atoll() in C – Syntax, Use and Example

Write a C program to input string from user and convert string to long long using atoll() library function. How to convert string to long long int type using atoll() library function in C programming?

atoll() library function

atoll() function converts string representation of integer to long long type. It is defined in stdlib.h header file.

It is a part of C11 standard. Therefore, you must have C11 supported compiler installed to use this function.

Syntax of atoll() function

long long int atoll(const char* str);

• It accepts a pointer to constant character str. Which points to string representation of integer.
• On success it return converted integer as long long int type. On failure it return 0LL.

Note: The function behaviour is undefined if given integer underflows or overflows long long range. To handle such situations use more reliable and cross-platform strtoll() library function.

Example program to use atoll() function

/**
 * C program to convert string to long long using atoll() library function. 
 */

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


int main()
{
    char number[30];
    long long int bigNum;


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


    /* Convert string representation of number to integer */
    bigNum = atoll(number);


    /* Use %lld or %lli to print long long type */
    printf("Converted long long int = %lld\n", bigNum);


    return 0;
}

See all format specifiers in C programming.

Enter any integer: 9223372036854775807
Converted long long int = 9223372036854775807

Enter any integer: -9223372036854775808
Converted long long int = -9223372036854775808

Enter any integer: 922337203685477580799
Converted long long int = 9223372036854775807

Enter any integer: -922337203685477580899999999
Converted long long int = -9223372036854775808

Enter any integer: 2018
Converted long long int = 2018

Enter any integer: error
Converted long long int = 0

Happy coding 😉