FOPEN(S) UNIX System V FOPEN(S)
Name
fopen, freopen, fdopen - open a stream
Syntax
#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
The fopen function opens the file named by filename and
associates a stream with it. The fopen function 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-
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(S),
which open files but do not return pointers to a FILE
structure stream. Streams are necessary input for many
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 (that is, when type is "a"
or "a+"), it is impossible to overwrite information already
in the file. The fseek function 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(S), dup(S), open(S), pipe(S), fclose(S), fseek(S),
stdio(S)
Diagnostics
fopen, fdopen, and freopen return a NULL pointer on failure.
Standards Conformance
fdopen is conformant with:
AT&T SVID Issue 2, Select Code 307-127;
The X/Open Portability Guide II of January 1987;
IEEE POSIX Std 1003.1-1988 with C Standard Language-
Dependent System Support;
and NIST FIPS 151-1.
fopen and freopen are conformant with:
AT&T SVID Issue 2, Select Code 307-127;
The X/Open Portability Guide II of January 1987;
ANSI X3.159-198X C Language Draft Standard, May 13,
1988;
IEEE POSIX Std 1003.1-1988 with C Standard Language-
Dependent System Support;
and NIST FIPS 151-1.
(printed 6/20/89)