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;
}

I don't remember off the top of my head; will -Wall warn you that you're using an undeclared function in C? — Wug, Jun 18 '13 at 16:27
`gcc -Wall temp.c` `warning: implicit declaration of function ‘strerror’ [-Wimplicit-function-declaration]` — Karoly Horvath, Jun 18 '13 at 16:29

score 4 · Answer 1 · answered Jun 18 '13 at 16:26

4

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.

answered Jun 18 '13 at 16:26

Zan Lynx

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score 4 · Accepted Answer · answered Jun 18 '13 at 16:26

4

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.

answered Jun 18 '13 at 16:26

Well, if you're interested what **exactly** the problem is, use `-Wall` – Karoly Horvath Jun 18 '13 at 16:31
1

@KarolyHorvath One **does always** use `-Wall`. (And I use `-Wextra -pedantic -pedantic-errors` as well, but maybe I'm just masochistic.) – Jun 18 '13 at 16:32
1

Actually C99 dropped the default `int` rule; calling a function with no visible declaration is just illegal (actually a constraint violation). But many compilers, including gcc, operate under the old rules by default. – Keith Thompson Jun 18 '13 at 17:28
@KeithThompson Yes, that's correct, both `gcc` and `clang` refuse to issue a diagnostic unless `-std=c99 -pedantic` is present, actually. – Jun 18 '13 at 17:30
1

Software would have a lot fewer bugs if they made this a fatal error except when `-std=c89` is specified. – R.. GitHub STOP HELPING ICE Jun 18 '13 at 17:47

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

temp.c

2 Answers2