4.4BSD snapshot (revision 8.1)

[unix-history] / usr / src / lib / libc / stdio / fseek.3

.\" Copyright (c) 1990, 1991 The Regents of the University of California.
.\" All rights reserved.
.\"
.\" This code is derived from software contributed to Berkeley by
.\" Chris Torek and the American National Standards Committee X3,
.\" on Information Processing Systems.
.\"
.\" %sccs.include.redist.man%
.\"
.\"     @(#)fseek.3	6.12 (Berkeley) %G%
.\"
.Dd 
.Dt FSEEK 3
.Os
.Sh NAME
.Nm fgetpos ,
.Nm fseek ,
.Nm fsetpos ,
.Nm ftell ,
.Nm rewind
.Nd reposition a stream
.Sh SYNOPSIS
.Fd #include <stdio.h>
.Ft int
.Fn fseek "FILE *stream" "long offset" "int whence"
.Ft long
.Fn ftell "FILE *stream"
.Ft void
.Fn rewind "FILE *stream"
.Ft int
.Fn fgetpos "FILE *stream" "fpos_t *pos"
.Ft int
.Fn fsetpos "FILE *stream" "fpos_t *pos"
.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 ftell
function
obtains the current value of the file position indicator for the
stream pointed to by
.Fa stream .
.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
.Pq non- Ns Tn UNIX
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.
Upon successful completion,
.Fn fgetpos ,
.Fn fseek ,
.Fn fsetpos
return 0,
and
.Fn ftell
returns the current offset.
Otherwise, \-1 is returned and the global variable errno is set to
indicate the error.
.Sh ERRORS
.Bl -tag -width [EINVAL]
.It Bq Er EBADF
The
.Fa stream
specified
is not a seekable stream.
.It Bq Er EINVAL
The
.Fa whence
argument to 
.Fn fseek
was not
.Dv SEEK_SET ,
.Dv SEEK_END ,
or
.Dv SEEK_CUR .
.El
.Pp
The function
.Fn fgetpos ,
.Fn fseek ,
.Fn fsetpos ,
and 
.Fn ftell
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 .