libio/fmemopen.c

   1 /* Fmemopen implementation.
   2    Copyright (C) 2000-2015 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 <errno.h>
  70 #include <libio.h>
  71 #include <stdio.h>
  72 #include <stdlib.h>
  73 #include <stdint.h>
  74 #include <string.h>
  75 #include <sys/types.h>
  76 #include "libioP.h"
  77
  78
  79 typedef struct fmemopen_cookie_struct fmemopen_cookie_t;
  80 struct fmemopen_cookie_struct
  81 {
  82   char *buffer;
  83   int mybuffer;
  84   int binmode;
  85   size_t size;
  86   _IO_off64_t pos;
  87   size_t maxpos;
  88 };
  89
  90
  91 static ssize_t
  92 fmemopen_read (void *cookie, char *b, size_t s)
  93 {
  94   fmemopen_cookie_t *c;
  95
  96   c = (fmemopen_cookie_t *) cookie;
  97
  98   if (c->pos + s > c->size)
  99     {
 100       if ((size_t) c->pos == c->size)
 101         return 0;
 102       s = c->size - c->pos;
 103     }
 104
 105   memcpy (b, &(c->buffer[c->pos]), s);
 106
 107   c->pos += s;
 108   if ((size_t) c->pos > c->maxpos)
 109     c->maxpos = c->pos;
 110
 111   return s;
 112 }
 113
 114
 115 static ssize_t
 116 fmemopen_write (void *cookie, const char *b, size_t s)
 117 {
 118   fmemopen_cookie_t *c;
 119   int addnullc;
 120
 121   c = (fmemopen_cookie_t *) cookie;
 122
 123   addnullc = c->binmode == 0 && (s == 0 || b[s - 1] != '\0');
 124
 125   if (c->pos + s + addnullc > c->size)
 126     {
 127       if ((size_t) (c->pos + addnullc) == c->size)
 128         {
 129           __set_errno (ENOSPC);
 130           return 0;
 131         }
 132       s = c->size - c->pos - addnullc;
 133     }
 134
 135   memcpy (&(c->buffer[c->pos]), b, s);
 136
 137   c->pos += s;
 138   if ((size_t) c->pos > c->maxpos)
 139     {
 140       c->maxpos = c->pos;
 141       if (addnullc)
 142         c->buffer[c->maxpos] = '\0';
 143     }
 144
 145   return s;
 146 }
 147
 148
 149 static int
 150 fmemopen_seek (void *cookie, _IO_off64_t *p, int w)
 151 {
 152   _IO_off64_t np;
 153   fmemopen_cookie_t *c;
 154
 155   c = (fmemopen_cookie_t *) cookie;
 156
 157   switch (w)
 158     {
 159     case SEEK_SET:
 160       np = *p;
 161       break;
 162
 163     case SEEK_CUR:
 164       np = c->pos + *p;
 165       break;
 166
 167     case SEEK_END:
 168       np = (c->binmode ? c->size : c->maxpos) - *p;
 169       break;
 170
 171     default:
 172       return -1;
 173     }
 174
 175   if (np < 0 || (size_t) np > c->size)
 176     return -1;
 177
 178   *p = c->pos = np;
 179
 180   return 0;
 181 }
 182
 183
 184 static int
 185 fmemopen_close (void *cookie)
 186 {
 187   fmemopen_cookie_t *c;
 188
 189   c = (fmemopen_cookie_t *) cookie;
 190
 191   if (c->mybuffer)
 192     free (c->buffer);
 193   free (c);
 194
 195   return 0;
 196 }
 197
 198
 199 FILE *
 200 fmemopen (void *buf, size_t len, const char *mode)
 201 {
 202   cookie_io_functions_t iof;
 203   fmemopen_cookie_t *c;
 204
 205   if (__glibc_unlikely (len == 0))
 206     {
 207     einval:
 208       __set_errno (EINVAL);
 209       return NULL;
 210     }
 211
 212   c = (fmemopen_cookie_t *) malloc (sizeof (fmemopen_cookie_t));
 213   if (c == NULL)
 214     return NULL;
 215
 216   c->mybuffer = (buf == NULL);
 217
 218   if (c->mybuffer)
 219     {
 220       c->buffer = (char *) malloc (len);
 221       if (c->buffer == NULL)
 222         {
 223           free (c);
 224           return NULL;
 225         }
 226       c->buffer[0] = '\0';
 227       c->maxpos = 0;
 228     }
 229   else
 230     {
 231       if (__glibc_unlikely ((uintptr_t) len > -(uintptr_t) buf))
 232         {
 233           free (c);
 234           goto einval;
 235         }
 236
 237       c->buffer = buf;
 238
 239       if (mode[0] == 'w')
 240         c->buffer[0] = '\0';
 241
 242       c->maxpos = strnlen (c->buffer, len);
 243     }
 244
 245   c->size = len;
 246
 247   if (mode[0] == 'a')
 248     c->pos = c->maxpos;
 249   else
 250     c->pos = 0;
 251
 252   c->binmode = mode[0] != '\0' && mode[1] == 'b';
 253
 254   iof.read = fmemopen_read;
 255   iof.write = fmemopen_write;
 256   iof.seek = fmemopen_seek;
 257   iof.close = fmemopen_close;
 258
 259   return _IO_fopencookie (c, mode, iof);
 260 }
 261 libc_hidden_def (fmemopen)