strtol() in C – Syntax, Use and Example

Write a C program to input string representation of integer and convert string to long using strtol() library function. How to convert string to long using strtol() library function in C programming?

strtol() library function

strtol() function is used to convert string representation of integer to long type. It is defined in stdlib.h header file. It is more flexible and reliable then atol() function.

  • strtol() lets you to specify base of input string. You can use this function to convert integer string of any base to long type.
  • Using strtol() you can easily distinguish between successful or failed conversion.

Syntax of strtol() function

long strtol(const char* str, char** endPtr, int base);
  • It accepts pointer to constant character str i.e. string representaiton of integer to be converted.
  • Next it accepts a pointer to char pointer endPtr. On success endPtr points to first character after number otherwise is a NULL pointer.
  • Finally it accept an integer base of input string.
  • The function on success return converted integer as long type and sets endPtr to point to first character after converted integer. Otherwise returns zero (0L) if no valid conversion can be performed and sets endPtr to NULL pointer.

Note: Unlike atol(), it efficiently handles integer underflow and overflows. It returns LONG_MIN or LONG_MAX on integer underflows or overflows respectively.

Also see how to find minimum and maximum limit of a data type?

Example program to use strtol() function

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

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

int main()
    char number[30];
    char* endPtr;

    long bigNumber;
    int base;

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

    /* Input base */
    printf("Enter base: ");
    scanf("%d", &base);

    /* Convert string representation of number to long */
    bigNumber = strtol(number, &endPtr, base);

    /* endPtr points to NULL for failed conversions */
        printf("Unable to convert '%s' to base %d.", number, base);
        printf("Converted long = %ld\n", bigNumber);

    return 0;


Enter any number: 22
Enter base: 10
Converted long = 22

Enter any number: CA10
Enter base: 16
Converted long = 51728

Enter any number: 054
Enter base: 8
Converted long = 44

Enter any number: 1001
Enter base: 2
Converted long = 9

Enter any number: error
Enter base: 10
Unable to convert 'error' to base 10.

Happy coding 😉

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