atoi : <stdlib.h> library function

The function int atoi(const char *str); converts a string to a integer(int). The function skips all white-space characters at the beginning of the string until the first non-whitespace character is found. Then it converts the subsequent characters into a integer number until it finds a character which is not a digit. It also takes an optional initial plus or minus sign before first base-10 digit in string.

Function prototype of atoi

int atoi(const char *str);
  • str : A pointer to a string beginning with the representation of an integer number.

Return value of atoi

This function returns the converted integer number as an int value otherwise it returns zero(0), If no valid conversion could be performed.

C program to show the use of atoi function

The following program shows the use of atoi function for string to integer conversion.

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

int main(){
    char string[100];
    int value;
    printf("Enter a string\n");
    scanf("%s", &string);
    /* String to Integer conversion */
    value = atoi(string);
    printf("Integer value %d\n", value);
    return 0;

Program Output
Enter a string
Integer value 1234
Enter a string
Integer value 1234
Enter a string
Integer value 1234
Enter a string
Integer value 0