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 *path, const char *mode);
12 FILE *fdopen(int fd, const char *mode);
14 FILE *freopen(const char *path, const char *mode, FILE *stream);
16 Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
18 fdopen(): _POSIX_C_SOURCE >= 1 || _XOPEN_SOURCE || _POSIX_SOURCE
20
22 The fopen() function opens the file whose name is the string pointed to
23 by path and associates a stream with it.
24 The argument mode points to a string beginning with one of the follow‐
26 ing sequences (Additional characters may follow these sequences.):
27 r Open text file for reading. The stream is positioned at the
29 beginning of the file.
30 r+ Open for reading and writing. The stream is positioned at the
32 beginning of the file.
33 w Truncate file to zero length or create text file for writing.
35 The stream is positioned at the beginning of the file.
36 w+ Open for reading and writing. The file is created if it does
38 not exist, otherwise it is truncated. The stream is positioned
39 at the beginning of the file.
40 a Open for appending (writing at end of file). The file is cre‐
42 ated if it does not exist. The stream is positioned at the end
43 of the file.
44 a+ Open for reading and appending (writing at end of file). The
46 file is created if it does not exist. The initial file position
47 for reading is at the beginning of the file, but output is
48 always appended to the end of the file.
49 The mode string can also include the letter 'b' either as a last char‐
51 acter or as a character between the characters in any of the two-char‐
52 acter strings described above. This is strictly for compatibility with
53 C89 and has no effect; the 'b' is ignored on all POSIX conforming sys‐
54 tems, including Linux. (Other systems may treat text files and binary
55 files differently, and adding the 'b' may be a good idea if you do I/O
56 to a binary file and expect that your program may be ported to non-Unix
57 environments.)
58 See NOTES below for details of glibc extensions for mode.
60 Any created files will have mode S_IRUSR | S_IWUSR | S_IRGRP | S_IWGRP
62 | S_IROTH | S_IWOTH (0666), as modified by the process's umask value
63 (see umask(2)).
64 Reads and writes may be intermixed on read/write streams in any order.
66 Note that ANSI C requires that a file positioning function intervene
67 between output and input, unless an input operation encounters end-of-
68 file. (If this condition is not met, then a read is allowed to return
69 the result of writes other than the most recent.) Therefore it is good
70 practice (and indeed sometimes necessary under Linux) to put an
71 fseek(3) or fgetpos(3) operation between write and read operations on
72 such a stream. This operation may be an apparent no-op (as in
73 fseek(..., 0L, SEEK_CUR) called for its synchronizing side effect.
74 Opening a file in append mode (a as the first character of mode) causes
76 all subsequent write operations to this stream to occur at end-of-file,
77 as if preceded by an
78 fseek(stream,0,SEEK_END);
80 call.
82 The fdopen() function associates a stream with the existing file
84 descriptor, fd. The mode of the stream (one of the values "r", "r+",
85 "w", "w+", "a", "a+") must be compatible with the mode of the file
86 descriptor. The file position indicator of the new stream is set to
87 that belonging to fd, and the error and end-of-file indicators are
88 cleared. Modes "w" or "w+" do not cause truncation of the file. The
89 file descriptor is not dup'ed, and will be closed when the stream cre‐
90 ated by fdopen() is closed. The result of applying fdopen() to a
91 shared memory object is undefined.
92 The freopen() function opens the file whose name is the string pointed
94 to by path and associates the stream pointed to by stream with it. The
95 original stream (if it exists) is closed. The mode argument is used
96 just as in the fopen() function. The primary use of the freopen()
97 function is to change the file associated with a standard text stream
98 (stderr, stdin, or stdout).
99
101 Upon successful completion fopen(), fdopen() and freopen() return a
102 FILE pointer. Otherwise, NULL is returned and errno is set to indicate
103 the error.
104
106 EINVAL The mode provided to fopen(), fdopen(), or freopen() was
107 invalid.
108 The fopen(), fdopen() and freopen() functions may also fail and set
110 errno for any of the errors specified for the routine malloc(3).
111 The fopen() function may also fail and set errno for any of the errors
113 specified for the routine open(2).
114 The fdopen() function may also fail and set errno for any of the errors
116 specified for the routine fcntl(2).
117 The freopen() function may also fail and set errno for any of the
119 errors specified for the routines open(2), fclose(3) and fflush(3).
120
122 The fopen() and freopen() functions conform to C89. The fdopen() func‐
123 tion conforms to POSIX.1-1990.
124
126 Glibc Notes
127 The GNU C library allows the following extensions for the string speci‐
128 fied in mode:
129 c (since glibc 2.3.3)
131 Do not make the open operation, or subsequent read and write
132 operations, thread cancellation points.
133 e (since glibc 2.7)
135 Open the file with the O_CLOEXEC flag. See open(2) for more
136 information.
137 m (since glibc 2.3)
139 Attempt to access the file using mmap(2), rather than I/O system
140 calls (read(2), write(2)). Currently, use of mmap(2) is only
141 attempted for a file opened for reading.
142 x Open the file exclusively (like the O_EXCL flag of open(2)). If
144 the file already exists, fopen() fails, and sets errno to EEX‐
145 IST. This flag is ignored for fdopen().
146
148 open(2), fclose(3), fileno(3), fmemopen(3), fopencookie(3)
149
151 This page is part of release 3.25 of the Linux man-pages project. A
152 description of the project, and information about reporting bugs, can
153 be found at http://www.kernel.org/doc/man-pages/.
154GNU 2009-02-23 FOPEN(3)