From 0986c53a7069b5aaad9d4c1c1d52e0c5561b5502 Mon Sep 17 00:00:00 2001
From: Benjamin Barenblat <benjamin@barenblat.name>
Date: Wed, 13 Sep 2017 15:26:32 -0400
Subject:

---
 underlying.c | 103 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 103 insertions(+)
 create mode 100644 underlying.c

diff --git a/underlying.c b/underlying.c
new file mode 100644
index 0000000..cdaaf56
--- /dev/null
+++ b/underlying.c
@@ -0,0 +1,103 @@
+// 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;
+}
\ No newline at end of file
-- 
cgit v1.2.3