freebsd-dev/lib/libc/stdio/fopen.3

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.\"     @(#)fopen.3	8.1 (Berkeley) 6/4/93
.\" $FreeBSD$
.\"
.Dd October 17, 2011
.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 * restrict path" "const char * restrict mode"
.Ft FILE *
.Fn fdopen "int fildes" "const char *mode"
.Ft FILE *
.Fn freopen "const char *path" "const char *mode" "FILE *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 letters:
.Bl -tag -width indent
.It Dq Li r
Open for reading.
The stream is positioned at the beginning of the file.
Fail if the file does not exist.
.It Dq Li w
Open for writing.
The stream is positioned at the beginning of the file.
Create the file if it does not exist.
.It Dq Li a
Open for writing.
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.
Create the file if it does not exist.
.El
.Pp
An optional
.Dq Li +
following
.Dq Li r ,
.Dq Li w ,
or
.Dq Li a
opens the file for both reading and writing.
An optional
.Dq Li x
following
.Dq Li w
or
.Dq Li w+
causes the
.Fn fopen
call to fail if the file already exists.
.Pp
The
.Fa mode
string can also include the letter
.Dq Li b
after either the
.Dq Li +
or the first letter.
This is strictly for compatibility with
.St -isoC
and has no effect; the ``b'' is ignored.
.Pp
Any created files will have mode
.Do Dv S_IRUSR
\&|
.Dv S_IWUSR
\&|
.Dv S_IRGRP
\&|
.Dv S_IWGRP
\&|
.Dv S_IROTH
\&|
.Dv S_IWOTH Dc
.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.
The
.Dq Li x
mode option is ignored.
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 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.
.Pp
If the
.Fa path
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:
Streams open for reading can only be re-opened for reading,
streams open for writing can only be re-opened for writing,
and streams open for reading and writing can be re-opened in any mode.
The
.Dq Li x
mode option is not meaningful in this context.
.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 ,
with the exception of the
.Dq Li x
mode option which conforms to
.St -isoC-2011 .
The
.Fn fdopen
function
conforms to
.St -p1003.1-88 .