1FMEMOPEN(3) Linux Programmer's Manual FMEMOPEN(3)
2
6 fmemopen, open_memstream, open_wmemstream - open memory as stream
7
9 #define _GNU_SOURCE
10 #include <stdio.h>
11 FILE *fmemopen(void *buf, size_t size, const char *mode);
13 FILE *open_memstream(char **ptr, size_t *sizeloc);
15 #define _GNU_SOURCE
17 #include <wchar.h>
18 FILE *open_wmemstream(wchar_t **ptr, size_t *sizeloc);
20
22 The fmemopen() function opens a stream that permits the access speci‐
23 fied by mode. The stream allows I/O to be performed on the string or
24 memory buffer pointed to by buf. This buffer must be at least size
25 bytes long.
26 The argument mode is the same as for fopen(3). If mode specifies an
28 append mode, then the initial file position is set to the location of
29 the first null byte ('\0') in the buffer; otherwise the initial file
30 position is set to the start of the buffer. Since glibc 2.9, the let‐
31 ter 'b' may be specified as the second character in mode. This pro‐
32 vides "binary" mode: writes don't implicitly add a terminating null
33 byte, and fseek(3) SEEK_END is relative to the end of the buffer (i.e.,
34 the value specified by the size argument), rather than the current
35 string length.
36 When a stream that has been opened for writing is flushed (fflush(3))
38 or closed (fclose(3)), a null byte is written at the end of the buffer
39 if there is space. The caller should ensure that an extra byte is
40 available in the buffer (and that size counts that byte) to allow for
41 this.
42 Attempts to write more than size bytes to the buffer result in an
44 error. (By default, such errors will only be visible when the stdio
45 buffer is flushed. Disabling buffering with setbuf(fp, NULL) may be
46 useful to detect errors at the time of an output operation. Alterna‐
47 tively, the caller can explicitly set buf as the stdio stream buffer,
48 at the same time informing stdio of the buffer's size, using set‐
49 buffer(fp, buf, size).)
50 In a stream opened for reading, null bytes ('\0') in the buffer do not
52 cause read operations to return an end-of-file indication. A read from
53 the buffer will only indicate end-of-file when the file pointer
54 advances size bytes past the start of the buffer.
55 If buf is specified as NULL, then fmemopen() dynamically allocates a
57 buffer size bytes long. This is useful for an application that wants
58 to write data to a temporary buffer and then read it back again. The
59 buffer is automatically freed when the stream is closed. Note that the
60 caller has no way to obtain a pointer to the temporary buffer allocated
61 by this call (but see open_memstream() below).
62 The open_memstream() function opens a stream for writing to a buffer.
64 The buffer is dynamically allocated (as with malloc(3)), and automati‐
65 cally grows as required. After closing the stream, the caller should
66 free(3) this buffer.
67 When the stream is closed (fclose(3)) or flushed (fflush(3)), the loca‐
69 tions pointed to by ptr and sizeloc are updated to contain, respec‐
70 tively, a pointer to the buffer and the current size of the buffer.
71 These values remain valid only as long as the caller performs no fur‐
72 ther output on the stream. If further output is performed, then the
73 stream must again be flushed before trying to access these variables.
74 A null byte is maintained at the end of the buffer. This byte is not
76 included in the size value stored at sizeloc.
77 The stream's file position can be changed with fseek(3) or fseeko(3).
79 Moving the file position past the end of the data already written fills
80 the intervening space with zeros.
81 The open_wmemstream() is similar to open_memstream(), but operates on
83 wide characters instead of bytes.
84
86 Upon successful completion fmemopen(), open_memstream() and open_wmem‐
87 stream() return a FILE pointer. Otherwise, NULL is returned and errno
88 is set to indicate the error.
89
91 fmemopen() and open_memstream() were already available in glibc 1.0.x.
92 open_wmemstream() is available since glibc 2.4.
93
95 POSIX.1-2008. These functions are not specified in POSIX.1-2001, and
96 are not widely available on other systems.
97
99 There is no file descriptor associated with the file stream returned by
100 these functions (i.e., fileno(3) will return an error if called on the
101 returned stream).
102
104 In glibc before version 2.7, seeking past the end of a stream created
105 by open_memstream() does not enlarge the buffer; instead the fseek()
106 call fails, returning -1.
107
109 The program below uses fmemopen() to open an input buffer, and
110 open_memstream() to open a dynamically sized output buffer. The pro‐
111 gram scans its input string (taken from the program's first command-
112 line argument) reading integers, and writes the squares of these inte‐
113 gers to the output buffer. An example of the output produced by this
114 program is the following:
115 $ ./a.out '1 23 43'
117 size=11; ptr=1 529 1849
118 Program source
120 #define _GNU_SOURCE
122 #include <string.h>
123 #include <stdio.h>
124 #include <stdlib.h>
125 #define handle_error(msg) \
127 do { perror(msg); exit(EXIT_FAILURE); } while (0)
128 int
130 main(int argc, char *argv[])
131 {
132 FILE *out, *in;
133 int v, s;
134 size_t size;
135 char *ptr;
136 if (argc != 2) {
138 fprintf(stderr, "Usage: %s <file>\n", argv[0]);
139 exit(EXIT_FAILURE);
140 }
141 in = fmemopen(argv[1], strlen(argv[1]), "r");
143 if (in == NULL)
144 handle_error("fmemopen");
145 out = open_memstream(&ptr, &size);
147 if (out == NULL)
148 handle_error("open_memstream");
149 for (;;) {
151 s = fscanf(in, "%d", &v);
152 if (s <= 0)
153 break;
154 s = fprintf(out, "%d ", v * v);
156 if (s == -1)
157 handle_error("fprintf");
158 }
159 fclose(in);
160 fclose(out);
161 printf("size=%ld; ptr=%s\n", (long) size, ptr);
162 free(ptr);
163 exit(EXIT_SUCCESS);
164 }
165
167 fopen(3), fopencookie(3), feature_test_macros(7)
168
170 This page is part of release 3.25 of the Linux man-pages project. A
171 description of the project, and information about reporting bugs, can
172 be found at http://www.kernel.org/doc/man-pages/.
173GNU 2010-06-10 FMEMOPEN(3)