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