.\"	$OpenBSD: fseek.3,v 1.19 2022/09/11 06:38:11 jmc Exp $
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.Dd $Mdocdate: September 11 2022 $
.Dt FSEEK 3
.Os
.Sh NAME
.Nm fgetpos ,
.Nm fseek ,
.Nm fseeko ,
.Nm fsetpos ,
.Nm ftell ,
.Nm ftello ,
.Nm rewind
.Nd reposition a stream
.Sh SYNOPSIS
.In stdio.h
.Ft int
.Fn fgetpos "FILE *stream" "fpos_t *pos"
.Ft int
.Fn fseek "FILE *stream" "long offset" "int whence"
.Ft int
.Fn fseeko "FILE *stream" "off_t offset" "int whence"
.Ft int
.Fn fsetpos "FILE *stream" "const fpos_t *pos"
.Ft long
.Fn ftell "FILE *stream"
.Ft off_t
.Fn ftello "FILE *stream"
.Ft void
.Fn rewind "FILE *stream"
.Sh DESCRIPTION
The
.Fn fseek
function sets the file position indicator for the stream pointed to by
.Fa stream .
The new position, measured in bytes, is obtained by adding
.Fa offset
bytes to the position specified by
.Fa whence .
If
.Fa whence
is set to
.Dv SEEK_SET ,
.Dv SEEK_CUR ,
or
.Dv SEEK_END ,
the offset is relative to the
start of the file, the current position indicator, or end-of-file,
respectively.
A successful call to the
.Fn fseek
function clears the end-of-file indicator for the stream and undoes
any effects of the
.Xr ungetc 3
function on the same stream.
.Pp
The
.Fn fseeko
function is identical to
.Fn fseek
except that it takes an
.Vt off_t
as its
.Fa offset .
.Pp
The
.Fn ftell
function obtains the current value of the file position indicator for the
stream pointed to by
.Fa stream .
.Pp
The
.Fn ftello
function is identical to
.Fn ftell
except that its return value is of type
.Vt off_t .
.Pp
The
.Fn rewind
function sets the file position indicator for the stream pointed
to by
.Fa stream
to the beginning of the file.
It is equivalent to:
.Pp
.Dl (void)fseek(stream, 0L, SEEK_SET)
.Pp
except that the error indicator for the stream is also cleared
(see
.Xr clearerr 3 ) .
.Pp
The
.Fn fgetpos
and
.Fn fsetpos
functions are alternate interfaces equivalent to
.Fn ftell
and
.Fn fseek
(with whence set to
.Dv SEEK_SET ) ,
setting and storing the current value of
the file offset into or from the object referenced by
.Fa pos .
On some systems an
.Dq Fa fpos_t
object may be a complex object
and these routines may be the only way to portably reposition a text stream.
.Sh RETURN VALUES
The
.Fn rewind
function returns no value.
Prefer
.Fn fseek ,
which is just as portable, and does not hide errors.
Upon successful completion,
.Fn fgetpos ,
.Fn fseek ,
.Fn fseeko ,
and
.Fn fsetpos
return 0 and
.Fn ftell
and
.Fn ftello
return the current offset.
Otherwise,
.Fn fseek ,
.Fn fseeko ,
.Fn ftell ,
and
.Fn ftello
return \-1 and
.Fn fgetpos
and
.Fn fsetpos
return a non-zero value and the global variable
.Va errno
is set to indicate the error.
.Sh ERRORS
The
.Fn fgetpos ,
.Fn fseek ,
.Fn fseeko ,
.Fn fsetpos ,
.Fn ftell ,
.Fn ftello ,
and
.Fn rewind
functions will fail if:
.Bl -tag -width Er
.It Bq Er EBADF
The
.Fa stream
specified is not a seekable stream.
.El
.Pp
Additionally, the
.Fn fseek
and
.Fn fseeko
functions will fail if:
.Bl -tag -width Er
.It Bq Er EINVAL
The
.Fa whence
argument was not
.Dv SEEK_SET ,
.Dv SEEK_END ,
or
.Dv SEEK_CUR .
.El
.Pp
Additionally, the
.Fn ftell
function will fail if:
.Bl -tag -width Er
.It Bq Er EOVERFLOW
The value of the file position indicator is too large to be represented by a
.Vt long .
.El
.Pp
Finally, the functions
.Fn fgetpos ,
.Fn fseek ,
.Fn fseeko ,
.Fn fsetpos ,
.Fn ftell ,
and
.Fn ftello
may also fail and set
.Va errno
for any of the errors specified for the routines
.Xr fflush 3 ,
.Xr fstat 2 ,
.Xr lseek 2 ,
and
.Xr malloc 3 .
.Sh SEE ALSO
.Xr lseek 2
.Sh STANDARDS
The
.Fn fgetpos ,
.Fn fsetpos ,
.Fn fseek ,
.Fn ftell ,
and
.Fn rewind
functions conform to
.St -ansiC
and
.St -xpg4 .
.Pp
The
.Fn fseeko
and
.Fn ftello
functions conform to
.St -xpg4 .
.Sh HISTORY
The functions
.Fn fseek ,
.Fn ftell ,
and
.Fn rewind
first appeared in
.At v7 .