1FOPEN(3) Linux Programmer's Manual FOPEN(3)
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6 fopen, fdopen, freopen - stream open functions
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9 #include <stdio.h>
10 FILE *fopen(const char *path, const char *mode);
12 FILE *fdopen(int fildes, const char *mode);
13 FILE *freopen(const char *path, const char *mode, FILE *stream);
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16 The fopen() function opens the file whose name is the string pointed to
17 by path and associates a stream with it.
18 The argument mode points to a string beginning with one of the follow‐
20 ing sequences (Additional characters may follow these sequences.):
21 r Open text file for reading. The stream is positioned at the
23 beginning of the file.
24 r+ Open for reading and writing. The stream is positioned at the
26 beginning of the file.
27 w Truncate file to zero length or create text file for writing.
29 The stream is positioned at the beginning of the file.
30 w+ Open for reading and writing. The file is created if it does
32 not exist, otherwise it is truncated. The stream is positioned
33 at the beginning of the file.
34 a Open for appending (writing at end of file). The file is cre‐
36 ated if it does not exist. The stream is positioned at the end
37 of the file.
38 a+ Open for reading and appending (writing at end of file). The
40 file is created if it does not exist. The initial file position
41 for reading is at the beginning of the file, but output is
42 always appended to the end of the file.
43 The mode string can also include the letter ``b'' either as a last
45 character or as a character between the characters in any of the two-
46 character strings described above. This is strictly for compatibility
47 with C89 and has no effect; the ``b'' is ignored on all POSIX conform‐
48 ing systems, including Linux. (Other systems may treat text files and
49 binary files differently, and adding the ``b'' may be a good idea if
50 you do I/O to a binary file and expect that your program may be ported
51 to non-Unix environments.)
52 Any created files will have mode
54 S_IRUSR|S_IWUSR|S_IRGRP|S_IWGRP|S_IROTH|S_IWOTH [22m(0666), as modified by
55 the process' umask value (see umask(2)).
56 Reads and writes may be intermixed on read/write streams in any order.
58 Note that ANSI C requires that a file positioning function intervene
59 between output and input, unless an input operation encounters end-of-
60 file. (If this condition is not met, then a read is allowed to return
61 the result of writes other than the most recent.) Therefore it is good
62 practice (and indeed sometimes necessary under Linux) to put an fseek()
63 or fgetpos() operation between write and read operations on such a
64 stream. This operation may be an apparent no-op (as in fseek(..., 0L,
65 SEEK_CUR) called for its synchronizing side effect.
66 Opening a file in append mode (a as the first character of mode) causes
68 all subsequent write operations to this stream to occur at end-of-file,
69 as if preceded by an
70 fseek(stream,0,SEEK_END);
71 call.
72 The fdopen() function associates a stream with the existing file
74 descriptor, fildes. The mode of the stream (one of the values "r",
75 "r+", "w", "w+", "a", "a+") must be compatible with the mode of the
76 file descriptor. The file position indicator of the new stream is set
77 to that belonging to fildes, and the error and end-of-file indicators
78 are cleared. Modes "w" or "w+" do not cause truncation of the file.
79 The file descriptor is not dup'ed, and will be closed when the stream
80 created by fdopen() is closed. The result of applying fdopen() to a
81 shared memory object is undefined.
82 The freopen() function opens the file whose name is the string pointed
84 to by path and associates the stream pointed to by stream with it. The
85 original stream (if it exists) is closed. The mode argument is used
86 just as in the fopen() function. The primary use of the freopen()
87 function is to change the file associated with a standard text stream
88 (stderr, stdin, or stdout).
89
91 Upon successful completion fopen(), fdopen() and freopen() return a
92 FILE pointer. Otherwise, NULL is returned and the global variable
93 errno is set to indicate the error.
94
96 EINVAL The mode provided to fopen(), fdopen(), or freopen() was
97 invalid.
98 The fopen(), fdopen() and freopen() functions may also fail and set
100 errno for any of the errors specified for the routine malloc(3).
101 The fopen() function may also fail and set errno for any of the errors
103 specified for the routine open(2).
104 The fdopen() function may also fail and set errno for any of the errors
106 specified for the routine fcntl(2).
107 The freopen() function may also fail and set errno for any of the
109 errors specified for the routines open(2), fclose(3) and fflush(3).
110
112 The fopen() and freopen() functions conform to C89. The fdopen() func‐
113 tion conforms to POSIX.1-1990.
114
116 The GNU C library allows the following extensions for the string speci‐
117 fied in mode:
118 c (since glibc 2.3.3)
120 Do not make the open operation, or subsequent read and write
121 operations, thread cancellation points.
122 m (since glibc 2.3)
124 Attempt to access the file using mmap(2), rather than I/O system
125 calls (read(2), write(2)). Currently, use of mmap(2) is only
126 attempted for a file opened for reading.
127 x Open the file exclusively (like the O_EXCL flag of open(2)). If
129 the file already exists, fopen() fails, and sets errno to EEX‐
130 IST. This flag is ignored for fdopen().
131
133 open(2), fclose(3), fileno(3)
134BSD MANPAGE 2006-05-04 FOPEN(3)