libio/fmemopen.c

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