1 /* Fmemopen implementation.
2 Copyright (C) 2000-2023 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