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https://www.bell-labs.com/usr/dmr/www/man31.pdf

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.\"     @(#)fopen.3	8.1 (Berkeley) 6/4/93
.\"
.Dd September 2, 2019
.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 * restrict path" "const char * restrict 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, which may be followed by additional modifiers
as indicated below:
.Bl -tag -width 4n
.It Dq Li a
Append; open an existing or new file for writing in append mode.
The file is created if it does not exist.
.It Dq Li a+
Open for reading and writing in append mode.
The file is created if it does not exist.
.It Dq Li r
Read; open an existing file for reading.
.It Dq Li r+
Open an existing file for reading and writing.
.It Dq Li w
Write; open an empty file for writing.
Truncate an existing file to zero length or create a new file.
.It Dq Li w+
Open an empty file for reading and writing.
Truncate file to zero length or create file.
.El
.Pp
After one of those, the
.Fa mode
string can also include one or more of the following modifier letters:
.Bl -tag -width 4n
.It Sq b
The letter
.Sq b
may appear 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 ,
where it means open in
.Dq binary
mode which is identical to text mode here,
so has no effect; the
.Sq b
is ignored.
.It Sq e
The letter
.Sq e
in the mode string sets the close-on-exec
.Pq Dv O_CLOEXEC
flag of the file descriptor, which means that it will not be available
after an
.Xr exec 3
system call.
This is a non
.St -ansiC
extension.
.It Sq f
The letter
.Sq f
in the mode string restricts
.Fn fopen
to regular files; if the file opened is not a regular file,
.Fn fopen
will close it, and fail.
This is a non
.St -ansiC
extension.
.It Sq l
The letter
.Sq l
in the mode string turns the don't-follow-symlinks
.Pq Dv O_NOFOLLOW
flag of the file descriptor, which means that if the last path component
is a symbolic link, it will not be followed.
This is a non
.St -ansiC
extension.
.It Sq x
The letter
.Sq x
in the mode turns on exclusive open mode to the file
.Pq Dv O_EXCL
which means that the file will not be created if it already exists.
In that case
.Fn fopen
will fail.
.El
.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'
.Xr umask 2
value.
.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 ) .
.Pp
Input and output against the opened stream will be fully buffered, unless
it refers to an interactive terminal device, or a different kind of buffering
is specified in the environment.
See
.Xr setvbuf 3
for additional details.
.Sh RETURN VALUES
Upon successful completion
.Fn fopen ,
.Fn fdopen
and
.Fn freopen
return a FILE pointer.
Otherwise,
.Dv NULL
is returned and the global variable
.Va errno
is set to indicate the error.
.Sh ERRORS
The functions may fail if:
.Bl -tag -width Er
.It Bq Er EFTYPE
The file is not a regular file and the character ``f'' is specified
in the mode.
.It Bq Er EINVAL
The specified
.Fa mode
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 -ansiC .
All three functions are specified in
.St -p1003.1-2008 .
.Sh HISTORY
An
.Fn fopen
function appeared in
.At v1 .
.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