NetBSD/lib/libc/stdio/fopen.3

.\"	$NetBSD: fopen.3,v 1.21 2003/04/16 13:34:44 wiz Exp $
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.\" This code is derived from software contributed to Berkeley by
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.\"     @(#)fopen.3	8.1 (Berkeley) 6/4/93
.\"
.Dd June 4, 1993
.Dt FOPEN 3
.Os
.Sh NAME
.Nm fopen ,
.Nm fdopen ,
.Nm freopen
.Nd stream open functions
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In stdio.h
.Ft FILE *
.Fn fopen "const char *path" "const char *mode"
.Ft FILE *
.Fn fdopen "int fildes" "const char *mode"
.Ft FILE *
.Fn freopen "const char *path" "const char *mode" "FILE * restrict stream"
.Sh DESCRIPTION
The
.Fn fopen
function
opens the file whose name is the string pointed to by
.Fa path
and associates a stream with it.
.Pp
The argument
.Fa mode
points to a string beginning with one of the following
sequences (Additional characters may follow these sequences.):
.Bl -tag -width indent
.It Dq Li r
Open for reading.
.It Dq Li r+
Open for reading and writing.
.It Dq Li w
Open for writing.
Truncate file to zero length or create file.
.It Dq Li w+
Open for reading and writing.
Truncate file to zero length or create file.
.It Dq Li a
Append; open for writing.
The file is created if it does not exist.
.It Dq Li a+
Append; open for reading and writing.
The file is created if it does not exist.
.El
.Pp
The
.Fa mode
string can also include the letter ``b'' either as a last character or
as a character between the characters in any of the two-character strings
described above.
This is strictly for compatibility with
.St -ansiC
and has no effect; the ``b'' is ignored.
.Pp
The letter ``f'' in the mode string restricts fopen to regular
files; if the file opened is not a regular file,
.Fn fopen
will fail.
This is a non
.St -ansiC
extension.
.Pp
Any created files will have mode
.Pf \\*q Dv S_IRUSR
\&|
.Dv S_IWUSR
\&|
.Dv S_IRGRP
\&|
.Dv S_IWGRP
\&|
.Dv S_IROTH
\&|
.Dv S_IWOTH Ns \\*q
.Pq Li 0666 ,
as modified by the process'
umask value (see
.Xr umask 2 ) .
.Pp
Opening a file with append mode causes all subsequent writes to it
to be forced to the then current end of file, regardless of intervening
repositioning of the stream.
.Pp
The
.Fn fopen
and
.Fn freopen
functions initially position the stream at the start of the file
unless the file is opened with append mode,
in which case the stream is initially positioned at the end of the file.
.\" PR 6072 claims this paragraph is not correct.
.\" .Pp
.\" Reads and writes may be intermixed on read/write streams in any order,
.\" and do not require an intermediate seek as in previous versions of
.\" .Em stdio .
.\" This is not portable to other systems, however;
.\" .Tn ANSI C
.\" requires that
.\" a file positioning function intervene between output and input, unless
.\" an input operation encounters end-of-file.
.Pp
The
.Fn fdopen
function associates a stream with the existing file descriptor,
.Fa fildes .
The
.Fa mode
of the stream must be compatible with the mode of the file descriptor.
The stream is positioned at the file offset of the file descriptor.
.Pp
The
.Fn freopen
function
opens the file whose name is the string pointed to by
.Fa path
and associates the stream pointed to by
.Fa stream
with it.
The original stream (if it exists) is closed.
The
.Fa mode
argument is used just as in the
.Fn fopen
function.
The primary use of the
.Fn freopen
function
is to change the file associated with a
standard text stream
.Pf ( Em stderr ,
.Em stdin ,
or
.Em stdout ) .
.Sh RETURN VALUES
Upon successful completion
.Fn fopen ,
.Fn fdopen
and
.Fn freopen
return a
.Tn FILE
pointer.
Otherwise,
.Dv NULL
is returned and the global variable
.Va errno
is set to indicate the error.
.Sh ERRORS
.Bl -tag -width Er
.It Bq Er EINVAL
The
.Fa mode
provided to
.Fn fopen ,
.Fn fdopen ,
or
.Fn freopen
was invalid.
.It Bq Er EFTYPE
The file is not a regular file and the character ``f'' is specified
in the mode.
.El
.Pp
The
.Fn fopen ,
.Fn fdopen
and
.Fn freopen
functions
may also fail and set
.Va errno
for any of the errors specified for the routine
.Xr malloc 3 .
.Pp
The
.Fn fopen
function
may also fail and set
.Va errno
for any of the errors specified for the routine
.Xr open 2 .
.Pp
The
.Fn fdopen
function
may also fail and set
.Va errno
for any of the errors specified for the routine
.Xr fcntl 2 .
.Pp
The
.Fn freopen
function
may also fail and set
.Va errno
for any of the errors specified for the routines
.Xr open 2 ,
.Xr fclose 3
and
.Xr fflush 3 .
.Sh SEE ALSO
.Xr open 2 ,
.Xr fclose 3 ,
.Xr fileno 3 ,
.Xr fseek 3 ,
.Xr funopen 3
.Sh STANDARDS
The
.Fn fopen
and
.Fn freopen
functions
conform to
.St -ansiC .
The
.Fn fdopen
function conforms to
.St -p1003.1-90 .
.Sh CAVEATS
Proper code using
.Fn fdopen
with error checking should
.Xr close 2
.Fa fildes
in case of failure, and
.Xr fclose 3
the resulting FILE * in case of success.
.Bd -literal
	FILE *file;
	int fd;

	if ((file = fdopen(fd, "r")) != NULL) {
		/* perform operations on the FILE * */
		fclose(file);
	} else {
		/* failure, report the error */
		close(fd);
	}
.Ed