fopen(3S) UNIX System V(C Development Set) fopen(3S)
NAME
fopen, freopen, fdopen - open a stream
SYNOPSIS
#include <stdio.h>
FILE *fopen (const char *filename, const char *type);
FILE *freopen (const char *filename, const char *type, FILE
*stream);
FILE *fdopen (int fildes, const char *type);
DESCRIPTION
fopen opens the file named by filename and associates a stream with it.
fopen returns a pointer to the FILE structure associated with the stream.
filename points to a character string that contains the name of the file
to be opened.
type is a character string beginning with one of the following sequences:
"r" or "rb"
open for reading
"w" or "wb"
truncate to zero length or create for writing
"a" or "ab"
append; open for writing at end of file, or create for writing
"r+", "r+b" or "rb+"
open for update (reading and writing)
"w+", "w+b" or "wb+"
truncate or create for update
"a+", "a+b" or "ab+"
append; open or create for update at end-of-file
The ``b'' is ignored in the above types. The ``b'' exists to distinguish
binary files from text files. However, there is no distinction between
these types of files on a UNIX system.
freopen substitutes the named file in place of the open stream. A flush
is first attempted, and then the original stream is closed, regardless of
whether the open ultimately succeeds. Failure to flush or close stream
successfully is ignored. freopen returns a pointer to the FILE structure
associated with stream.
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fopen(3S) UNIX System V(C Development Set) fopen(3S)
freopen is typically used to attach the preopened streams associated with
stdin, stdout, and stderr to other files. stderr is by default
unbuffered, but the use of freopen will cause it to become buffered or
line-buffered.
fdopen associates a stream with a file descriptor. File descriptors are
obtained from open, dup, creat, or pipe, which open files but do not
return pointers to a FILE structure stream. Streams are necessary input
for almost all of the Section 3S library routines. The type of stream
must agree with the mode of the open file. The file position indicator
associated with stream is set to the position indicated by the file
offset associated with fildes.
When a file is opened for update, both input and output may be done on
the resulting stream. However, output may not be directly followed by
input without an intervening fflush, fseek, fsetpos, or rewind, and input
may not be directly followed by output without an intervening fseek,
fsetpos, or rewind, or an input operation that encounters end-of-file.
When a file is opened for append (i.e., when type is "a", "ab", "a+", or
"ab+"), it is impossible to overwrite information already in the file.
fseek may be used to reposition the file pointer to any position in the
file, but when output is written to the file, the current file pointer is
disregarded. All output is written at the end of the file and causes the
file pointer to be repositioned at the end of the output. If two
separate processes open the same file for append, each process may write
freely to the file without fear of destroying output being written by the
other. The output from the two processes will be intermixed in the file
in the order in which it is written.
When opened, a stream is fully buffered if and only if it can be
determined not to refer to an interactive device. The error and end-of-
file indicators are cleared for the stream.
SEE ALSO
close(2), creat(2), dup(2), open(2), pipe(2), write(2), fclose(3S),
fseek(3S), setbuf(3S), stdio(3S).
DIAGNOSTICS
The functions fopen and freopen return a null pointer if path cannot be
accessed, or if type is invalid, or if the file cannot be opened.
The function fdopen returns a null pointer if fildes is not an open file
descriptor, or if type is invalid, or if the file cannot be opened.
The functions fopen or fdopen may fail and not set errno if there are no
free stdio streams.
File descriptors used by fdopen must be less than 255.
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