.\" Copyright (c) 1990, 1991, 1993
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.\" This code is derived from software contributed to Berkeley by
.\" Chris Torek and the American National Standards Committee X3,
.\" on Information Processing Systems.
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.\" modification, are permitted provided that the following conditions
.\" are met:
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.\" 4. Neither the name of the University nor the names of its contributors
.\"    may be used to endorse or promote products derived from this software
.\"    without specific prior written permission.
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.\"     @(#)fopen.3	8.1 (Berkeley) 6/4/93
.\" $FreeBSD: src/lib/libc/stdio/fopen.3,v 1.21 2009/09/09 19:38:19 ed Exp $
.\"
.Dd January 26, 2003
.Dt FOPEN 3
.Os
.Sh NAME
.Nm fdopen ,
.Nm fopen ,
.Nm freopen
.Nd stream open functions
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In stdio.h
.Ft FILE *
.Fo fdopen
.Fa "int fildes"
.Fa "const char *mode"
.Fc
.Ft FILE *
.Fo fopen
.Fa "const char *restrict filename"
.Fa "const char *restrict mode"
.Fc
.Ft FILE *
.Fo freopen
.Fa "const char *restrict filename"
.Fa "const char *restrict mode"
.Fa "FILE *restrict stream"
.Fc
.Sh DESCRIPTION
The
.Fn fopen
function
opens the file whose name is the string pointed to by
.Fa filename
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 text file for reading.
The stream is positioned at the beginning of the file.
.It Dq Li r+
Open for reading and writing.
The stream is positioned at the beginning of the file.
.It Dq Li w
Truncate to zero length or create text file for writing.
The stream is positioned at the beginning of the file.
.It Dq Li w+
Open for reading and writing.
The file is created if it does not exist, otherwise it is truncated.
The stream is positioned at the beginning of the file.
.It Dq Li a
Open for writing.
The file is created if it does not exist.
The stream is positioned at the end of the file.
Subsequent writes to the file will always end up at the then current
end of file, irrespective of any intervening
.Xr fseek 3
or similar.
.It Dq Li a+
Open for reading and writing.
The file is created if it does not exist.
The stream is positioned at the end of the file.
Subsequent writes to the file will always end up at the then current
end of file, irrespective of any intervening
.Xr fseek 3
or similar.
.El
.Pp
The
.Fa mode
string can also include the letter ``b'' either as 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 -isoC
and has no effect; the ``b'' is ignored.
.Pp
Finally, as an extension to the standards (and thus may not be portable),
.Fa mode
string may end with the letter ``x'', which insists on creating a new file
when used with ``w'' or ``a''.
If
.Fa path
exists, then an error is returned (this is the equivalent of specifying
.Dv O_EXCL
with
.Xr open 2 ) .
.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
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 mode
of the stream must be compatible with the mode of the file descriptor.
When the stream is closed via
.Xr fclose 3 ,
.Fa fildes
is closed also.
.Pp
The
.Fn freopen
function
opens the file whose name is the string pointed to by
.Fa filename
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.
.Pp
If the
.Fa filename
argument is
.Dv NULL ,
.Fn freopen
attempts to re-open the file associated with
.Fa stream
with a new mode.
The new mode must be compatible with the mode that the stream was originally
opened with:
.Bl -bullet -offset indent
.It
Streams originally opened with mode
.Dq Li r
can only be reopened with that same mode.
.It
Streams originally opened with mode
.Dq Li a
can be reopened with the same mode, or mode
.Dq Li w .
.It
Streams originally opened with mode
.Dq Li w
can be reopened with the same mode, or mode
.Dq Li a .
.It
Streams originally opened with mode
.Dq Li r+ ,
.Dq Li w+ ,
or
.Dq Li a+
can be reopened with any mode.
.El
.Pp
The primary use of the
.Fn freopen
function
is to change the file associated with a
standard text stream
.Dv ( stderr , stdin ,
or
.Dv 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
argument
to
.Fn fopen ,
.Fn fdopen ,
or
.Fn freopen
was invalid.
.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 -isoC .
The
.Fn fdopen
function
conforms to
.St -p1003.1-88 .