In another question, the answer states that on Unixes with /proc, the really straight and reliable way is to readlink("/proc/self/exe", buf, bufsize) and it then proceeds to give backup solutions as follows:

On Unixes without /proc (i.e. if above fails):

• If argv[0] starts with "/" (absolute path) this is the path.
• Otherwise if argv[0] contains "/" (relative path) append it to cwd (assuming it hasn't been changed yet).

getcwd(buf, bufsize); strncat(buf, "/", bufsize-strlen(buf)-1);
strncat(buf, argv[0], bufsize-strlen(buf)-1);

• Otherwise search directories in $PATH for executable argv[0].

Afterward it may be reasonable to check whether the executable isn't actually a symlink. If it is resolve it relative to the symlink directory.

Now in my case, unfortunately, none of the above works:

1. /proc/self/exe exists but fail to readlink() due to permission denied errno 13.
2. The argv[0] has no / for absolute or relative path.
3. The $PATH does not contain the executable found in argv[0].

It appears this issue is faced also when sgid applications run. In my case, it is not sgid, but an inetd launch.

Try looking in /proc from a suid binary.