fopen(3S) SDK R4.11 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.
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.
Errors
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.
File descriptors used by fdopen must be less than 255.
Considerations for Threads Programming
+---------+-----------------------------+
| | async- |
|function | reentrant cancel cancel |
| | point safe |
+---------+-----------------------------+
|fdopen | Y N N |
|fopen | Y Y N |
|freopen | Y Y N |
+---------+-----------------------------+
REFERENCES
close(2), creat(2), dup(2), open(2), pipe(2), write(2), reentrant(3),
fclose(3S), fseek(3S), setbuf(3S), stdio(3S)
Licensed material--property of copyright holder(s)