1 .\" Copyright (c) 1990, 1991 The Regents of the University of California.
2 .\" All rights reserved.
4 .\" This code is derived from software contributed to Berkeley by
5 .\" Chris Torek and the American National Standards Committee X3,
6 .\" on Information Processing Systems.
8 .\" %%%LICENSE_START(BSD_4_CLAUSE_UCB)
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20 .\" California, Berkeley and its contributors.
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38 .\" @(#)fopen.3 6.8 (Berkeley) 6/29/91
40 .\" Converted for Linux, Mon Nov 29 15:22:01 1993, faith@cs.unc.edu
41 .\" Modified, aeb, 960421, 970806
42 .\" Modified, joey, aeb, 2002-01-03
44 .TH FOPEN 3 2017-09-15 "GNU" "Linux Programmer's Manual"
46 fopen, fdopen, freopen \- stream open functions
51 .BI "FILE *fopen(const char *" pathname ", const char *" mode );
53 .BI "FILE *fdopen(int " fd ", const char *" mode );
55 .BI "FILE *freopen(const char *" pathname ", const char *" mode ", FILE *" stream );
59 Feature Test Macro Requirements for glibc (see
60 .BR feature_test_macros (7)):
68 function opens the file whose name is the string pointed to by
70 and associates a stream with it.
74 points to a string beginning with one of the following sequences
75 (possibly followed by additional characters, as described below):
78 Open text file for reading.
79 The stream is positioned at the beginning of the file.
82 Open for reading and writing.
83 The stream is positioned at the beginning of the file.
86 Truncate file to zero length or create text file for writing.
87 The stream is positioned at the beginning of the file.
90 Open for reading and writing.
91 The file is created if it does not exist, otherwise it is truncated.
92 The stream is positioned at the beginning of
96 Open for appending (writing at end of file).
97 The file is created if it does not exist.
98 The stream is positioned at the end of the file.
101 Open for reading and appending (writing at end of file).
102 The file is created if it does not exist.
103 Output is always appended to the end of the file.
104 POSIX is silent on what the initial read position is when using this mode.
105 For glibc, the initial file position for reading is at
106 the beginning of the file, but for Android/BSD/MacOS, the
107 initial file position for reading is at the end of the file.
111 string can also include the letter \(aqb\(aq either as a last character or as
112 a character between the characters in any of the two-character strings
114 This is strictly for compatibility with C89
115 and has no effect; the \(aqb\(aq is ignored on all POSIX
116 conforming systems, including Linux.
117 (Other systems may treat text files and binary files differently,
118 and adding the \(aqb\(aq may be a good idea if you do I/O to a binary
119 file and expect that your program may be ported to non-UNIX
122 See NOTES below for details of glibc extensions for
125 Any created file will have the mode
126 .BR S_IRUSR " | " S_IWUSR " | " S_IRGRP " | " S_IWGRP " | " S_IROTH " | " S_IWOTH
127 (0666), as modified by the process's umask value (see
130 Reads and writes may be intermixed on read/write streams in any order.
131 Note that ANSI C requires that a file positioning function intervene
132 between output and input, unless an input operation encounters end-of-file.
133 (If this condition is not met, then a read is allowed to return the
134 result of writes other than the most recent.)
135 Therefore it is good practice (and indeed sometimes necessary
136 under Linux) to put an
140 operation between write and read operations on such a stream.
141 This operation may be an apparent no-op
142 (as in \fIfseek(..., 0L, SEEK_CUR)\fP
143 called for its synchronizing side effect).
145 Opening a file in append mode (\fBa\fP as the first character of
147 causes all subsequent write operations to this stream to occur
148 at end-of-file, as if preceded the call:
152 fseek(stream, 0, SEEK_END);
156 The file descriptor associated with the stream is opened as if by a call to
158 with the following flags:
164 fopen() mode open() flags
166 \fIw\fP O_WRONLY | O_CREAT | O_TRUNC
167 \fIa\fP O_WRONLY | O_CREAT | O_APPEND
169 \fIw+\fP O_RDWR | O_CREAT | O_TRUNC
170 \fIa+\fP O_RDWR | O_CREAT | O_APPEND
177 function associates a stream with the existing file descriptor,
181 of the stream (one of the values "r", "r+", "w", "w+", "a", "a+")
182 must be compatible with the mode of the file descriptor.
183 The file position indicator of the new stream is set to that
186 and the error and end-of-file indicators are cleared.
187 Modes "w" or "w+" do not cause truncation of the file.
188 The file descriptor is not dup'ed, and will be closed when
189 the stream created by
192 The result of applying
194 to a shared memory object is undefined.
199 function opens the file whose name is the string pointed to by
201 and associates the stream pointed to by
204 The original stream (if it exists) is closed.
207 argument is used just as in the
213 argument is a null pointer,
215 changes the mode of the stream to that specified in
219 reopens the pathname that is associated with the stream.
220 The specification for this behavior was added in the C99 standard, which says:
224 the file descriptor associated with the stream need not be closed
228 It is implementation-defined which changes of mode are permitted (if any),
229 and under what circumstances.
232 The primary use of the
234 function is to change the file associated with a standard text stream
235 .RI ( stderr ", " stdin ", or " stdout ).
237 Upon successful completion
245 Otherwise, NULL is returned and
247 is set to indicate the error.
265 functions may also fail and set
267 for any of the errors specified for the routine
272 function may also fail and set
274 for any of the errors specified for the routine
279 function may also fail and set
281 for any of the errors specified for the routine
286 function may also fail and set
288 for any of the errors specified for the routines
294 For an explanation of the terms used in this section, see
300 Interface Attribute Value
305 T} Thread safety MT-Safe
310 POSIX.1-2001, POSIX.1-2008, C89, C99.
313 POSIX.1-2001, POSIX.1-2008.
316 The GNU C library allows the following extensions for the string specified in
319 .BR c " (since glibc 2.3.3)"
320 Do not make the open operation,
321 or subsequent read and write operations,
322 thread cancellation points.
323 This flag is ignored for
326 .BR e " (since glibc 2.7)"
327 Open the file with the
332 for more information.
333 This flag is ignored for
336 .BR m " (since glibc 2.3)"
337 Attempt to access the file using
339 rather than I/O system calls
346 is attempted only for a file opened for reading.
350 .\" FIXME . C11 specifies this flag
351 Open the file exclusively
356 If the file already exists,
362 This flag is ignored for
365 In addition to the above characters,
369 support the following syntax
377 is taken as the name of a coded character set and
378 the stream is marked as wide-oriented.
379 Thereafter, internal conversion functions convert I/O
380 to and from the character set
384 syntax is not specified,
385 then the wide-orientation of the stream is
386 determined by the first file operation.
387 If that operation is a wide-character operation,
388 the stream is marked wide-oriented,
389 and functions to convert to the coded character set are loaded.
391 When parsing for individual flag characters in
393 (i.e., the characters preceding the "ccs" specification),
394 the glibc implementation of
395 .\" FIXME . http://sourceware.org/bugzilla/show_bug.cgi?id=12685
399 limits the number of characters examined in
401 to 7 (or, in glibc versions before 2.14, to 6,
402 which was not enough to include possible specifications such as "rb+cmxe").
403 The current implementation of
405 parses at most 5 characters in
413 .BR open_memstream (3)