libio/oldfmemopen.c

   1 /* Fmemopen implementation.
   2    Copyright (C) 2000-2017 Free Software Foundation, Inc.
   3    This file is part of the GNU C Library.
   4    Contributed by Hanno Mueller, kontakt@hanno.de, 2000.
   5
   6    The GNU C Library is free software; you can redistribute it and/or
   7    modify it under the terms of the GNU Lesser General Public
   8    License as published by the Free Software Foundation; either
   9    version 2.1 of the License, or (at your option) any later version.
  10
  11    The GNU C Library 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 GNU
  14    Lesser General Public License for more details.
  15
  16    You should have received a copy of the GNU Lesser General Public
  17    License along with the GNU C Library; if not, see
  18    <http://www.gnu.org/licenses/>.  */
  19
  20 /*
  21  * fmemopen() - "my" version of a string stream
  22  * Hanno Mueller, kontakt@hanno.de
  23  *
  24  *
  25  * I needed fmemopen() for an application that I currently work on,
  26  * but couldn't find it in libio. The following snippet of code is an
  27  * attempt to implement what glibc's documentation describes.
  28  *
  29  *
  30  *
  31  * I already see some potential problems:
  32  *
  33  * - I never used the "original" fmemopen(). I am sure that "my"
  34  *   fmemopen() behaves differently than the original version.
  35  *
  36  * - The documentation doesn't say wether a string stream allows
  37  *   seeks. I checked the old fmemopen implementation in glibc's stdio
  38  *   directory, wasn't quite able to see what is going on in that
  39  *   source, but as far as I understand there was no seek there. For
  40  *   my application, I needed fseek() and ftell(), so it's here.
  41  *
  42  * - "append" mode and fseek(p, SEEK_END) have two different ideas
  43  *   about the "end" of the stream.
  44  *
  45  *   As described in the documentation, when opening the file in
  46  *   "append" mode, the position pointer will be set to the first null
  47  *   character of the string buffer (yet the buffer may already
  48  *   contain more data). For fseek(), the last byte of the buffer is
  49  *   used as the end of the stream.
  50  *
  51  * - It is unclear to me what the documentation tries to say when it
  52  *   explains what happens when you use fmemopen with a NULL
  53  *   buffer.
  54  *
  55  *   Quote: "fmemopen [then] allocates an array SIZE bytes long. This
  56  *   is really only useful if you are going to write things to the
  57  *   buffer and then read them back in again."
  58  *
  59  *   What does that mean if the original fmemopen() did not allow
  60  *   seeking? How do you read what you just wrote without seeking back
  61  *   to the beginning of the stream?
  62  *
  63  * - I think there should be a second version of fmemopen() that does
  64  *   not add null characters for each write. (At least in my
  65  *   application, I am not actually using strings but binary data and
  66  *   so I don't need the stream to add null characters on its own.)
  67  */
  68
  69 #include "libioP.h"
  70
  71 #if SHLIB_COMPAT (libc, GLIBC_2_2, GLIBC_2_22)
  72
  73 #include <errno.h>
  74 #include <libio.h>
  75 #include <stdio.h>
  76 #include <stdlib.h>
  77 #include <stdint.h>
  78 #include <string.h>
  79 #include <sys/types.h>
  80
  81
  82 typedef struct fmemopen_cookie_struct fmemopen_cookie_t;
  83 struct fmemopen_cookie_struct
  84 {
  85   char *buffer;
  86   int mybuffer;
  87   int binmode;
  88   size_t size;
  89   _IO_off64_t pos;
  90   size_t maxpos;
  91 };
  92
  93
  94 static ssize_t
  95 fmemopen_read (void *cookie, char *b, size_t s)
  96 {
  97   fmemopen_cookie_t *c;
  98
  99   c = (fmemopen_cookie_t *) cookie;
 100
 101   if (c->pos + s > c->size)
 102     {
 103       if ((size_t) c->pos == c->size)
 104         return 0;
 105       s = c->size - c->pos;
 106     }
 107
 108   memcpy (b, &(c->buffer[c->pos]), s);
 109
 110   c->pos += s;
 111   if ((size_t) c->pos > c->maxpos)
 112     c->maxpos = c->pos;
 113
 114   return s;
 115 }
 116
 117
 118 static ssize_t
 119 fmemopen_write (void *cookie, const char *b, size_t s)
 120 {
 121   fmemopen_cookie_t *c;
 122   int addnullc;
 123
 124   c = (fmemopen_cookie_t *) cookie;
 125
 126   addnullc = c->binmode == 0 && (s == 0 || b[s - 1] != '\0');
 127
 128   if (c->pos + s + addnullc > c->size)
 129     {
 130       if ((size_t) (c->pos + addnullc) >= c->size)
 131         {
 132           __set_errno (ENOSPC);
 133           return 0;
 134         }
 135       s = c->size - c->pos - addnullc;
 136     }
 137
 138   memcpy (&(c->buffer[c->pos]), b, s);
 139
 140   c->pos += s;
 141   if ((size_t) c->pos > c->maxpos)
 142     {
 143       c->maxpos = c->pos;
 144       if (addnullc)
 145         c->buffer[c->maxpos] = '\0';
 146     }
 147
 148   return s;
 149 }
 150
 151
 152 static int
 153 fmemopen_seek (void *cookie, _IO_off64_t *p, int w)
 154 {
 155   _IO_off64_t np;
 156   fmemopen_cookie_t *c;
 157
 158   c = (fmemopen_cookie_t *) cookie;
 159
 160   switch (w)
 161     {
 162     case SEEK_SET:
 163       np = *p;
 164       break;
 165
 166     case SEEK_CUR:
 167       np = c->pos + *p;
 168       break;
 169
 170     case SEEK_END:
 171       np = (c->binmode ? c->size : c->maxpos) - *p;
 172       break;
 173
 174     default:
 175       return -1;
 176     }
 177
 178   if (np < 0 || (size_t) np > c->size)
 179     return -1;
 180
 181   *p = c->pos = np;
 182
 183   return 0;
 184 }
 185
 186
 187 static int
 188 fmemopen_close (void *cookie)
 189 {
 190   fmemopen_cookie_t *c;
 191
 192   c = (fmemopen_cookie_t *) cookie;
 193
 194   if (c->mybuffer)
 195     free (c->buffer);
 196   free (c);
 197
 198   return 0;
 199 }
 200
 201
 202 FILE *
 203 __old_fmemopen (void *buf, size_t len, const char *mode)
 204 {
 205   cookie_io_functions_t iof;
 206   fmemopen_cookie_t *c;
 207   FILE *result;
 208
 209   if (__glibc_unlikely (len == 0))
 210     {
 211     einval:
 212       __set_errno (EINVAL);
 213       return NULL;
 214     }
 215
 216   c = (fmemopen_cookie_t *) malloc (sizeof (fmemopen_cookie_t));
 217   if (c == NULL)
 218     return NULL;
 219
 220   c->mybuffer = (buf == NULL);
 221
 222   if (c->mybuffer)
 223     {
 224       c->buffer = (char *) malloc (len);
 225       if (c->buffer == NULL)
 226         {
 227           free (c);
 228           return NULL;
 229         }
 230       c->buffer[0] = '\0';
 231       c->maxpos = 0;
 232     }
 233   else
 234     {
 235       if (__glibc_unlikely ((uintptr_t) len > -(uintptr_t) buf))
 236         {
 237           free (c);
 238           goto einval;
 239         }
 240
 241       c->buffer = buf;
 242
 243       if (mode[0] == 'w')
 244         c->buffer[0] = '\0';
 245
 246       c->maxpos = strnlen (c->buffer, len);
 247     }
 248
 249   c->size = len;
 250
 251   if (mode[0] == 'a')
 252     c->pos = c->maxpos;
 253   else
 254     c->pos = 0;
 255
 256   c->binmode = mode[0] != '\0' && mode[1] == 'b';
 257
 258   iof.read = fmemopen_read;
 259   iof.write = fmemopen_write;
 260   iof.seek = fmemopen_seek;
 261   iof.close = fmemopen_close;
 262
 263   result = _IO_fopencookie (c, mode, iof);
 264   if (__glibc_unlikely (result == NULL))
 265     {
 266       if (c->mybuffer)
 267         free (c->buffer);
 268
 269       free (c);
 270     }
 271
 272   return result;
 273 }
 274 compat_symbol (libc, __old_fmemopen, fmemopen, GLIBC_2_2);
 275 #endif