Thread: How to determine if a file is executable or not?

Hi fellows,

I have been given the darness task of determining whether a file is an executable or not. I know there is a shell command called file that does this. I was wondering if it can be done in c, or a combination of the two, since I need to perform some things to the executable file inside the c program.

I have been googling alot but have found only that files have magic number at the header that tells the kernel what type it is. However there is nothing I could find that tells me how to access this number or what number it is that determines if the file is an executable or not. I need an answer asap please. thanks.

oh btw, i'm working under the ext2 file system (linux) if that is of any relevance.

supernewb.

A simple way would be to use the access() function (I think it's in <unistd.h>). It takes two parameters, the file name, and what you're looking for. It returns 0 on success.

Code:

if(access("~/test",X_OK)) {
  printf("not executable");
} else {
  printf("executable");
}

Did you know that (under DOS and Windows) all executables begin with MZ? Maybe in Linux there is a similarly distinguishing label in executable headers...

Here's another method I just discovered.

Code:

FILE *pipe = popen("file ~/test | grep -i executable","r");
char buffer[2048];
fgets(buffer,2048,pipe);
if(strlen(buffer) > 5) printf("woot\n");
else printf("doh\n");

man file

file is a fairly standard unix utility to tell you the type of files, and it has a nice large database of lots of magic numbers which signify the start of certain file types.