misc/realpath.c

   1 /*
   2  * realpath.c -- canonicalize pathname by removing symlinks
   3  * Copyright (C) 1993 Rick Sladkey <jrs@world.std.com>
   4  *
   5  * This program is free software; you can redistribute it and/or modify
   6  * it under the terms of the GNU Library Public License as published by
   7  * the Free Software Foundation; either version 2, or (at your option)
   8  * any later version.
   9  *
  10  * This program is distributed in the hope that it will be useful,
  11  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  12  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  13  * GNU Library Public License for more details.
  14  */
  15
  16 #include "system.h"
  17
  18 #ifndef STDC_HEADERS
  19 extern int errno;
  20 #endif
  21
  22 #define MAX_READLINKS 32
  23
  24 #ifdef __STDC__
  25 char *realpath(const char *path, char resolved_path [])
  26 #else
  27 char *realpath(path, resolved_path)
  28 const char *path;
  29 char resolved_path [];
  30 #endif
  31 {
  32         char copy_path[PATH_MAX];
  33         char link_path[PATH_MAX];
  34         char *new_path = resolved_path;
  35         char *max_path;
  36         int readlinks = 0;
  37         int n;
  38
  39         /* Make a copy of the source path since we may need to modify it. */
  40         strcpy(copy_path, path);
  41         path = copy_path;
  42         max_path = copy_path + PATH_MAX - 2;
  43         /* If it's a relative pathname use getwd for starters. */
  44         if (*path != '/') {
  45 #ifdef HAVE_GETCWD
  46                 getcwd(new_path, PATH_MAX - 1);
  47 #else
  48                 getwd(new_path);
  49 #endif
  50                 new_path += strlen(new_path);
  51                 if (new_path[-1] != '/')
  52                         *new_path++ = '/';
  53         }
  54         else {
  55                 *new_path++ = '/';
  56                 path++;
  57         }
  58         /* Expand each slash-separated pathname component. */
  59         while (*path != '\0') {
  60                 /* Ignore stray "/". */
  61                 if (*path == '/') {
  62                         path++;
  63                         continue;
  64                 }
  65                 if (*path == '.') {
  66                         /* Ignore ".". */
  67                         if (path[1] == '\0' || path[1] == '/') {
  68                                 path++;
  69                                 continue;
  70                         }
  71                         if (path[1] == '.') {
  72                                 if (path[2] == '\0' || path[2] == '/') {
  73                                         path += 2;
  74                                         /* Ignore ".." at root. */
  75                                         if (new_path == resolved_path + 1)
  76                                                 continue;
  77                                         /* Handle ".." by backing up. */
  78                                         while ((--new_path)[-1] != '/');
  79                                         continue;
  80                                 }
  81                         }
  82                 }
  83                 /* Safely copy the next pathname component. */
  84                 while (*path != '\0' && *path != '/') {
  85                         if (path > max_path) {
  86                                 errno = ENAMETOOLONG;
  87                                 return NULL;
  88                         }
  89                         *new_path++ = *path++;
  90                 }
  91 #ifdef S_IFLNK
  92                 /* Protect against infinite loops. */
  93                 if (readlinks++ > MAX_READLINKS) {
  94                         errno = ELOOP;
  95                         return NULL;
  96                 }
  97                 /* See if latest pathname component is a symlink. */
  98                 *new_path = '\0';
  99                 n = readlink(resolved_path, link_path, PATH_MAX - 1);
 100                 if (n < 0) {
 101                         /* EINVAL means the file exists but isn't a symlink. */
 102                         if (errno != EINVAL)
 103                                 return NULL;
 104                 }
 105                 else {
 106                         /* Note: readlink doesn't add the null byte. */
 107                         link_path[n] = '\0';
 108                         if (*link_path == '/')
 109                                 /* Start over for an absolute symlink. */
 110                                 new_path = resolved_path;
 111                         else
 112                                 /* Otherwise back up over this component. */
 113                                 while (*(--new_path) != '/');
 114                         /* Safe sex check. */
 115                         if (strlen(path) + n >= PATH_MAX) {
 116                                 errno = ENAMETOOLONG;
 117                                 return NULL;
 118                         }
 119                         /* Insert symlink contents into path. */
 120                         strcat(link_path, path);
 121                         strcpy(copy_path, link_path);
 122                         path = copy_path;
 123                 }
 124 #endif /* S_IFLNK */
 125                 *new_path++ = '/';
 126         }
 127         /* Delete trailing slash but don't whomp a lone slash. */
 128         if (new_path != resolved_path + 1 && new_path[-1] == '/')
 129                 new_path--;
 130         /* Make sure it's null terminated. */
 131         *new_path = '\0';
 132         return resolved_path;
 133 }