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
13 open_memstream, open_wmemstream — open a dynamic memory buffer stream
14
16 #include <stdio.h>
17 FILE *open_memstream(char **bufp, size_t *sizep);
19 #include <wchar.h>
21 FILE *open_wmemstream(wchar_t **bufp, size_t *sizep);
23
25 The open_memstream() and open_wmemstream() functions shall create an
26 I/O stream associated with a dynamically allocated memory buffer. The
27 stream shall be opened for writing and shall be seekable.
28 The stream associated with a call to open_memstream() shall be byte-
30 oriented.
31 The stream associated with a call to open_wmemstream() shall be wide-
33 oriented.
34 The stream shall maintain a current position in the allocated buffer
36 and a current buffer length. The position shall be initially set to
37 zero (the start of the buffer). Each write to the stream shall start at
38 the current position and move this position by the number of success‐
39 fully written bytes for open_memstream() or the number of successfully
40 written wide characters for open_wmemstream(). The length shall be
41 initially set to zero. If a write moves the position to a value larger
42 than the current length, the current length shall be set to this posi‐
43 tion. In this case a null character for open_memstream() or a null wide
44 character for open_wmemstream() shall be appended to the current buf‐
45 fer. For both functions the terminating null is not included in the
46 calculation of the buffer length.
47 After a successful fflush() or fclose(), the pointer referenced by bufp
49 shall contain the address of the buffer, and the variable pointed to by
50 sizep shall contain the smaller of the current buffer length and the
51 number of bytes for open_memstream(), or the number of wide characters
52 for open_wmemstream(), between the beginning of the buffer and the cur‐
53 rent file position indicator.
54 After a successful fflush() the pointer referenced by bufp and the
56 variable referenced by sizep remain valid only until the next write
57 operation on the stream or a call to fclose().
58
60 Upon successful completion, these functions shall return a pointer to
61 the object controlling the stream. Otherwise, a null pointer shall be
62 returned, and errno shall be set to indicate the error.
63
65 These functions may fail if:
66 EINVAL bufp or sizep are NULL.
68 EMFILE {FOPEN_MAX} streams are currently open in the calling process.
70 ENOMEM Memory for the stream or the buffer could not be allocated.
72 The following sections are informative.
74
76 #include <stdio.h>
77 #include <stdlib.h>
78 int
80 main (void)
81 {
82 FILE *stream;
83 char *buf;
84 size_t len;
85 off_t eob;
86 stream = open_memstream (&buf, &len);
88 if (stream == NULL)
89 /* handle error */ ;
90 fprintf (stream, "hello my world");
91 fflush (stream);
92 printf ("buf=%s, len=%zu\n", buf, len);
93 eob = ftello(stream);
94 fseeko (stream, 0, SEEK_SET);
95 fprintf (stream, "good-bye");
96 fseeko (stream, eob, SEEK_SET);
97 fclose (stream);
98 printf ("buf=%s, len=%zu\n", buf, len);
99 free (buf);
100 return 0;
101 }
102 This program produces the following output:
104 buf=hello my world, len=14
106 buf=good-bye world, len=14
107
109 The buffer created by these functions should be freed by the applica‐
110 tion after closing the stream, by means of a call to free().
111
113 These functions are similar to fmemopen() except that the memory is
114 always allocated dynamically by the function, and the stream is opened
115 only for output.
116
118 None.
119
121 fclose(), fdopen(), fflush(), fmemopen(), fopen(), free(), freopen()
122 The Base Definitions volume of POSIX.1‐2008, <stdio.h>, <wchar.h>
124
126 Portions of this text are reprinted and reproduced in electronic form
127 from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
128 -- Portable Operating System Interface (POSIX), The Open Group Base
129 Specifications Issue 7, Copyright (C) 2013 by the Institute of Electri‐
130 cal and Electronics Engineers, Inc and The Open Group. (This is
131 POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) In the
132 event of any discrepancy between this version and the original IEEE and
133 The Open Group Standard, the original IEEE and The Open Group Standard
134 is the referee document. The original Standard can be obtained online
135 at http://www.unix.org/online.html .
136 Any typographical or formatting errors that appear in this page are
138 most likely to have been introduced during the conversion of the source
139 files to man page format. To report such errors, see https://www.ker‐
140 nel.org/doc/man-pages/reporting_bugs.html .
141IEEE/The Open Group 2013 OPEN_MEMSTREAM(3P)