Using stat to check if a file is executable in C

Question

For homework I have to write a C program and one of the things it has to do is check to see a file exists and if it is executable by the owner.

Using (stat(path[j], &sb) >= 0 I'm able to see if the file indicated by path[j] exists.

I've looked through man pages, a lot of questions and answers on stackoverflow, and several websites but I'm not able to wrap my head around exactly how to check if a file is executable using stat. I thought it would be as simple as ((stat(path[j], &sb) >= 0) && (sb.st_mode > 0) && (S_IEXEC) but as far as I can tell by testing it, it seems to ignore the fact that these files aren't executable.

I think that perhaps stat doesn't work the way I think it does. Assuming I use stat, how can I go about fixing this?

Answer 1

You can indeed use stat to do this. You just have to use S_IXUSR (S_IEXEC is an old synonym of S_IXUSR) to check if you have execute permission. Bitwise AND operator (&) checks whether the bits of S_IXUSR are set or not.

if (stat(file, &sb) == 0 && sb.st_mode & S_IXUSR) 
    /* executable */
else  
    /* non-executable */

Example:

#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <unistd.h>

int main(int argc, char **argv)
{
    if (argc > 1) {
        struct stat sb;
        printf("%s is%s executable.\n", argv[1], stat(argv[1], &sb) == 0 &&
                                                 sb.st_mode & S_IXUSR ? 
                                                 "" : " not");
    }
    return 0;
}   

Answer 2

Try:

((stat(path[j], &sb) >= 0) && (sb.st_mode > 0) && (S_IEXEC & sb.st_mode)

Answer 3

We can leverage the libmagic.so library which comes along with the file(1) utility. It can detect all executable like ELF, bash/python/perl scripts etc

Here's my code:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include "magic.h"

int
main(int argc, char **argv)
{
   struct magic_set *ms;
   const char *result;
   char *desired;
   size_t desired_len;
   int i;
   FILE *fp;

   ms = magic_open(MAGIC_RAW);
   if (ms == NULL) {
      (void)fprintf(stderr, "ERROR opening MAGIC_NONE: out of memory\n");
      return -1;
   }
   if (magic_load(ms, NULL) == -1) {
      (void)fprintf(stderr, "ERROR loading with NULL file: %s\n", magic_error(ms));
      return 11;
   }

   if (argc > 1) {
      if (argc != 2) {
         (void)fprintf(stderr, "Usage:  ./a.out </path/to/file>\n");
      } else {
         if ((result = magic_file(ms, argv[1])) == NULL) {
            (void)fprintf(stderr, "ERROR loading file %s: %s\n", argv[1], magic_error(ms));
            return -1;
         } else {
             if (strstr(result, (const char *)"executable")) {
                printf("%s: is executable\n", argv[1], result);
             }
         }
      }
   }
   magic_close(ms);
   return 0;
}

$ gcc test.c -I/path/to/magic.h /usr/lib/libmagic.so.1

./a.out /bin/ls

./a.out a.out

./a.out test.c