fopen(3S) LIBRARY FUNCTIONS 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 struc-
ture 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 follow-
ing 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 sys-
tem.
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
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fopen(3S) LIBRARY FUNCTIONS fopen(3S)
ignored. freopen returns a pointer to the FILE structure
associated with stream.
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 struc-
ture 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 informa-
tion already in the file. fseek may be used to reposition
the file pointer to any position in the file, but when out-
put 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.
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fopen(3S) LIBRARY FUNCTIONS fopen(3S)
The function fdopen returns a null pointer if fildes is not
an open file descriptor, or if type is invalid, of 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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