strptime throws Segmentation fault when passing char array to const char *

Question

I am getting segmentation fault when I try to pass char array to const char *

#include <stdio.h>
#include <stdint.h>
#include <stdlib.h>
#include <string.h>
#include <time.h>

static char *
get_valid_date_format(const char *date) {
    struct tm result;
    char *ret;
    char **f;
    
    char *formats[] = {"%Y", "%Y-%m", "%y-%m", "%Y-%m-%d", "%y-%m-%d",
        "%Y%m%d", "%y%m%d", "%Y-%m-%d %T", "%y-%m-%d %T", "%Y%m%d%H%M%S",
        "%y%m%d%H%M%S", "%Y-%m-%dT%T", "%y-%m-%dT%T", "%Y-%m-%dT%TZ",
        "%y-%m-%dT%TZ", "%Y-%m-%d %TZ", "%y-%m-%d %TZ", "%Y%m%dT%TZ",
        "%y%m%dT%TZ", "%Y%m%d %TZ", "%y%m%d %TZ", NULL };
        
    
    memset(&result, 0, sizeof(result));
    for (f = formats; f && *f; f++)
    {
        printf("check format: %s\n", *f);
        ret = strptime(date, *f, &result);
        if (ret && *ret == '\0')
        {
            printf("found Format: %s\n\n", *f);
            return *f;
        }
    }

    return (char *)0;
}


void main()
{
    char *format;
    char *date = "2020-07-25T00:10:58";
    char date2[] = "2020-07-25T00:10:58";
    char *date3 = "2020-07-25T00:10:58.000Z";
    
    date2[3] = '1';

    format = get_valid_date_format(date2);
    if (format == NULL) {
        printf("format is NULL\n");
        return;
    } else {
        printf("format found = %s\n", format);
    }
}

If I call get_valid_date_format(date), then it works fine. So calling with char * works find but when passing char[] then I get segmentation fault. I need to use array as I need to modify it before calling get_valid_date_format.

Answer 1

As M. Nejat Aydin commented, strptime is not a standard C function, it comes from BSD and is part of the POSIX standard used by Unix. To ensure it is loaded, add -D_XOPEN_SOURCE=700 to load X/Open and POSIX extensions.

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Compiler warnings will tell you about a host of other potential issues. If you're using a command line compiler like gcc or clang, you have to run compiler warnings on. There's a host of options, I recommend -Wall -Wshadow -Wwrite-strings -Wextra -Wconversion -std=c11 -pedantic.

char *formats[] declares an array of char * local to the function. You're returning one of those elements. Normally returning a local variable can cause a memory fault because that memory can be deallocated when the function exits. But formats consists of string literals which are not deallocated. But you've incorrectly declared them writable char *. You can go from non-const to const, but you can't go from const to non-const.

formats should be const char *formats[], an array of read-only const char *. They are read-only string literals which are never deallocated (static) and can be safely returned from the function. For belt-and-suspenders, declare formats static because it only needs ever to be allocated once.

Similarly, get_valid_date_format should return static const char *, format should be const char *, and f is a const char **.

Though some compilers will allow void main, that is not valid. main returns an int.

char *foo = "..."; is incorrect. It is a char foo[] = "..." or a const char *foo if it's read-only. This applies to all your date variables. const char *date = ..., const char *date3 = "..." and because you modify date2, char date2[] = ....

There is no need to check if f is true in the loop, just *f, but it doesn't hurt.

return (char *)0; is better written as return NULL;

There is no need to initialize result with memset since you're just passing it to strptime which will overwrite with its own values.