root/maint/gnulib/lib/areadlink-with-size.c

/* */

DEFINITIONS

1. areadlink_with_size

   1 /* readlink wrapper to return the link name in malloc'd storage.
   2    Unlike xreadlink and xreadlink_with_size, don't ever call exit.
   3 
   4    Copyright (C) 2001, 2003-2007, 2009-2021 Free Software Foundation, Inc.
   5 
   6    This file is free software: you can redistribute it and/or modify
   7    it under the terms of the GNU Lesser General Public License as
   8    published by the Free Software Foundation; either version 2.1 of the
   9    License, or (at your option) any later version.
  10 
  11    This file is distributed in the hope that it will be useful,
  12    but WITHOUT ANY WARRANTY; without even the implied warranty of
  13    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  14    GNU Lesser General Public License for more details.
  15 
  16    You should have received a copy of the GNU Lesser General Public License
  17    along with this program.  If not, see <https://www.gnu.org/licenses/>.  */
  18 
  19 /* Written by Jim Meyering <jim@meyering.net>  */
  20 
  21 #include <config.h>
  22 
  23 #include "areadlink.h"
  24 
  25 #include <errno.h>
  26 #include <limits.h>
  27 #include <stdint.h>
  28 #include <stdlib.h>
  29 #include <string.h>
  30 #include <unistd.h>
  31 
  32 #ifndef SSIZE_MAX
  33 # define SSIZE_MAX ((ssize_t) (SIZE_MAX / 2))
  34 #endif
  35 
  36 /* SYMLINK_MAX is used only for an initial memory-allocation sanity
  37    check, so it's OK to guess too small on hosts where there is no
  38    arbitrary limit to symbolic link length.  */
  39 #ifndef SYMLINK_MAX
  40 # define SYMLINK_MAX 1024
  41 #endif
  42 
  43 #define MAXSIZE (SIZE_MAX < SSIZE_MAX ? SIZE_MAX : SSIZE_MAX)
  44 
  45 /* Call readlink to get the symbolic link value of FILE.
  46    SIZE is a hint as to how long the link is expected to be;
  47    typically it is taken from st_size.  It need not be correct.
  48    Return a pointer to that NUL-terminated string in malloc'd storage.
  49    If readlink fails, malloc fails, or if the link value is longer
  50    than SSIZE_MAX, return NULL (caller may use errno to diagnose).  */
  51 
  52 char *
  53 areadlink_with_size (char const *file, size_t size)
     /*  */
  54 {
  55   /* Some buggy file systems report garbage in st_size.  Defend
  56      against them by ignoring outlandish st_size values in the initial
  57      memory allocation.  */
  58   size_t symlink_max = SYMLINK_MAX;
  59   size_t INITIAL_LIMIT_BOUND = 8 * 1024;
  60   size_t initial_limit = (symlink_max < INITIAL_LIMIT_BOUND
  61                           ? symlink_max + 1
  62                           : INITIAL_LIMIT_BOUND);
  63 
  64   enum { stackbuf_size = 128 };
  65 
  66   /* The initial buffer size for the link value.  */
  67   size_t buf_size = (size == 0 ? stackbuf_size
  68                      : size < initial_limit ? size + 1 : initial_limit);
  69 
  70   while (1)
  71     {
  72       ssize_t r;
  73       size_t link_length;
  74       char stackbuf[stackbuf_size];
  75       char *buf = stackbuf;
  76       char *buffer = NULL;
  77 
  78       if (! (size == 0 && buf_size == stackbuf_size))
  79         {
  80           buf = buffer = malloc (buf_size);
  81           if (!buffer)
  82             {
  83               errno = ENOMEM;
  84               return NULL;
  85             }
  86         }
  87 
  88       r = readlink (file, buf, buf_size);
  89       link_length = r;
  90 
  91       if (r < 0)
  92         {
  93           free (buffer);
  94           return NULL;
  95         }
  96 
  97       if (link_length < buf_size)
  98         {
  99           buf[link_length] = 0;
 100           if (!buffer)
 101             {
 102               buffer = malloc (link_length + 1);
 103               if (buffer)
 104                 return memcpy (buffer, buf, link_length + 1);
 105             }
 106           else if (link_length + 1 < buf_size)
 107             {
 108               /* Shrink BUFFER before returning it.  */
 109               char *shrinked_buffer = realloc (buffer, link_length + 1);
 110               if (shrinked_buffer != NULL)
 111                 buffer = shrinked_buffer;
 112             }
 113           return buffer;
 114         }
 115 
 116       free (buffer);
 117       if (buf_size <= MAXSIZE / 2)
 118         buf_size *= 2;
 119       else if (buf_size < MAXSIZE)
 120         buf_size = MAXSIZE;
 121       else
 122         {
 123           errno = ENOMEM;
 124           return NULL;
 125         }
 126     }
 127 }

/* */