path: root/lib/libc/stdio/fopen.3

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.Dd $Mdocdate: September 28 2023 $
.Dt FOPEN 3
.Os
.Sh NAME
.Nm fopen ,
.Nm fdopen ,
.Nm freopen
.Nd stream open functions
.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 *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 Do Li r Dc or Do Li rb Dc
Open file for reading.
.It Do Li r+ Dc or Do Li rb+ Dc or Do Li r+b Dc
Open for reading and writing.
.It Do Li w Dc or Do Li wb Dc
Open for writing.
The file is created if it does not exist, otherwise it is truncated.
.It Do Li w+ Dc or Do Li wb+ Dc or Do Li w+b Dc
Open for reading and writing.
The file is created if it does not exist, otherwise it is truncated.
.It Do Li a Dc or Do Li ab Dc
Open for writing.
The file is created if it does not exist.
.It Do Li a+ Dc or Do Li ab+ Dc or Do Li a+b Dc
Open for reading and writing.
The file is created if it does not exist.
.El
.Pp
The letter
.Dq b
in the
.Fa mode
strings above is strictly for compatibility with
.St -ansiC
and has no effect; the
.Dq b
is ignored.
.Pp
After any of the above prefixes, the
.Fa mode
string can also include zero or more of the following:
.Bl -tag -width indent
.It Dq Li e
The close-on-exec flag is set on the underlying file descriptor of the new
.Vt FILE .
.It Dq Li x
If the
.Fa mode
string starts with
.Dq w
or
.Dq a
then the function shall fail if the file specified by
.Fa path
already exists, as if the
.Dv O_EXCL
flag was passed to the
.Xr open 2
function.
It has no effect if used with
.Fn fdopen
or the
.Fa mode
string begins with
.Dq r .
.El
.Pp
The
.Fn fopen
and
.Fn freopen
functions initially position the stream at the start of the file
unless the file is opened in append mode
.Po
.Sq a
or
.Sq a+
.Pc ,
in which case the stream is initially positioned at the end of the file.
.Pp
Opening a file in append mode causes all subsequent writes to it
to be forced to the current end-of-file, regardless of intervening
repositioning of the stream.
.Pp
Any created files will have mode
.Qq Dv S_IRUSR | S_IWUSR | S_IRGRP | S_IWGRP | S_IROTH | S_IWOTH
.Pq Li 0666 ,
as modified by the process'
umask value (see
.Xr umask 2 ) .
.Pp
Reads and writes cannot be arbitrarily intermixed on read/write streams.
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.
If
.Fn fdopen
fails, the file descriptor
.Fa fildes
is not affected in any way.
.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 always closed, even if
.Fn freopen
fails.
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
.Vt 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.
.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 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 -p1003.1-88 .
.Sh HISTORY
The
.Fn fopen
function first appeared in
.At v1 .
The
.Fn fdopen
and
.Fn freopen
functions first appeared in
.At v7 .
.Pp
Support for the
.Dq e
and
.Dq x
mode letters appeared in
.Ox 5.7 .
.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
.Vt 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