1OPEN_MEMSTREAM(3P) POSIX Programmer's Manual OPEN_MEMSTREAM(3P)
2
6 This manual page is part of the POSIX Programmer's Manual. The Linux
7 implementation of this interface may differ (consult the corresponding
8 Linux manual page for details of Linux behavior), or the interface may
9 not be implemented on Linux.
10
12 open_memstream, open_wmemstream — open a dynamic memory buffer stream
13
15 #include <stdio.h>
16 FILE *open_memstream(char **bufp, size_t *sizep);
18 #include <wchar.h>
20 FILE *open_wmemstream(wchar_t **bufp, size_t *sizep);
22
24 The open_memstream() and open_wmemstream() functions shall create an
25 I/O stream associated with a dynamically allocated memory buffer. The
26 stream shall be opened for writing and shall be seekable.
27 The stream associated with a call to open_memstream() shall be byte-
29 oriented.
30 The stream associated with a call to open_wmemstream() shall be wide-
32 oriented.
33 The stream shall maintain a current position in the allocated buffer
35 and a current buffer length. The position shall be initially set to
36 zero (the start of the buffer). Each write to the stream shall start at
37 the current position and move this position by the number of success‐
38 fully written bytes for open_memstream() or the number of successfully
39 written wide characters for open_wmemstream(). The length shall be
40 initially set to zero. If a write moves the position to a value larger
41 than the current length, the current length shall be set to this posi‐
42 tion. In this case a null character for open_memstream() or a null wide
43 character for open_wmemstream() shall be appended to the current buf‐
44 fer. For both functions the terminating null is not included in the
45 calculation of the buffer length.
46 After a successful fflush() or fclose(), the pointer referenced by bufp
48 shall contain the address of the buffer, and the variable pointed to by
49 sizep shall contain the smaller of the current buffer length and the
50 number of bytes for open_memstream(), or the number of wide characters
51 for open_wmemstream(), between the beginning of the buffer and the cur‐
52 rent file position indicator.
53 After a successful fflush() the pointer referenced by bufp and the
55 variable referenced by sizep remain valid only until the next write
56 operation on the stream or a call to fclose().
57 After a successful fclose(), the pointer referenced by bufp can be
59 passed to free().
60
62 Upon successful completion, these functions shall return a pointer to
63 the object controlling the stream. Otherwise, a null pointer shall be
64 returned, and errno shall be set to indicate the error.
65
67 These functions shall fail if:
68 EMFILE {STREAM_MAX} streams are currently open in the calling process.
70 These functions may fail if:
72 EINVAL bufp or sizep are NULL.
74 EMFILE {FOPEN_MAX} streams are currently open in the calling process.
76 ENOMEM Memory for the stream or the buffer could not be allocated.
78 The following sections are informative.
80
82 #include <stdio.h>
83 #include <stdlib.h>
84 int
86 main (void)
87 {
88 FILE *stream;
89 char *buf;
90 size_t len;
91 off_t eob;
92 stream = open_memstream (&buf, &len);
94 if (stream == NULL)
95 /* handle error */ ;
96 fprintf (stream, "hello my world");
97 fflush (stream);
98 printf ("buf=%s, len=%zu\n", buf, len);
99 eob = ftello(stream);
100 fseeko (stream, 0, SEEK_SET);
101 fprintf (stream, "good-bye");
102 fseeko (stream, eob, SEEK_SET);
103 fclose (stream);
104 printf ("buf=%s, len=%zu\n", buf, len);
105 free (buf);
106 return 0;
107 }
108 This program produces the following output:
110 buf=hello my world, len=14
113 buf=good-bye world, len=14
114
116 The buffer created by these functions should be freed by the applica‐
117 tion after closing the stream, by means of a call to free().
118
120 These functions are similar to fmemopen() except that the memory is
121 always allocated dynamically by the function, and the stream is opened
122 only for output.
123
125 None.
126
128 fclose(), fdopen(), fflush(), fmemopen(), fopen(), free(), freopen()
129 The Base Definitions volume of POSIX.1‐2017, <stdio.h>, <wchar.h>
131
133 Portions of this text are reprinted and reproduced in electronic form
134 from IEEE Std 1003.1-2017, Standard for Information Technology -- Por‐
135 table Operating System Interface (POSIX), The Open Group Base Specifi‐
136 cations Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of
137 Electrical and Electronics Engineers, Inc and The Open Group. In the
138 event of any discrepancy between this version and the original IEEE and
139 The Open Group Standard, the original IEEE and The Open Group Standard
140 is the referee document. The original Standard can be obtained online
141 at http://www.opengroup.org/unix/online.html .
142 Any typographical or formatting errors that appear in this page are
144 most likely to have been introduced during the conversion of the source
145 files to man page format. To report such errors, see https://www.ker‐
146 nel.org/doc/man-pages/reporting_bugs.html .
147IEEE/The Open Group 2017 OPEN_MEMSTREAM(3P)