1FMEMOPEN(3) Linux Programmer's Manual FMEMOPEN(3)
2
6 fmemopen, open_memstream, open_wmemstream - open memory as stream
7
9 #include <stdio.h>
10 FILE *fmemopen(void *buf, size_t size, const char *mode);
12 FILE *open_memstream(char **ptr, size_t *sizeloc);
14 #include <wchar.h>
16 FILE *open_wmemstream(wchar_t **ptr, size_t *sizeloc);
18 Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
20 fmemopen(), open_memstream(), open_wmemstream():
22 Since glibc 2.10:
23 _XOPEN_SOURCE >= 700 || _POSIX_C_SOURCE >= 200809L
24 Before glibc 2.10:
25 _GNU_SOURCE
26
28 The fmemopen() function opens a stream that permits the access speci‐
29 fied by mode. The stream allows I/O to be performed on the string or
30 memory buffer pointed to by buf. This buffer must be at least size
31 bytes long.
32 The argument mode is the same as for fopen(3). If mode specifies an
34 append mode, then the initial file position is set to the location of
35 the first null byte ('\0') in the buffer; otherwise the initial file
36 position is set to the start of the buffer. Since glibc 2.9, the let‐
37 ter 'b' may be specified as the second character in mode. This pro‐
38 vides "binary" mode: writes don't implicitly add a terminating null
39 byte, and fseek(3) SEEK_END is relative to the end of the buffer (i.e.,
40 the value specified by the size argument), rather than the current
41 string length.
42 When a stream that has been opened for writing is flushed (fflush(3))
44 or closed (fclose(3)), a null byte is written at the end of the buffer
45 if there is space. The caller should ensure that an extra byte is
46 available in the buffer (and that size counts that byte) to allow for
47 this.
48 Attempts to write more than size bytes to the buffer result in an
50 error. (By default, such errors will be visible only when the stdio
51 buffer is flushed. Disabling buffering with setbuf(fp, NULL) may be
52 useful to detect errors at the time of an output operation. Alterna‐
53 tively, the caller can explicitly set buf as the stdio stream buffer,
54 at the same time informing stdio of the buffer's size, using set‐
55 buffer(fp, buf, size).)
56 In a stream opened for reading, null bytes ('\0') in the buffer do not
58 cause read operations to return an end-of-file indication. A read from
59 the buffer will only indicate end-of-file when the file pointer
60 advances size bytes past the start of the buffer.
61 If buf is specified as NULL, then fmemopen() dynamically allocates a
63 buffer size bytes long. This is useful for an application that wants
64 to write data to a temporary buffer and then read it back again. The
65 buffer is automatically freed when the stream is closed. Note that the
66 caller has no way to obtain a pointer to the temporary buffer allocated
67 by this call (but see open_memstream() below).
68 The open_memstream() function opens a stream for writing to a buffer.
70 The buffer is dynamically allocated (as with malloc(3)), and automati‐
71 cally grows as required. After closing the stream, the caller should
72 free(3) this buffer.
73 When the stream is closed (fclose(3)) or flushed (fflush(3)), the loca‐
75 tions pointed to by ptr and sizeloc are updated to contain, respec‐
76 tively, a pointer to the buffer and the current size of the buffer.
77 These values remain valid only as long as the caller performs no fur‐
78 ther output on the stream. If further output is performed, then the
79 stream must again be flushed before trying to access these variables.
80 A null byte is maintained at the end of the buffer. This byte is not
82 included in the size value stored at sizeloc.
83 The stream's file position can be changed with fseek(3) or fseeko(3).
85 Moving the file position past the end of the data already written fills
86 the intervening space with zeros.
87 The open_wmemstream() is similar to open_memstream(), but operates on
89 wide characters instead of bytes.
90
92 Upon successful completion fmemopen(), open_memstream() and open_wmem‐
93 stream() return a FILE pointer. Otherwise, NULL is returned and errno
94 is set to indicate the error.
95
97 fmemopen() and open_memstream() were already available in glibc 1.0.x.
98 open_wmemstream() is available since glibc 2.4.
99
101 POSIX.1-2008. These functions are not specified in POSIX.1-2001, and
102 are not widely available on other systems.
103 POSIX.1-2008 specifies that 'b' in mode shall be ignored. However,
105 Technical Corrigendum 1 adjusts the standard to allow implementation-
106 specific treatment for this case, thus permitting the glibc treatment
107 of 'b'.
108
110 There is no file descriptor associated with the file stream returned by
111 these functions (i.e., fileno(3) will return an error if called on the
112 returned stream).
113
115 In glibc before version 2.7, seeking past the end of a stream created
116 by open_memstream() does not enlarge the buffer; instead the fseek(3)
117 call fails, returning -1.
118 If size is specified as zero, fmemopen() fails with the error EINVAL.
120 It would be more consistent if this case successfully created a stream
121 that then returned end of file on the first attempt at reading. Fur‐
122 thermore, POSIX.1-2008 does not specify a failure for this case.
123 Specifying append mode ("a" or "a+") for fmemopen() sets the initial
125 file position to the first null byte, but (if the file offset is reset
126 to a location other than the end of the stream) does not force subse‐
127 quent writes to append at the end of the stream.
128 If the mode argument to fmemopen() specifies append ("a" or "a+"), and
130 the size argument does not cover a null byte in buf then, according to
131 POSIX.1-2008, the initial file position should be set to the next byte
132 after the end of the buffer. However, in this case the glibc fmemo‐
133 pen() sets the file position to -1.
134 To specify binary mode for fmemopen() the 'b' must be the second char‐
136 acter in mode. Thus, for example, "wb+" has the desired effect, but
137 "w+b" does not. This is inconsistent with the treatment of mode by
138 fopen(3).
139 The glibc 2.9 addition of "binary" mode for fmemopen() silently changed
141 the ABI: previously, fmemopen() ignored 'b' in mode.
142
144 The program below uses fmemopen() to open an input buffer, and
145 open_memstream() to open a dynamically sized output buffer. The pro‐
146 gram scans its input string (taken from the program's first command-
147 line argument) reading integers, and writes the squares of these inte‐
148 gers to the output buffer. An example of the output produced by this
149 program is the following:
150 $ ./a.out '1 23 43'
152 size=11; ptr=1 529 1849
153 Program source
155 #define _GNU_SOURCE
157 #include <string.h>
158 #include <stdio.h>
159 #include <stdlib.h>
160 #define handle_error(msg) \
162 do { perror(msg); exit(EXIT_FAILURE); } while (0)
163 int
165 main(int argc, char *argv[])
166 {
167 FILE *out, *in;
168 int v, s;
169 size_t size;
170 char *ptr;
171 if (argc != 2) {
173 fprintf(stderr, "Usage: %s <file>\n", argv[0]);
174 exit(EXIT_FAILURE);
175 }
176 in = fmemopen(argv[1], strlen(argv[1]), "r");
178 if (in == NULL)
179 handle_error("fmemopen");
180 out = open_memstream(&ptr, &size);
182 if (out == NULL)
183 handle_error("open_memstream");
184 for (;;) {
186 s = fscanf(in, "%d", &v);
187 if (s <= 0)
188 break;
189 s = fprintf(out, "%d ", v * v);
191 if (s == -1)
192 handle_error("fprintf");
193 }
194 fclose(in);
195 fclose(out);
196 printf("size=%ld; ptr=%s\n", (long) size, ptr);
197 free(ptr);
198 exit(EXIT_SUCCESS);
199 }
200
202 fopen(3), fopencookie(3)
203
205 This page is part of release 3.53 of the Linux man-pages project. A
206 description of the project, and information about reporting bugs, can
207 be found at http://www.kernel.org/doc/man-pages/.
208GNU 2012-04-28 FMEMOPEN(3)