find the underlying program in a wrapper script

underlying.c

// Copyright 2017 Google Inc.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

#define _POSIX_C_SOURCE 200809L

#include <stdbool.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#include <libgen.h>
#include <limits.h>
#include <unistd.h>

// Like `strchr`, but if `c` is not found, returns a pointer to the null at the
// end of `s` rather than `NULL` itself. (In glibc, this is `strchrnul`.)
static const char* FindCharacterOrEndOfString(const char* s, int c) {
  while (*s != c && *s != '\0') {
    s++;
  }
  return s;
}

// Splits a colon-separated list (e.g., `PATH`) into head and tail. Places the
// head in `dir` and stores `strlen(dir)` in `dir_len`. Returns a pointer to the
// tail, or a pointer to an empty string if the tail is empty.
static const char* GetNextEntry(const char* const restrict path,
                                char* const restrict dir,
                                size_t* const restrict dir_len) {
  const char* const end = FindCharacterOrEndOfString(path, ':');
  *dir_len = (size_t)(end - path);
  memcpy(dir, path, *dir_len);
  dir[*dir_len] = '\0';
  if (*end == '\0') {
    // Empty tail.
    return end;
  } else {
    // Skip over the ':'.
    return end + 1;
  }
}

int main(const int argc, char* const argv[]) {
  if (argc != 2) {
    fprintf(stderr,
            "usage: underlying <full_path>\n"
            "example: exec \"$(underlying \"$(readlink -f \"$0\")\")\"\n");
    return EXIT_FAILURE;
  }

  const char* const parent_path = argv[1];

  // Determine the base name of the caller. basename may modify its argument, so
  // give it a copy.
  char parent_path2[PATH_MAX] = {0};
  strcpy(parent_path2, parent_path);
  const char* const parent_name = basename(parent_path2);

  char underlying_path[PATH_MAX] = {0};
  bool underlying_path_found = false;

  // Look for parent_name in each directory in PATH. Find the first one that's
  // below parent_path.
  const char* path_environment = getenv("PATH");
  if (path_environment == NULL) {
    perror("getenv");
    return EXIT_FAILURE;
  }
  while (*path_environment != '\0') {
    // Generate a candidate path.
    char candidate_path[PATH_MAX] = {0};
    size_t candidate_path_len = 0;
    path_environment =
        GetNextEntry(path_environment, candidate_path, &candidate_path_len);
    stpcpy(stpcpy(candidate_path + candidate_path_len, "/"), parent_name);

    if (strcmp(candidate_path, parent_path) == 0) {
      // We're looking at ourself. Invalidate any work we've done so far.
      underlying_path_found = false;
    } else if (!underlying_path_found && access(candidate_path, F_OK) == 0) {
      strcpy(underlying_path, candidate_path);
      underlying_path_found = true;
    }
  }

  if (!underlying_path_found) {
    return EXIT_FAILURE;
  }

  puts(underlying_path);
  return EXIT_SUCCESS;
}