FOPEN(3S)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. The initial portion of type must consist of
one of the following character sequences:
r or rb open for reading
w or wb truncate 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 for update at end-of-file or create for
update
As this implementation does not distinguish between binary and text
files, the character b in the string type, (which is used to indicate
that the file being opened is a binary file) is inconsequential.
Opening a file for reading (when r is the first character of type) will
fail if the file does not exist or is unreadable.
When a file is opened for update (when + appears as the second (or third,
in the case of fopen or freopen) character of type) both input and output
may be done on the resulting stream. However, output may not be directly
followed by input without an intervening fseek, fsetpos, or rewind.
Similarly, input may not be directly followed by output without an
intervening call to one of these functions, unless the input operation
left the file positioned at end-of-file. (See note under BUGS below.)
Page 1
FOPEN(3S)FOPEN(3S)
When a file is opened for append (i.e., when type is a, ab, a+b, ab+ 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.
freopen substitutes the named file in place of the open stream. An
attempt is made to close the original stream, using fclose(3s). If this
close attempt is unsuccessful, the failure is ignored. freopen then
attempts to open the file indicated by filename, returning the result.
The character string type must have the same form as for fopen. If the
open is successful, the end-of-file and error indicators for stream are
cleared.
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
initialize a stream, which is the object manipulated by many of the
Section 3S library routines. fdopen initializes a stream for the open
file descriptor fildes, and returns a pointer to the corresponding FILE
structure. The character string type has the same form as that for
fopen, with the exception that the (superfluous) binary file
specification character b, is not allowed. (This restricts the initial
portion of type to one of r, w, a, r+, w+, or a+.) The type specified
for the stream must agree with the mode of the open file indicated by
fildes (see open(2)).
SEE ALSOcreat(2), close(2), dup(2), open(2), pipe(2), write(2), fclose(3S),
fseek(3S), setbuf(3s), stdio(3S).
DIAGNOSTICS
fopen, fdopen, and freopen return a NULL pointer on failure.
Depending on which ABI a program is compiled in, there may be a limit on
the number of open stdio streams, or a limit on which file descriptors
may be associated with stdio streams. When compiling in n32 or n64
modes, there are no limits, however in o32 mode the functions fopen or
fdopen may fail and not set errno if there are no free stdio streams. No
more than 255 files may be opened via fopen, and only file descriptors 0
through 255 are valid with stdio streams.
In o32 mode file descriptors used by fdopen must be less than 255.
Page 2
FOPEN(3S)FOPEN(3S)BUGS
When operating on a file opened for update on which the last operation
was output, an input operation may be performed if there is an
intervening call to a file positioning function. An input operation
should also be possible under these circumstances if an intervening call
is made to fflush. If this sequence of operations (i.e., output, fflush,
input) is performed, however, the input operation fails with the
misleading error EBADF.
Page 3
