fopen, fdopen, freopen - stream open functions
The fopen() function opens the file whose name is
the string pointed to by
pathname and associates a stream with
mode points to a string beginning with one of
the following sequences (possibly followed by additional characters, as
Open text file for reading. The stream is positioned at the beginning of the file.
Open for reading and writing. The stream is positioned at the beginning of the file.
Truncate file to zero length or create text file for writing. The stream is positioned at the beginning of the file.
Open for reading and writing. The file is created if it does not exist, otherwise it is truncated. The stream is positioned at the beginning of the file.
Open for appending (writing at end of file). The file is created if it does not exist. The stream is positioned at the end of the file.
Open for reading and appending (writing at end of file). The file is created if it does not exist. Output is always appended to the end of the file. POSIX is silent on what the initial read position is when using this mode. For glibc, the initial file position for reading is at the beginning of the file, but for Android/BSD/MacOS, the initial file position for reading is at the end of the file.
mode string can also include the letter 'b' either as a
last character or as a character between the characters in any of the
two-character strings described above. This is strictly for
compatibility with C89 and has no effect; the 'b' is ignored on all
POSIX conforming systems, including Linux. (Other systems may treat text
files and binary files differently, and adding the 'b' may be a good
idea if you do I/O to a binary file and expect that your program may be
ported to non-UNIX environments.)
See NOTES below for details of glibc extensions for
Any created file will have the mode S_IRUSR | S_IWUSR | S_IRGRP | S_IWGRP | S_IROTH | S_IWOTH (0666), as modified by the process's umask value (see umask(2)).
Reads and writes may be intermixed on read/write streams in any
order. Note that ANSI C requires that a file positioning function
intervene between output and input, unless an input operation encounters
end-of-file. (If this condition is not met, then a read is allowed to
return the result of writes other than the most recent.) Therefore it is
good practice (and indeed sometimes necessary under Linux) to put an
fseek(3) or fgetpos(3) operation
between write and read operations on such a stream. This operation may
be an apparent no-op (as in
fseek(..., 0L, SEEK_CUR) called for
its synchronizing side effect).
Opening a file in append mode (a as the first
mode) causes all subsequent write operations to
this stream to occur at end-of-file, as if preceded the call:
fseek(stream, 0, SEEK_END);
The file descriptor associated with the stream is opened as if by a call to open(2) with the following flags:
fopen() mode open() flags
O_WRONLY | O_CREAT | O_TRUNC
O_WRONLY | O_CREAT | O_APPEND
O_RDWR | O_CREAT | O_TRUNC
O_RDWR | O_CREAT | O_APPEND
The fdopen() function associates a stream with the
existing file descriptor,
mode of the stream
(one of the values "r", "r+", "w", "w+", "a", "a+") must be compatible
with the mode of the file descriptor. The file position indicator of the
new stream is set to that belonging to
fd, and the error and
end-of-file indicators are cleared. Modes "w" or "w+" do not cause
truncation of the file. The file descriptor is not dup'ed, and will be
closed when the stream created by fdopen() is closed.
The result of applying fdopen() to a shared memory
object is undefined.
The freopen() function opens the file whose name is
the string pointed to by
pathname and associates the stream
pointed to by
stream with it. The original stream (if it
exists) is closed. The
mode argument is used just as in the
pathname argument is a null pointer,
freopen() changes the mode of the stream to that
mode; that is, freopen() reopens
the pathname that is associated with the stream. The specification for
this behavior was added in the C99 standard, which says:
In this case, the file descriptor associated with the stream need not be closed if the call to freopen() succeeds. It is implementation-defined which changes of mode are permitted (if any), and under what circumstances.
The primary use of the freopen() function is to
change the file associated with a standard text stream (
The GNU C library allows the following extensions for the string
Do not make the open operation, or subsequent read and write operations, thread cancellation points. This flag is ignored for fdopen().
string is taken as the name of a coded character
set and the stream is marked as wide-oriented. Thereafter, internal
conversion functions convert I/O to and from the character set
string. If the ,ccs=
string syntax is
not specified, then the wide-orientation of the stream is determined by
the first file operation. If that operation is a wide-character
operation, the stream is marked wide-oriented, and functions to convert
to the coded character set are loaded.
When parsing for individual flag characters in
the characters preceding the "ccs" specification), the glibc
implementation of fopen() and
freopen() limits the number of characters examined in
mode to 7 (or, in glibc versions before 2.14, to 6, which was
not enough to include possible specifications such as "rb+cmxe"). The
current implementation of fdopen() parses at most 5
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