CODEWITHSUNDEEP
ccompiler-constructionif-statement
banjo67xxx
banjo67xxx

Reputation: 145

if statement ambiguity

I'm trying to write my own code to walk the PATH to find an executable as a learning exercise in C programming. (after sucess I might replace it with someone else's code, but for now I want to understand my mistakes).

The following section of code is not jumping to the else statement I expect ...

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

#define EXECUTABLE S_IXOTH /* executable by others */
#define MAX_PATH_LEN 1024

void message (const char *msg)
{
  fprintf(stdout, "INFO: %s\n", *msg);
}

int main (int argc, char *argv[], char *envp[])
{
  char *editor;
  struct stat editor_stat;
  char full_path[MAX_PATH_LEN];
  int found_path;

  memset(full_path,0,MAX_PATH_LEN);
  strcpy(full_path,"/bin/ed");
  found_path=stat(full_path,&editor_stat);

  if (found_path!=0) {
    editor=NULL;
    message("The EDITOR specified is not found in the PATH. Using default editor");
  } else {
    if (editor_stat.st_mode&EXECUTABLE==0) {
      editor=NULL;
      message("The EDITOR specified must have world execute permission. using default editoe");
    } else {
      editor=full_path;
    }
  }

}

When I track it with gdb I see it jumps to the 2nd else instead of the first one, and doesn't execute the check for executable ...

(gdb) file /tmp/sample2
Reading symbols from /tmp/sample2...done.
(gdb) b 28
Breakpoint 1 at 0x400688: file /home/ken/c/shorter_sample.c, line 28.
(gdb) r
Starting program: /tmp/sample2 

Breakpoint 1, main (argc=1, argv=0x7fffffffe1f8, envp=0x7fffffffe208)
    at /home/ken/c/shorter_sample.c:28
28        if (found_path!=0) {
Missing separate debuginfos, use: debuginfo-install glibc-2.13-2.x86_64
(gdb) p found_path
$1 = 0
(gdb) s
36            editor=full_path;
(gdb)

it should jump to line 32 not 36.

I've tried searcing here for C ambiguity and I've read the sections from "C" by Kernighan & Ritchie that are referenced in the index under C ambiguity, and stuck as many curly braces as I can in the code but the compiler isn't doing what I intend.

FYI, I'm using gcc-4.5.1-4.fc14.x86_64 with kernel 2.6.35.14-106.fc14.x86_64 on Fedora 14.

Upvotes: 2

Views: 193

Answers (1)

MSN
MSN

Reputation: 54554

& has a lower operator precedence than ==; that means the second if statement is equivalent to:

    if (editor_stat.st_mode&(EXECUTABLE==0))

I'm going to go out on a limb and say EXECUTABLE is not 0, which makes the if equivalent to:

    if (editor_stat.st_mode & 0)

or 

    if (0)

The second if statement should be: 

    if ((editor_stat.st_mode&EXECUTABLE)==0)

Upvotes: 7

Related Questions