Why can't the result of strerror() be returned

Question

According to the man page strerror(errnum) returns a char *, but I get the following warning:

gcc temp.c -o temp
temp.c: In function ‘mystrerror’:
temp.c:10:4: warning: return makes pointer from integer without a cast [enabled by default]

I get a segfault When I run it with ./temp 0 but not ./temp 256.

Could someone explain why this happens and how to fix it (if possible)?

temp.c

#include <stdio.h>
#include <errno.h>

char *mystrerror(int errnum)
{
    switch (errnum) {
    case 256:
        return "My test";
    default:
        return strerror(errnum);
    }
}

int main(int argc, char **argv)
{
    int err;

    if (argc > 1) {
        err = atoi(argv[1]);
        printf("test error (%d) %s\n", err, mystrerror(err));
    }

    return 0;
}

Answer 1

You are missing the inclusion of the <string.h> header file.

Documentation.

The compiler even tells you what exactly your problem is:

return makes pointer from integer without a cast

If there's no prototype present for a function, then it's assumed to return int. And it appears that on your platform, a pointer to char does not fit into an int, hence its truncated, and then printf() tries to dereference the thus invalid pointer.

Answer 2

According to the strerror manpage its include is #include <string.h>.

Without a function definition C assumes every function returns int, which is why you are getting that compiler error.