1FMEMOPEN(3) Linux Programmer's Manual FMEMOPEN(3)
2
6 fmemopen - open memory as stream
7
9 #include <stdio.h>
10 FILE *fmemopen(void *buf, size_t size, const char *mode);
12 Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
14 fmemopen():
16 Since glibc 2.10:
17 _POSIX_C_SOURCE >= 200809L
18 Before glibc 2.10:
19 _GNU_SOURCE
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.
25 The mode argument specifies the semantics of I/O on the stream, and is
27 one of the following:
28 r The stream is opened for reading.
30 w The stream is opened for writing.
32 a Append; open the stream for writing, with the initial buffer po‐
34 sition set to the first null byte.
35 r+ Open the stream for reading and writing.
37 w+ Open the stream for reading and writing. The buffer contents
39 are truncated (i.e., '\0' is placed in the first byte of the
40 buffer).
41 a+ Append; open the stream for reading and writing, with the ini‐
43 tial buffer position set to the first null byte.
44 The stream maintains the notion of a current position, the location
46 where the next I/O operation will be performed. The current position
47 is implicitly updated by I/O operations. It can be explicitly updated
48 using fseek(3), and determined using ftell(3). In all modes other than
49 append, the initial position is set to the start of the buffer. In ap‐
50 pend mode, if no null byte is found within the buffer, then the initial
51 position is size+1.
52 If buf is specified as NULL, then fmemopen() allocates a buffer of size
54 bytes. This is useful for an application that wants to write data to a
55 temporary buffer and then read it back again. The initial position is
56 set to the start of the buffer. The buffer is automatically freed when
57 the stream is closed. Note that the caller has no way to obtain a
58 pointer to the temporary buffer allocated by this call (but see
59 open_memstream(3)).
60 If buf is not NULL, then it should point to a buffer of at least len
62 bytes allocated by the caller.
63 When a stream that has been opened for writing is flushed (fflush(3))
65 or closed (fclose(3)), a null byte is written at the end of the buffer
66 if there is space. The caller should ensure that an extra byte is
67 available in the buffer (and that size counts that byte) to allow for
68 this.
69 In a stream opened for reading, null bytes ('\0') in the buffer do not
71 cause read operations to return an end-of-file indication. A read from
72 the buffer will indicate end-of-file only when the current buffer posi‐
73 tion advances size bytes past the start of the buffer.
74 Write operations take place either at the current position (for modes
76 other than append), or at the current size of the stream (for append
77 modes).
78 Attempts to write more than size bytes to the buffer result in an er‐
80 ror. By default, such errors will be visible (by the absence of data)
81 only when the stdio buffer is flushed. Disabling buffering with the
82 following call may be useful to detect errors at the time of an output
83 operation:
84 setbuf(stream, NULL);
86
88 Upon successful completion, fmemopen() returns a FILE pointer. Other‐
89 wise, NULL is returned and errno is set to indicate the error.
90
92 fmemopen() was already available in glibc 1.0.x.
93
95 For an explanation of the terms used in this section, see at‐
96 tributes(7).
97 ┌────────────────────────────────────────────┬───────────────┬─────────┐
99 │Interface │ Attribute │ Value │
100 ├────────────────────────────────────────────┼───────────────┼─────────┤
101 │fmemopen(), │ Thread safety │ MT-Safe │
102 └────────────────────────────────────────────┴───────────────┴─────────┘
103
105 POSIX.1-2008. This function is not specified in POSIX.1-2001, and is
106 not widely available on other systems.
107 POSIX.1-2008 specifies that 'b' in mode shall be ignored. However,
109 Technical Corrigendum 1 adjusts the standard to allow implementation-
110 specific treatment for this case, thus permitting the glibc treatment
111 of 'b'.
112
114 There is no file descriptor associated with the file stream returned by
115 this function (i.e., fileno(3) will return an error if called on the
116 returned stream).
117 With version 2.22, binary mode (see below) was removed, many longstand‐
119 ing bugs in the implementation of fmemopen() were fixed, and a new ver‐
120 sioned symbol was created for this interface.
121 Binary mode
123 From version 2.9 to 2.21, the glibc implementation of fmemopen() sup‐
124 ported a "binary" mode, enabled by specifying the letter 'b' as the
125 second character in mode. In this mode, writes don't implicitly add a
126 terminating null byte, and fseek(3) SEEK_END is relative to the end of
127 the buffer (i.e., the value specified by the size argument), rather
128 than the current string length.
129 An API bug afflicted the implementation of binary mode: to specify bi‐
131 nary mode, the 'b' must be the second character in mode. Thus, for ex‐
132 ample, "wb+" has the desired effect, but "w+b" does not. This is in‐
133 consistent with the treatment of mode by fopen(3).
134 Binary mode was removed in glibc 2.22; a 'b' specified in mode has no
136 effect.
137
139 In versions of glibc before 2.22, if size is specified as zero, fmemo‐
140 pen() fails with the error EINVAL. It would be more consistent if this
141 case successfully created a stream that then returned end-of-file on
142 the first attempt at reading; since version 2.22, the glibc implementa‐
143 tion provides that behavior.
144 In versions of glibc before 2.22, specifying append mode ("a" or "a+")
146 for fmemopen() sets the initial buffer position to the first null byte,
147 but (if the current position is reset to a location other than the end
148 of the stream) does not force subsequent writes to append at the end of
149 the stream. This bug is fixed in glibc 2.22.
150 In versions of glibc before 2.22, if the mode argument to fmemopen()
152 specifies append ("a" or "a+"), and the size argument does not cover a
153 null byte in buf, then, according to POSIX.1-2008, the initial buffer
154 position should be set to the next byte after the end of the buffer.
155 However, in this case the glibc fmemopen() sets the buffer position to
156 -1. This bug is fixed in glibc 2.22.
157 In versions of glibc before 2.22, when a call to fseek(3) with a whence
159 value of SEEK_END was performed on a stream created by fmemopen(), the
160 offset was subtracted from the end-of-stream position, instead of being
161 added. This bug is fixed in glibc 2.22.
162 The glibc 2.9 addition of "binary" mode for fmemopen() silently changed
164 the ABI: previously, fmemopen() ignored 'b' in mode.
165
167 The program below uses fmemopen() to open an input buffer, and
168 open_memstream(3) to open a dynamically sized output buffer. The pro‐
169 gram scans its input string (taken from the program's first command-
170 line argument) reading integers, and writes the squares of these inte‐
171 gers to the output buffer. An example of the output produced by this
172 program is the following:
173 $ ./a.out '1 23 43'
175 size=11; ptr=1 529 1849
176 Program source
178 #define _GNU_SOURCE
180 #include <string.h>
181 #include <stdio.h>
182 #include <stdlib.h>
183 #define handle_error(msg) \
185 do { perror(msg); exit(EXIT_FAILURE); } while (0)
186 int
188 main(int argc, char *argv[])
189 {
190 FILE *out, *in;
191 int v, s;
192 size_t size;
193 char *ptr;
194 if (argc != 2) {
196 fprintf(stderr, "Usage: %s '<num>...'\n", argv[0]);
197 exit(EXIT_FAILURE);
198 }
199 in = fmemopen(argv[1], strlen(argv[1]), "r");
201 if (in == NULL)
202 handle_error("fmemopen");
203 out = open_memstream(&ptr, &size);
205 if (out == NULL)
206 handle_error("open_memstream");
207 for (;;) {
209 s = fscanf(in, "%d", &v);
210 if (s <= 0)
211 break;
212 s = fprintf(out, "%d ", v * v);
214 if (s == -1)
215 handle_error("fprintf");
216 }
217 fclose(in);
219 fclose(out);
220 printf("size=%zu; ptr=%s\n", size, ptr);
222 free(ptr);
224 exit(EXIT_SUCCESS);
225 }
226
228 fopen(3), fopencookie(3), open_memstream(3)
229
231 This page is part of release 5.12 of the Linux man-pages project. A
232 description of the project, information about reporting bugs, and the
233 latest version of this page, can be found at
234 https://www.kernel.org/doc/man-pages/.
235GNU 2021-03-22 FMEMOPEN(3)