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// 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;
}
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