atoi() in C – Syntax, Use and Example

Write a C program to input string representation of integer and convert string to int data type. C program to convert string to integer using atoi() library function.

In programming it is often required to convert string representation of integer to int type. In this post I will explain how to convert string to integer using pre-defined atoi() library function.

atoi() library function

C programming supports a pre-defined library function atoi() to handle string to integer conversion. The function is defined in stdlib.h header file.

Syntax of atoi() function

int atoi(const char * str);
  • It accepts a pointer to constant character str. Which is string representation of integer.
  • It return an integer. If str is valid integer string then returns that number as int type otherwise returns 0.

Note: atoi() function behaviour is undefined if converted integer underflows or overflows integer range. Use more robust and reliable strtol() function to convert string to integer.

Example program to use atoi() function

 * C program to convert string to integer using atoi() library function.

#include <stdio.h>
#include <stdlib.h>     // Used for atoi()

int main()
    char number[30];
    int  num;

    /* Input string representation of integer from user. */
    printf("Enter any integer: ");
    fgets(number, 30, stdin);

    /* Convert string representation of number to integer */
    num = atoi(number);

    /* Print converted integer */
    printf("Converted integer = %d\n", num);

    return 0;


Enter any integer: a
Converted integer = 0

Enter any integer: 12
Converted integer = 12

Enter any integer: 1.5
Converted integer = 1

Enter any integer: 1111111111111
Converted integer = -1285418553

Enter any integer: -99999999999999999
Converted integer = -1569325055

Happy coding 😉

About Pankaj

Pankaj Prakash is the founder, editor and blogger at Codeforwin. He loves to learn new techs and write programming articles especially for beginners. He works at Vasudhaika Software Sols. as a Software Design Engineer and manages Codeforwin. In short Pankaj is Web developer, Blogger, Learner, Tech and Music lover.

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