libio/oldfmemopen.c

   1 /* Fmemopen implementation.
   2    Copyright (C) 2000-2024 Free Software Foundation, Inc.
   3    This file is part of the GNU C Library.
   4
   5    The GNU C Library is free software; you can redistribute it and/or
   6    modify it under the terms of the GNU Lesser General Public
   7    License as published by the Free Software Foundation; either
   8    version 2.1 of the License, or (at your option) any later version.
   9
  10    The GNU C Library 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 GNU
  13    Lesser General Public License for more details.
  14
  15    You should have received a copy of the GNU Lesser General Public
  16    License along with the GNU C Library; if not, see
  17    <https://www.gnu.org/licenses/>.  */
  18
  19 /*
  20  * fmemopen() - "my" version of a string stream
  21  * Hanno Mueller, kontakt@hanno.de
  22  *
  23  *
  24  * I needed fmemopen() for an application that I currently work on,
  25  * but couldn't find it in libio. The following snippet of code is an
  26  * attempt to implement what glibc's documentation describes.
  27  *
  28  *
  29  *
  30  * I already see some potential problems:
  31  *
  32  * - I never used the "original" fmemopen(). I am sure that "my"
  33  *   fmemopen() behaves differently than the original version.
  34  *
  35  * - The documentation doesn't say whether a string stream allows
  36  *   seeks. I checked the old fmemopen implementation in glibc's stdio
  37  *   directory, wasn't quite able to see what is going on in that
  38  *   source, but as far as I understand there was no seek there. For
  39  *   my application, I needed fseek() and ftell(), so it's here.
  40  *
  41  * - "append" mode and fseek(p, SEEK_END) have two different ideas
  42  *   about the "end" of the stream.
  43  *
  44  *   As described in the documentation, when opening the file in
  45  *   "append" mode, the position pointer will be set to the first null
  46  *   character of the string buffer (yet the buffer may already
  47  *   contain more data). For fseek(), the last byte of the buffer is
  48  *   used as the end of the stream.
  49  *
  50  * - It is unclear to me what the documentation tries to say when it
  51  *   explains what happens when you use fmemopen with a NULL
  52  *   buffer.
  53  *
  54  *   Quote: "fmemopen [then] allocates an array SIZE bytes long. This
  55  *   is really only useful if you are going to write things to the
  56  *   buffer and then read them back in again."
  57  *
  58  *   What does that mean if the original fmemopen() did not allow
  59  *   seeking? How do you read what you just wrote without seeking back
  60  *   to the beginning of the stream?
  61  *
  62  * - I think there should be a second version of fmemopen() that does
  63  *   not add null characters for each write. (At least in my
  64  *   application, I am not actually using strings but binary data and
  65  *   so I don't need the stream to add null characters on its own.)
  66  */
  67
  68 #include "libioP.h"
  69
  70 #if SHLIB_COMPAT (libc, GLIBC_2_2, GLIBC_2_22)
  71
  72 #include <errno.h>
  73 #include <stdio.h>
  74 #include <stdlib.h>
  75 #include <stdint.h>
  76 #include <string.h>
  77 #include <sys/types.h>
  78
  79
  80 typedef struct fmemopen_cookie_struct fmemopen_cookie_t;
  81 struct fmemopen_cookie_struct
  82 {
  83   char *buffer;
  84   int mybuffer;
  85   int binmode;
  86   size_t size;
  87   off64_t pos;
  88   size_t maxpos;
  89 };
  90
  91
  92 static ssize_t
  93 fmemopen_read (void *cookie, char *b, size_t s)
  94 {
  95   fmemopen_cookie_t *c;
  96
  97   c = (fmemopen_cookie_t *) cookie;
  98
  99   if (c->pos + s > c->size)
 100     {
 101       if ((size_t) c->pos == c->size)
 102         return 0;
 103       s = c->size - c->pos;
 104     }
 105
 106   memcpy (b, &(c->buffer[c->pos]), s);
 107
 108   c->pos += s;
 109   if ((size_t) c->pos > c->maxpos)
 110     c->maxpos = c->pos;
 111
 112   return s;
 113 }
 114
 115
 116 static ssize_t
 117 fmemopen_write (void *cookie, const char *b, size_t s)
 118 {
 119   fmemopen_cookie_t *c;
 120   int addnullc;
 121
 122   c = (fmemopen_cookie_t *) cookie;
 123
 124   addnullc = c->binmode == 0 && (s == 0 || b[s - 1] != '\0');
 125
 126   if (c->pos + s + addnullc > c->size)
 127     {
 128       if ((size_t) (c->pos + addnullc) >= c->size)
 129         {
 130           __set_errno (ENOSPC);
 131           return 0;
 132         }
 133       s = c->size - c->pos - addnullc;
 134     }
 135
 136   memcpy (&(c->buffer[c->pos]), b, s);
 137
 138   c->pos += s;
 139   if ((size_t) c->pos > c->maxpos)
 140     {
 141       c->maxpos = c->pos;
 142       if (addnullc)
 143         c->buffer[c->maxpos] = '\0';
 144     }
 145
 146   return s;
 147 }
 148
 149
 150 static int
 151 fmemopen_seek (void *cookie, off64_t *p, int w)
 152 {
 153   off64_t np;
 154   fmemopen_cookie_t *c;
 155
 156   c = (fmemopen_cookie_t *) cookie;
 157
 158   switch (w)
 159     {
 160     case SEEK_SET:
 161       np = *p;
 162       break;
 163
 164     case SEEK_CUR:
 165       np = c->pos + *p;
 166       break;
 167
 168     case SEEK_END:
 169       np = (c->binmode ? c->size : c->maxpos) - *p;
 170       break;
 171
 172     default:
 173       return -1;
 174     }
 175
 176   if (np < 0 || (size_t) np > c->size)
 177     return -1;
 178
 179   *p = c->pos = np;
 180
 181   return 0;
 182 }
 183
 184
 185 static int
 186 fmemopen_close (void *cookie)
 187 {
 188   fmemopen_cookie_t *c;
 189
 190   c = (fmemopen_cookie_t *) cookie;
 191
 192   if (c->mybuffer)
 193     free (c->buffer);
 194   free (c);
 195
 196   return 0;
 197 }
 198
 199
 200 FILE *
 201 __old_fmemopen (void *buf, size_t len, const char *mode)
 202 {
 203   cookie_io_functions_t iof;
 204   fmemopen_cookie_t *c;
 205   FILE *result;
 206
 207   if (__glibc_unlikely (len == 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       c->maxpos = 0;
 230     }
 231   else
 232     {
 233       if (__glibc_unlikely ((uintptr_t) len > -(uintptr_t) buf))
 234         {
 235           free (c);
 236           goto einval;
 237         }
 238
 239       c->buffer = buf;
 240
 241       if (mode[0] == 'w')
 242         c->buffer[0] = '\0';
 243
 244       c->maxpos = strnlen (c->buffer, len);
 245     }
 246
 247   c->size = len;
 248
 249   if (mode[0] == 'a')
 250     c->pos = c->maxpos;
 251   else
 252     c->pos = 0;
 253
 254   c->binmode = mode[0] != '\0' && mode[1] == 'b';
 255
 256   iof.read = fmemopen_read;
 257   iof.write = fmemopen_write;
 258   iof.seek = fmemopen_seek;
 259   iof.close = fmemopen_close;
 260
 261   result = _IO_fopencookie (c, mode, iof);
 262   if (__glibc_unlikely (result == NULL))
 263     {
 264       if (c->mybuffer)
 265         free (c->buffer);
 266
 267       free (c);
 268     }
 269
 270   return result;
 271 }
 272 compat_symbol (libc, __old_fmemopen, fmemopen, GLIBC_2_2);
 273 #endif