1FOPEN(3) Linux Programmer's Manual FOPEN(3)
2
6 fopen, fdopen, freopen - stream open functions
7
9 #include <stdio.h>
10 FILE *fopen(const char *pathname, const char *mode);
12 FILE *fdopen(int fd, const char *mode);
14 FILE *freopen(const char *pathname, const char *mode, FILE *stream);
16 Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
18 fdopen(): _POSIX_C_SOURCE
20
22 The fopen() function opens the file whose name is the string pointed to
23 by pathname and associates a stream with it.
24 The argument mode points to a string beginning with one of the follow‐
26 ing sequences (possibly followed by additional characters, as described
27 below):
28 r Open text file for reading. The stream is positioned at the
30 beginning of the file.
31 r+ Open for reading and writing. The stream is positioned at the
33 beginning of the file.
34 w Truncate file to zero length or create text file for writing.
36 The stream is positioned at the beginning of the file.
37 w+ Open for reading and writing. The file is created if it does
39 not exist, otherwise it is truncated. The stream is positioned
40 at the beginning of the file.
41 a Open for appending (writing at end of file). The file is cre‐
43 ated if it does not exist. The stream is positioned at the end
44 of the file.
45 a+ Open for reading and appending (writing at end of file). The
47 file is created if it does not exist. The initial file position
48 for reading is at the beginning of the file, but output is
49 always appended to the end of the file.
50 The mode string can also include the letter 'b' either as a last char‐
52 acter or as a character between the characters in any of the two-char‐
53 acter strings described above. This is strictly for compatibility with
54 C89 and has no effect; the 'b' is ignored on all POSIX conforming sys‐
55 tems, including Linux. (Other systems may treat text files and binary
56 files differently, and adding the 'b' may be a good idea if you do I/O
57 to a binary file and expect that your program may be ported to non-UNIX
58 environments.)
59 See NOTES below for details of glibc extensions for mode.
61 Any created file will have the mode S_IRUSR | S_IWUSR | S_IRGRP |
63 S_IWGRP | S_IROTH | S_IWOTH (0666), as modified by the process's umask
64 value (see umask(2)).
65 Reads and writes may be intermixed on read/write streams in any order.
67 Note that ANSI C requires that a file positioning function intervene
68 between output and input, unless an input operation encounters end-of-
69 file. (If this condition is not met, then a read is allowed to return
70 the result of writes other than the most recent.) Therefore it is good
71 practice (and indeed sometimes necessary under Linux) to put an
72 fseek(3) or fgetpos(3) operation between write and read operations on
73 such a stream. This operation may be an apparent no-op (as in
74 fseek(..., 0L, SEEK_CUR) called for its synchronizing side effect).
75 Opening a file in append mode (a as the first character of mode) causes
77 all subsequent write operations to this stream to occur at end-of-file,
78 as if preceded the call:
79 fseek(stream, 0, SEEK_END);
81 The file descriptor associated with the stream is opened as if by a
83 call to open(2) with the following flags:
84 ┌─────────────┬───────────────────────────────┐
86 │fopen() mode │ open() flags │
87 ├─────────────┼───────────────────────────────┤
88 │ r │ O_RDONLY │
89 ├─────────────┼───────────────────────────────┤
90 │ w │ O_WRONLY | O_CREAT | O_TRUNC │
91 ├─────────────┼───────────────────────────────┤
92 │ a │ O_WRONLY | O_CREAT | O_APPEND │
93 ├─────────────┼───────────────────────────────┤
94 │ r+ │ O_RDWR │
95 ├─────────────┼───────────────────────────────┤
96 │ w+ │ O_RDWR | O_CREAT | O_TRUNC │
97 ├─────────────┼───────────────────────────────┤
98 │ a+ │ O_RDWR | O_CREAT | O_APPEND │
99 └─────────────┴───────────────────────────────┘
100 fdopen()
101 The fdopen() function associates a stream with the existing file
102 descriptor, fd. The mode of the stream (one of the values "r", "r+",
103 "w", "w+", "a", "a+") must be compatible with the mode of the file
104 descriptor. The file position indicator of the new stream is set to
105 that belonging to fd, and the error and end-of-file indicators are
106 cleared. Modes "w" or "w+" do not cause truncation of the file. The
107 file descriptor is not dup'ed, and will be closed when the stream cre‐
108 ated by fdopen() is closed. The result of applying fdopen() to a
109 shared memory object is undefined.
110 freopen()
112 The freopen() function opens the file whose name is the string pointed
113 to by pathname and associates the stream pointed to by stream with it.
114 The original stream (if it exists) is closed. The mode argument is
115 used just as in the fopen() function.
116 If the pathname argument is a null pointer, freopen() changes the mode
118 of the stream to that specified in mode; that is, freopen() reopens the
119 pathname that is associated with the stream. The specification for
120 this behavior was added in the C99 standard, which says:
121 In this case, the file descriptor associated with the stream
123 need not be closed if the call to freopen() succeeds. It is
124 implementation-defined which changes of mode are permitted (if
125 any), and under what circumstances.
126 The primary use of the freopen() function is to change the file associ‐
128 ated with a standard text stream (stderr, stdin, or stdout).
129
131 Upon successful completion fopen(), fdopen() and freopen() return a
132 FILE pointer. Otherwise, NULL is returned and errno is set to indicate
133 the error.
134
136 EINVAL The mode provided to fopen(), fdopen(), or freopen() was
137 invalid.
138 The fopen(), fdopen() and freopen() functions may also fail and set
140 errno for any of the errors specified for the routine malloc(3).
141 The fopen() function may also fail and set errno for any of the errors
143 specified for the routine open(2).
144 The fdopen() function may also fail and set errno for any of the errors
146 specified for the routine fcntl(2).
147 The freopen() function may also fail and set errno for any of the
149 errors specified for the routines open(2), fclose(3), and fflush(3).
150
152 For an explanation of the terms used in this section, see
153 attributes(7).
154 ┌─────────────────────────────┬───────────────┬─────────┐
156 │Interface │ Attribute │ Value │
157 ├─────────────────────────────┼───────────────┼─────────┤
158 │fopen(), fdopen(), freopen() │ Thread safety │ MT-Safe │
159 └─────────────────────────────┴───────────────┴─────────┘
161 fopen(), freopen(): POSIX.1-2001, POSIX.1-2008, C89, C99.
162 fdopen(): POSIX.1-2001, POSIX.1-2008.
164
166 Glibc notes
167 The GNU C library allows the following extensions for the string speci‐
168 fied in mode:
169 c (since glibc 2.3.3)
171 Do not make the open operation, or subsequent read and write
172 operations, thread cancellation points. This flag is ignored
173 for fdopen().
174 e (since glibc 2.7)
176 Open the file with the O_CLOEXEC flag. See open(2) for more
177 information. This flag is ignored for fdopen().
178 m (since glibc 2.3)
180 Attempt to access the file using mmap(2), rather than I/O system
181 calls (read(2), write(2)). Currently, use of mmap(2) is
182 attempted only for a file opened for reading.
183 x Open the file exclusively (like the O_EXCL flag of open(2)). If
185 the file already exists, fopen() fails, and sets errno to EEX‐
186 IST. This flag is ignored for fdopen().
187 In addition to the above characters, fopen() and freopen() support the
189 following syntax in mode:
190 ,ccs=string
192 The given string is taken as the name of a coded character set and the
194 stream is marked as wide-oriented. Thereafter, internal conversion
195 functions convert I/O to and from the character set string. If the
196 ,ccs=string syntax is not specified, then the wide-orientation of the
197 stream is determined by the first file operation. If that operation is
198 a wide-character operation, the stream is marked wide-oriented, and
199 functions to convert to the coded character set are loaded.
200
202 When parsing for individual flag characters in mode (i.e., the charac‐
203 ters preceding the "ccs" specification), the glibc implementation of
204 fopen() and freopen() limits the number of characters examined in mode
205 to 7 (or, in glibc versions before 2.14, to 6, which was not enough to
206 include possible specifications such as "rb+cmxe"). The current imple‐
207 mentation of fdopen() parses at most 5 characters in mode.
208
210 open(2), fclose(3), fileno(3), fmemopen(3), fopencookie(3), open_mem‐
211 stream(3)
212
214 This page is part of release 4.15 of the Linux man-pages project. A
215 description of the project, information about reporting bugs, and the
216 latest version of this page, can be found at
217 https://www.kernel.org/doc/man-pages/.
218GNU 2017-09-15 FOPEN(3)