Why doesn't the execve command in C on macOS allow the 'which' command to work?

Question

Why does the execve command in C on macOS not allow the 'which' command to work? It works on non-Mac devices.

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

int main()
{
    int fd;
    char cmd[] = "/bin/cat";
    char cmd1[] = "/usr/bin/which";
    char *s[]={"which","ls",NULL};
    if (execve(cmd1, s, NULL) == -1)
        perror("oops ur wrong!!");
}

Expected output

 clang-7 -pthread -lm -o main main.c
 ./main
/bin/ls
 

but on a Mac, it returns nothing.

Answer 1

macOS

The code works. It doesn't work well, but it does work.

Given the null PATH in the environment (because you've used execve() and provided NULL as the environment), /usr/bin/which can't find ls — it has nowhere to look for it because PATH is not set.

On my machine (a MacBook Pro running macOS Big Sur 11.7.1 — it's a work machine and the company IT is behind the times), /usr/bin/which is a universal binary with two architectures. If I run /usr/bin/which ozymandias on the command line, there is no output (I don't have a command ozymandias anywhere), but the exit status is 1 (failure). That's an odd implementation — not reporting an error — but it works within its limits.

You can see this effect with:

$ (unset PATH; /usr/bin/which ls)
$ echo $?
1
$

If you use execv() instead of execve() and remove the , NULL from the argument list, the output is /bin/ls and the exit status is 0 .

Linux

Just for comparison, on a RHEL 7.4 machine, I get different results:

$ which -a which
which='alias | /usr/bin/which --tty-only --read-alias --show-dot --show-tilde'
    /usr/bin/alias
    /usr/bin/which
/usr/bin/which
$ file /usr/bin/which
/usr/bin/which: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), dynamically linked (uses shared libs), for GNU/Linux 2.6.32, BuildID[sha1]=317ba624d2914607bf9246993446803a977fbc18, stripped
$ /usr/bin/which which
/usr/bin/which
$ (unset PATH; /usr/bin/which which)
/usr/bin/which: no which in ((null))
$ /usr/bin/which ozymandias
/usr/bin/which: no ozymandias in (/work2/jleffler/bin:/u/jleffler/bin:/usr/perl/v5.34.0/bin:/usr/gcc/v12.2.0/bin:/usr/local/bin:/usr/bin:/usr/sbin)
$ /usr/bin/which --help
Usage: /usr/bin/which [options] [--] COMMAND [...]
Write the full path of COMMAND(s) to standard output.

  --version, -[vV] Print version and exit successfully.
  --help,          Print this help and exit successfully.
  --skip-dot       Skip directories in PATH that start with a dot.
  --skip-tilde     Skip directories in PATH that start with a tilde.
  --show-dot       Don't expand a dot to current directory in output.
  --show-tilde     Output a tilde for HOME directory for non-root.
  --tty-only       Stop processing options on the right if not on tty.
  --all, -a        Print all matches in PATH, not just the first
  --read-alias, -i Read list of aliases from stdin.
  --skip-alias     Ignore option --read-alias; don't read stdin.
  --read-functions Read shell functions from stdin.
  --skip-functions Ignore option --read-functions; don't read stdin.

Recommended use is to write the output of (alias; declare -f) to standard
input, so that which can show aliases and shell functions. See which(1) for
examples.

If the options --read-alias and/or --read-functions are specified then the
output can be a full alias or function definition, optionally followed by
the full path of each command used inside of those.

Report bugs to <which-bugs@gnu.org>.
$

^{PATH sanitized — radically shortened.}

The which command reports an error when it can't find the command. It is a standalone executable on this Linux machine, and the which alias feeds it the aliases so it can report on them. The -a option reports on all the things that could be known as which (the second which in which -a which ).

Answer 2

I found that adding the envp(path argument in main) to the arguments made it work

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

int main(int argv, char *argc[],char *envp[])
{
    int fd;
    char cmd1[] = "/usr/bin/which";
    char *s[] = {"which", "ls", NULL};
    if (execve(cmd1, s, envp) == -1)
        perror("oops ur wrong!!");
}

thanks anyways