open_memstream.c revision cf63d5d00f5a631a2905da7812b5c029b5211cf6 
1/*
2 * Copyright (C) 2010 The Android Open Source Project
3 *
4 * Licensed under the Apache License, Version 2.0 (the "License");
5 * you may not use this file except in compliance with the License.
6 * You may obtain a copy of the License at
7 *
8 *      http://www.apache.org/licenses/LICENSE-2.0
9 *
10 * Unless required by applicable law or agreed to in writing, software
11 * distributed under the License is distributed on an "AS IS" BASIS,
12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13 * See the License for the specific language governing permissions and
14 * limitations under the License.
15 */
16
17#ifndef HAVE_OPEN_MEMSTREAM
18
19/*
20 * Implementation of the POSIX open_memstream() function, which Linux has
21 * but BSD lacks.
22 *
23 * Summary:
24 * - Works like a file-backed FILE* opened with fopen(name, "w"), but the
25 *   backing is a chunk of memory rather than a file.
26 * - The buffer expands as you write more data.  Seeking past the end
27 *   of the file and then writing to it zero-fills the gap.
28 * - The values at "*bufp" and "*sizep" should be considered read-only,
29 *   and are only valid immediately after an fflush() or fclose().
30 * - A '\0' is maintained just past the end of the file. This is not included
31 *   in "*sizep".  (The behavior w.r.t. fseek() is not clearly defined.
32 *   The spec says the null byte is written when a write() advances EOF,
33 *   but it looks like glibc ensures the null byte is always found at EOF,
34 *   even if you just seeked backwards.  The example on the opengroup.org
35 *   page suggests that this is the expected behavior.  The null must be
36 *   present after a no-op fflush(), which we can't see, so we have to save
37 *   and restore it.  Annoying, but allows file truncation.)
38 * - After fclose(), the caller must eventually free(*bufp).
39 *
40 * This is built out of funopen(), which BSD has but Linux lacks.  There is
41 * no flush() operator, so we need to keep the user pointers up to date
42 * after each operation.
43 *
44 * I don't think Windows has any of the above, but we don't need to use
45 * them there, so we just supply a stub.
46 */
47#include <cutils/open_memstream.h>
48#include <stdlib.h>
49#include <stdio.h>
50#include <string.h>
51#include <errno.h>
52#include <assert.h>
53
54#if 0
55# define DBUG(x) printf x
56#else
57# define DBUG(x) ((void)0)
58#endif
59
60#ifdef HAVE_FUNOPEN
61
62/*
63 * Definition of a seekable, write-only memory stream.
64 */
65typedef struct {
66    char**      bufp;       /* pointer to buffer pointer */
67    size_t*     sizep;      /* pointer to eof */
68
69    size_t      allocSize;  /* size of buffer */
70    size_t      eof;        /* furthest point we've written to */
71    size_t      offset;     /* current write offset */
72    char        saved;      /* required by NUL handling */
73} MemStream;
74
75#define kInitialSize    1024
76
77/*
78 * Ensure that we have enough storage to write "size" bytes at the
79 * current offset.  We also have to take into account the extra '\0'
80 * that we maintain just past EOF.
81 *
82 * Returns 0 on success.
83 */
84static int ensureCapacity(MemStream* stream, int writeSize)
85{
86    DBUG(("+++ ensureCap off=%d size=%d\n", stream->offset, writeSize));
87
88    size_t neededSize = stream->offset + writeSize + 1;
89    if (neededSize <= stream->allocSize)
90        return 0;
91
92    size_t newSize;
93
94    if (stream->allocSize == 0) {
95        newSize = kInitialSize;
96    } else {
97        newSize = stream->allocSize;
98        newSize += newSize / 2;             /* expand by 3/2 */
99    }
100
101    if (newSize < neededSize)
102        newSize = neededSize;
103    DBUG(("+++ realloc %p->%p to size=%d\n",
104        stream->bufp, *stream->bufp, newSize));
105    char* newBuf = (char*) realloc(*stream->bufp, newSize);
106    if (newBuf == NULL)
107        return -1;
108
109    *stream->bufp = newBuf;
110    stream->allocSize = newSize;
111    return 0;
112}
113
114/*
115 * Write data to a memstream, expanding the buffer if necessary.
116 *
117 * If we previously seeked beyond EOF, zero-fill the gap.
118 *
119 * Returns the number of bytes written.
120 */
121static int write_memstream(void* cookie, const char* buf, int size)
122{
123    MemStream* stream = (MemStream*) cookie;
124
125    if (ensureCapacity(stream, size) < 0)
126        return -1;
127
128    /* seeked past EOF earlier? */
129    if (stream->eof < stream->offset) {
130        DBUG(("+++ zero-fill gap from %d to %d\n",
131            stream->eof, stream->offset-1));
132        memset(*stream->bufp + stream->eof, '\0',
133            stream->offset - stream->eof);
134    }
135
136    /* copy data, advance write pointer */
137    memcpy(*stream->bufp + stream->offset, buf, size);
138    stream->offset += size;
139
140    if (stream->offset > stream->eof) {
141        /* EOF has advanced, update it and append null byte */
142        DBUG(("+++ EOF advanced to %d, appending nul\n", stream->offset));
143        assert(stream->offset < stream->allocSize);
144        stream->eof = stream->offset;
145    } else {
146        /* within previously-written area; save char we're about to stomp */
147        DBUG(("+++ within written area, saving '%c' at %d\n",
148            *(*stream->bufp + stream->offset), stream->offset));
149        stream->saved = *(*stream->bufp + stream->offset);
150    }
151    *(*stream->bufp + stream->offset) = '\0';
152    *stream->sizep = stream->offset;
153
154    return size;
155}
156
157/*
158 * Seek within a memstream.
159 *
160 * Returns the new offset, or -1 on failure.
161 */
162static fpos_t seek_memstream(void* cookie, fpos_t offset, int whence)
163{
164    MemStream* stream = (MemStream*) cookie;
165    off_t newPosn = (off_t) offset;
166
167    if (whence == SEEK_CUR) {
168        newPosn += stream->offset;
169    } else if (whence == SEEK_END) {
170        newPosn += stream->eof;
171    }
172
173    if (newPosn < 0 || ((fpos_t)((size_t) newPosn)) != newPosn) {
174        /* bad offset - negative or huge */
175        DBUG(("+++ bogus seek offset %ld\n", (long) newPosn));
176        errno = EINVAL;
177        return (fpos_t) -1;
178    }
179
180    if (stream->offset < stream->eof) {
181        /*
182         * We were pointing to an area we'd already written to, which means
183         * we stomped on a character and must now restore it.
184         */
185        DBUG(("+++ restoring char '%c' at %d\n",
186            stream->saved, stream->offset));
187        *(*stream->bufp + stream->offset) = stream->saved;
188    }
189
190    stream->offset = (size_t) newPosn;
191
192    if (stream->offset < stream->eof) {
193        /*
194         * We're seeked backward into the stream.  Preserve the character
195         * at EOF and stomp it with a NUL.
196         */
197        stream->saved = *(*stream->bufp + stream->offset);
198        *(*stream->bufp + stream->offset) = '\0';
199        *stream->sizep = stream->offset;
200    } else {
201        /*
202         * We're positioned at, or possibly beyond, the EOF.  We want to
203         * publish the current EOF, not the current position.
204         */
205        *stream->sizep = stream->eof;
206    }
207
208    return newPosn;
209}
210
211/*
212 * Close the memstream.  We free everything but the data buffer.
213 */
214static int close_memstream(void* cookie)
215{
216    free(cookie);
217    return 0;
218}
219
220/*
221 * Prepare a memstream.
222 */
223FILE* open_memstream(char** bufp, size_t* sizep)
224{
225    FILE* fp;
226    MemStream* stream;
227
228    if (bufp == NULL || sizep == NULL) {
229        errno = EINVAL;
230        return NULL;
231    }
232
233    stream = (MemStream*) calloc(1, sizeof(MemStream));
234    if (stream == NULL)
235        return NULL;
236
237    fp = funopen(stream,
238        NULL, write_memstream, seek_memstream, close_memstream);
239    if (fp == NULL) {
240        free(stream);
241        return NULL;
242    }
243
244    *sizep = 0;
245    *bufp = NULL;
246    stream->bufp = bufp;
247    stream->sizep = sizep;
248
249    return fp;
250}
251
252#else /*not HAVE_FUNOPEN*/
253FILE* open_memstream(char** bufp, size_t* sizep)
254{
255    abort();
256}
257#endif /*HAVE_FUNOPEN*/
258
259
260
261#if 0
262#define _GNU_SOURCE
263#include <stdio.h>
264#include <stdlib.h>
265#include <string.h>
266
267/*
268 * Simple regression test.
269 *
270 * To test on desktop Linux with valgrind, it's possible to make a simple
271 * change to open_memstream() to use fopencookie instead:
272 *
273 *  cookie_io_functions_t iofuncs =
274 *      { NULL, write_memstream, seek_memstream, close_memstream };
275 *  fp = fopencookie(stream, "w", iofuncs);
276 *
277 * (Some tweaks to seek_memstream are also required, as that takes a
278 * pointer to an offset rather than an offset, and returns 0 or -1.)
279 */
280int testMemStream(void)
281{
282    FILE *stream;
283    char *buf;
284    size_t len;
285    off_t eob;
286
287    printf("Test1\n");
288
289    /* std example */
290    stream = open_memstream(&buf, &len);
291    fprintf(stream, "hello my world");
292    fflush(stream);
293    printf("buf=%s, len=%zu\n", buf, len);
294    eob = ftello(stream);
295    fseeko(stream, 0, SEEK_SET);
296    fprintf(stream, "good-bye");
297    fseeko(stream, eob, SEEK_SET);
298    fclose(stream);
299    printf("buf=%s, len=%zu\n", buf, len);
300    free(buf);
301
302    printf("Test2\n");
303
304    /* std example without final seek-to-end */
305    stream = open_memstream(&buf, &len);
306    fprintf(stream, "hello my world");
307    fflush(stream);
308    printf("buf=%s, len=%zu\n", buf, len);
309    eob = ftello(stream);
310    fseeko(stream, 0, SEEK_SET);
311    fprintf(stream, "good-bye");
312    //fseeko(stream, eob, SEEK_SET);
313    fclose(stream);
314    printf("buf=%s, len=%zu\n", buf, len);
315    free(buf);
316
317    printf("Test3\n");
318
319    /* fancy example; should expand buffer with writes */
320    static const int kCmpLen = 1024 + 128;
321    char* cmp = malloc(kCmpLen);
322    memset(cmp, 0, 1024);
323    memset(cmp+1024, 0xff, kCmpLen-1024);
324    sprintf(cmp, "This-is-a-tes1234");
325    sprintf(cmp + 1022, "abcdef");
326
327    stream = open_memstream (&buf, &len);
328    setvbuf(stream, NULL, _IONBF, 0);   /* note: crashes in glibc with this */
329    fprintf(stream, "This-is-a-test");
330    fseek(stream, -1, SEEK_CUR);    /* broken in glibc; can use {13,SEEK_SET} */
331    fprintf(stream, "1234");
332    fseek(stream, 1022, SEEK_SET);
333    fputc('a', stream);
334    fputc('b', stream);
335    fputc('c', stream);
336    fputc('d', stream);
337    fputc('e', stream);
338    fputc('f', stream);
339    fflush(stream);
340
341    if (memcmp(buf, cmp, len+1) != 0) {
342        printf("mismatch\n");
343    } else {
344        printf("match\n");
345    }
346
347    printf("Test4\n");
348    stream = open_memstream (&buf, &len);
349    fseek(stream, 5000, SEEK_SET);
350    fseek(stream, 4096, SEEK_SET);
351    fseek(stream, -1, SEEK_SET);        /* should have no effect */
352    fputc('x', stream);
353    if (ftell(stream) == 4097)
354        printf("good\n");
355    else
356        printf("BAD: offset is %ld\n", ftell(stream));
357
358    printf("DONE\n");
359
360    return 0;
361}
362
363/* expected output:
364Test1
365buf=hello my world, len=14
366buf=good-bye world, len=14
367Test2
368buf=hello my world, len=14
369buf=good-bye, len=8
370Test3
371match
372Test4
373good
374DONE
375*/
376
377#endif
378
379#endif /*!HAVE_OPEN_MEMSTREAM*/
380