FOPEN(3S) (C Programming Language Utilities) FOPEN(3S)
NAME
fopen, freopen, fdopen - open a stream
SYNOPSIS
#include <stdio.h>
FILE *fopen (filename, type)
char *filename, *type;
FILE *freopen (filename, type, stream)
char *filename, *type;
FILE *stream;
FILE *fdopen (fildes, type)
int fildes;
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 having one of the following
values:
"r" open for reading
"w" truncate or create for writing
"a" append; open for writing at end of file, or
create for writing
"r+" open for update (reading and writing)
"w+" truncate or create for update
"a+" append; open or create for update at end-of-
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FOPEN(3S) (C Programming Language Utilities) FOPEN(3S)
file
Freopen substitutes the named file in place of the open
stream. The original stream is closed, regardless of
whether the open ultimately succeeds. 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.
Fdopen associates a stream with a file descriptor. File
descriptors are obtained from open, dup, creat, or pipe(2),
which open files but do not return pointers to a FILE
structure stream. Streams are necessary input for many of
the Section 3S library routines. The type of stream must
agree with the mode of the open file.
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 fseek or
rewind, and input may not be directly followed by output
without an intervening fseek, rewind, or an input operation
which encounters end-of-file.
When a file is opened for append (i.e., when type is "a" or
"a+"), 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.
SEE ALSO
creat(2), dup(2), open(2), pipe(2), fclose(3S), fseek(3S),
stdio(3S).
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FOPEN(3S) (C Programming Language Utilities) FOPEN(3S)
DIAGNOSTICS
fopen, fdopen, and freopen return a NULL pointer on failure.
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