为什么“gcc -Wall”不对“if (ptr < 0)”发出警告？

Question

(A long story... you can directly jump to the question at the end...)

I need to use realpath(3) so I wrote a simple example to try it:

$> cat realpath.c
#include <stdio.h>
#include <limits.h>
#include <stdlib.h>

int main(int argc, char * argv[])
{
    char pathname[PATH_MAX];

    if (realpath(argv[1], pathname) < 0) {
        perror("realpath");
        return 1;
    }

    printf("%s\n", pathname);

    return 0;
}
$> gcc -Wall -o realpath realpath.c
$> ls /xxx
ls: cannot access '/xxx': No such file or directory
$> ./realpath /xxx/foo/bar
/xxx

The result of ./realpath /xxx/foo/bar surprised me. According to the manual it makes more sense to fail with ENOENT . I even referred to the POSIX and found no answer. After quite some time I reread the manual and found realpath(3) returns char * rather than int . I was really irritated by gcc .

Question

So why doesn't gcc (even with -Wall ) warn about if (ptr < 0) ?

Answer 1

gcc -Wall does not enable all of GCC's warnings! See this question for more information.

In your case, you need to add the -Wextra flag:

gcc -Wall -Wextra -o realpath realpath.c

According toGCC's documentation :

This enables some extra warning flags that are not enabled by -Wall .

The option -Wextra also prints warning messages for the following cases:

• A pointer is compared against integer zero with <, <=, >, or >=.

• [...]

Answer 2

So why doesn't gcc (even with -Wall ) report a warning for if (ptr < 0) ?

The name given to the -Wall flag is actually misleading, since it does not enable all compiler warnings.

You need to pass -Wextra . This way, you will get the following compiler warning:

warning: ordered comparison of pointer with integer zero [-Wextra]