#include <unistd.h>
#include <stdio.h>
#include <err.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <sys/un.h>
#include <sys/ucred.h>
#include <errno.h>
#include <pwd.h>

/* Compile with:
 * cc -W -Wall -O2 -s -pipe -o getpeercred getpeercred.c
 */
 
const char *help = "getpeercred [-hn]\n" \
"Prints the name of the user connected to the local unix stream socket on\n" \
"file descriptor 0, 1, or 2 (stdin, stdout, or stderr, in that order).\n"    \
"Prints the uid of that user if -n is given as commandline parameter.";

int main (int argc, char* argv[])
{
  int flag, numeric_flag = 0, fd;
  struct xucred cred;
  socklen_t len;
  struct passwd *pw;

  while ((flag = getopt(argc, argv, "hn")) != -1) {
    switch (flag) {
      case 'h':
        puts(help);
        return 0;
      case 'n':
        numeric_flag = 1;
        break;
      default:
        errx(1, "See -h for help");
    }
  }

  for (fd = 0; fd < 3; fd++) {
    len = sizeof(cred);
    if (getsockopt(fd, SOL_LOCAL, LOCAL_PEERCRED, &cred, &len) == 0)
      break;
  }

  if (fd >= 3)
    return -1;

  if (numeric_flag) {
    printf("%u\n", cred.cr_uid);
  } else {
    errno = 0;
    pw = getpwuid(cred.cr_uid);
    if (pw != NULL) printf("%s\n", pw->pw_name);
    else if (errno != 0) err(1, "UID %u", cred.cr_uid);
    else errx(1, "UID %u does not exist", cred.cr_uid);
  }

  return 0;
}