libio/fmemopen.c

   1 /* Fmemopen implementation.
   2    Copyright (C) 2000, 2002, 2005, 2006, 2008, 2009
   3    Free Software Foundation, Inc.
   4    This file is part of the GNU C Library.
   5    Contributed by Hanno Mueller, kontakt@hanno.de, 2000.
   6
   7    The GNU C Library is free software; you can redistribute it and/or
   8    modify it under the terms of the GNU Lesser General Public
   9    License as published by the Free Software Foundation; either
  10    version 2.1 of the License, or (at your option) any later version.
  11
  12    The GNU C Library is distributed in the hope that it will be useful,
  13    but WITHOUT ANY WARRANTY; without even the implied warranty of
  14    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  15    Lesser General Public License for more details.
  16
  17    You should have received a copy of the GNU Lesser General Public
  18    License along with the GNU C Library; if not, write to the Free
  19    Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA
  20    02111-1307 USA.  */
  21
  22 /*
  23  * fmemopen() - "my" version of a string stream
  24  * Hanno Mueller, kontakt@hanno.de
  25  *
  26  *
  27  * I needed fmemopen() for an application that I currently work on,
  28  * but couldn't find it in libio. The following snippet of code is an
  29  * attempt to implement what glibc's documentation describes.
  30  *
  31  *
  32  *
  33  * I already see some potential problems:
  34  *
  35  * - I never used the "original" fmemopen(). I am sure that "my"
  36  *   fmemopen() behaves differently than the original version.
  37  *
  38  * - The documentation doesn't say wether a string stream allows
  39  *   seeks. I checked the old fmemopen implementation in glibc's stdio
  40  *   directory, wasn't quite able to see what is going on in that
  41  *   source, but as far as I understand there was no seek there. For
  42  *   my application, I needed fseek() and ftell(), so it's here.
  43  *
  44  * - "append" mode and fseek(p, SEEK_END) have two different ideas
  45  *   about the "end" of the stream.
  46  *
  47  *   As described in the documentation, when opening the file in
  48  *   "append" mode, the position pointer will be set to the first null
  49  *   character of the string buffer (yet the buffer may already
  50  *   contain more data). For fseek(), the last byte of the buffer is
  51  *   used as the end of the stream.
  52  *
  53  * - It is unclear to me what the documentation tries to say when it
  54  *   explains what happens when you use fmemopen with a NULL
  55  *   buffer.
  56  *
  57  *   Quote: "fmemopen [then] allocates an array SIZE bytes long. This
  58  *   is really only useful if you are going to write things to the
  59  *   buffer and then read them back in again."
  60  *
  61  *   What does that mean if the original fmemopen() did not allow
  62  *   seeking? How do you read what you just wrote without seeking back
  63  *   to the beginning of the stream?
  64  *
  65  * - I think there should be a second version of fmemopen() that does
  66  *   not add null characters for each write. (At least in my
  67  *   application, I am not actually using strings but binary data and
  68  *   so I don't need the stream to add null characters on its own.)
  69  */
  70
  71 #include <errno.h>
  72 #include <libio.h>
  73 #include <stdio.h>
  74 #include <stdlib.h>
  75 #include <stdint.h>
  76 #include <string.h>
  77 #include <sys/types.h>
  78 #include "libioP.h"
  79
  80
  81 typedef struct fmemopen_cookie_struct fmemopen_cookie_t;
  82 struct fmemopen_cookie_struct
  83 {
  84   char *buffer;
  85   int mybuffer;
  86   int binmode;
  87   size_t size;
  88   _IO_off64_t pos;
  89   size_t maxpos;
  90 };
  91
  92
  93 static ssize_t
  94 fmemopen_read (void *cookie, char *b, size_t s)
  95 {
  96   fmemopen_cookie_t *c;
  97
  98   c = (fmemopen_cookie_t *) cookie;
  99
 100   if (c->pos + s > c->size)
 101     {
 102       if ((size_t) c->pos == c->size)
 103         return 0;
 104       s = c->size - c->pos;
 105     }
 106
 107   memcpy (b, &(c->buffer[c->pos]), s);
 108
 109   c->pos += s;
 110   if ((size_t) c->pos > c->maxpos)
 111     c->maxpos = c->pos;
 112
 113   return s;
 114 }
 115
 116
 117 static ssize_t
 118 fmemopen_write (void *cookie, const char *b, size_t s)
 119 {
 120   fmemopen_cookie_t *c;
 121   int addnullc;
 122
 123   c = (fmemopen_cookie_t *) cookie;
 124
 125   addnullc = c->binmode == 0 && (s == 0 || b[s - 1] != '\0');
 126
 127   if (c->pos + s + addnullc > c->size)
 128     {
 129       if ((size_t) (c->pos + addnullc) == c->size)
 130         {
 131           __set_errno (ENOSPC);
 132           return 0;
 133         }
 134       s = c->size - c->pos - addnullc;
 135     }
 136
 137   memcpy (&(c->buffer[c->pos]), b, s);
 138
 139   c->pos += s;
 140   if ((size_t) c->pos > c->maxpos)
 141     {
 142       c->maxpos = c->pos;
 143       if (addnullc)
 144         c->buffer[c->maxpos] = '\0';
 145     }
 146
 147   return s;
 148 }
 149
 150
 151 static int
 152 fmemopen_seek (void *cookie, _IO_off64_t *p, int w)
 153 {
 154   _IO_off64_t np;
 155   fmemopen_cookie_t *c;
 156
 157   c = (fmemopen_cookie_t *) cookie;
 158
 159   switch (w)
 160     {
 161     case SEEK_SET:
 162       np = *p;
 163       break;
 164
 165     case SEEK_CUR:
 166       np = c->pos + *p;
 167       break;
 168
 169     case SEEK_END:
 170       np = (c->binmode ? c->size : c->maxpos) - *p;
 171       break;
 172
 173     default:
 174       return -1;
 175     }
 176
 177   if (np < 0 || (size_t) np > c->size)
 178     return -1;
 179
 180   *p = c->pos = np;
 181
 182   return 0;
 183 }
 184
 185
 186 static int
 187 fmemopen_close (void *cookie)
 188 {
 189   fmemopen_cookie_t *c;
 190
 191   c = (fmemopen_cookie_t *) cookie;
 192
 193   if (c->mybuffer)
 194     free (c->buffer);
 195   free (c);
 196
 197   return 0;
 198 }
 199
 200
 201 FILE *
 202 fmemopen (void *buf, size_t len, const char *mode)
 203 {
 204   cookie_io_functions_t iof;
 205   fmemopen_cookie_t *c;
 206
 207   if (__builtin_expect (len == 0, 0))
 208     {
 209     einval:
 210       __set_errno (EINVAL);
 211       return NULL;
 212     }
 213
 214   c = (fmemopen_cookie_t *) malloc (sizeof (fmemopen_cookie_t));
 215   if (c == NULL)
 216     return NULL;
 217
 218   c->mybuffer = (buf == NULL);
 219
 220   if (c->mybuffer)
 221     {
 222       c->buffer = (char *) malloc (len);
 223       if (c->buffer == NULL)
 224         {
 225           free (c);
 226           return NULL;
 227         }
 228       c->buffer[0] = '\0';
 229     }
 230   else
 231     {
 232       if (__builtin_expect ((uintptr_t) len > -(uintptr_t) buf, 0))
 233         {
 234           free (c);
 235           goto einval;
 236         }
 237
 238       c->buffer = buf;
 239     }
 240
 241   c->size = len;
 242
 243   if (mode[0] == 'w')
 244     c->buffer[0] = '\0';
 245
 246   c->maxpos = strlen (c->buffer);
 247
 248   if (mode[0] == 'a')
 249     c->pos = c->maxpos;
 250   else
 251     c->pos = 0;
 252
 253   c->binmode = mode[0] != '\0' && mode[1] == 'b';
 254
 255   iof.read = fmemopen_read;
 256   iof.write = fmemopen_write;
 257   iof.seek = fmemopen_seek;
 258   iof.close = fmemopen_close;
 259
 260   return _IO_fopencookie (c, mode, iof);
 261 }
 262 libc_hidden_def (fmemopen)