libc/stdio/fopen.3

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.Dd September 1, 2023
.Dt FOPEN 3
.Os
.Sh NAME
.Nm fopen ,
.Nm fdopen ,
.Nm freopen ,
.Nm fmemopen
.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"
.Ft FILE *
.Fn fmemopen "void * restrict buf" "size_t size" "const char * restrict mode"
.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.
Truncate the file to zero length if it exists or 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.
An optional
.Dq Li e
following the above
causes the
.Fn fopen
call to set the
.Dv FD_CLOEXEC
flag on the underlying file descriptor.
.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 effect only for
.Fn fmemopen ;
otherwise
.Dq Li 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;
.St -isoC
and
.St -p1003.1
both require 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.
If the
.Dq Li e
mode option is present, the
.Dv FD_CLOEXEC
flag is set, otherwise it remains unchanged.
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 ) .
.Pp
The
.Fn fmemopen
function
associates the buffer given by the
.Fa buf
and
.Fa size
arguments with a stream.
The
.Fa buf
argument is either a null pointer or point to a buffer that
is at least
.Fa size
bytes long.
If a null pointer is specified as the
.Fa buf
argument,
.Fn fmemopen
allocates
.Fa size
bytes of memory.
This buffer is automatically freed when the stream is closed.
Buffers can be opened in text-mode (default) or binary-mode
(if
.Dq Li b
is present in the second or third position of the
.Fa mode
argument).
Buffers opened in text-mode make sure that writes are terminated with a
.Dv NULL
byte, if the last write hasn't filled up the whole buffer.
Buffers opened in binary-mode never append a
.Dv NULL
byte.
.Sh RETURN VALUES
Upon successful completion
.Fn fopen ,
.Fn fdopen ,
.Fn freopen
and
.Fn fmemopen
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 ,
.Fn freopen ,
or
.Fn fmemopen
was invalid.
.El
.Pp
The
.Fn fopen ,
.Fn fdopen ,
.Fn freopen
and
.Fn fmemopen
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 .
.Pp
The
.Fn fmemopen
function
may also fail and set
.Va errno
if the
.Fa size
argument is 0.
.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 .
The
.Dq Li e
mode option does not conform to any standard
but is also supported by glibc.
The
.Fn fmemopen
function
conforms to
.St -p1003.1-2008 .
The
.Dq Li b
mode does not conform to any standard
but is also supported by glibc.
.Sh HISTORY
An
.Fn fopen
function appeared in
.At v1 .