetic Conversion

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 −

image::https://www.iokays.com/tutorialspoint/cprogramming/_images/usual_arithmetic_conversion.png[Usual Arithmetic Conversion]

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 −

[source]

When the above code is compiled and executed, it produces the following result −

[source]

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 <error.h> 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.


=== errno, perror(). and strerror()

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.

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When the above code is compiled and executed, it produces the following result −

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=== Divide by Zero Errors

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 −

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When the above code is compiled and executed, it produces the following result −

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=== Program Exit Status

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 −

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When the above code is compiled and executed, it produces the following result −

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== 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.

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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.


=== Number Factorial

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

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When the above code is compiled and executed, it produces the following result −

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=== Fibonacci Series

The following example generates the Fibonacci series for a given number using a recursive function −

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When the above code is compiled and executed, it produces the following result −

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== 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.

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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 −

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When the above code is compiled and executed, it produces the following result. It should be noted that the function *average()* has been called twice and each time the first argument represents the total number of variable arguments being passed. Only ellipses will be used to pass variable number of arguments.

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== C - Memory Management

This chapter explains dynamic memory management in C. The C programming language provides several functions for memory allocation and management. These functions can be found in the *<stdlib.h>* header file.
[%autowidth]
|===

|Sr.No.|Function & Description
|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(size_t size);*
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*.

|===


=== Allocating Memory Dynamically

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 −

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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 −

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When the above code is compiled and executed, it produces the following result.

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Same program can be written using *calloc();* only thing is you need to replace malloc with calloc as follows −

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So you have complete control and you can pass any size value while allocating memory, unlike arrays where once the size defined, you cannot change it.


=== Resizing and Releasing Memory

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 −

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When the above code is compiled and executed, it produces the following result.

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You can try the above example without re-allocating extra memory, and strcat() function will give an error due to lack of available memory in description.


== 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 −

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When the above code is compiled and executed with single argument, it produces the following result.

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When the above code is compiled and executed with a two arguments, it produces the following result.

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When the above code is compiled and executed without passing any argument, it produces the following result.

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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 ''. Let us re-write above example once again where we will print program name and we also pass a command line argument by putting inside double quotes −

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When the above code is compiled and executed with a single argument separated by space but inside double quotes, it produces the following result.

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