Question

The program below converts a string to long, but based on my understanding it also returns an error. I am relying on the fact that if strtol successfully converted string to long, then the second parameter to strtol should be equal to NULL. When I run the below application with 55, I get the following message.

./convertToLong 55
Could not convert 55 to long and leftover string is: 55 as long is 55

How can I successfully detect errors from strtol? In my application, zero is a valid value.

Code:

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

static long parseLong(const char * str);

int main(int argc, char ** argv)
{
    printf("%s as long is %ld\n", argv[1], parseLong(argv[1]));
    return 0;
 }

static long parseLong(const char * str)
{
    long _val = 0;
    char * temp;

    _val = strtol(str, &temp, 0);

    if(temp != '\0')
            printf("Could not convert %s to long and leftover string is: %s", str, temp);

    return _val;
}

Answer 1

You’re almost there. temp itself will not be null, but it will point to a null character if the whole string is converted, so you need to dereference it:

if (*temp != '\0')