man3/fseek.3

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.\"     @(#)fseek.3	6.11 (Berkeley) 6/29/91
.\"
.\" Converted for Linux, Mon Nov 29 15:22:01 1993, faith@cs.unc.edu
.\"
.TH FSEEK 3  2021-03-22 "GNU" "Linux Programmer's Manual"
.SH NAME
fgetpos, fseek, fsetpos, ftell, rewind \- reposition a stream
.SH SYNOPSIS
.nf
.B #include <stdio.h>
.PP
.BI "int fseek(FILE *" stream ", long " offset ", int " whence );
.BI "long ftell(FILE *" stream );
.PP
.BI "void rewind(FILE *" stream );
.PP
.BI "int fgetpos(FILE *restrict " stream ", fpos_t *restrict " pos );
.BI "int fsetpos(FILE *" stream ", const fpos_t *" pos );
.fi
.SH DESCRIPTION
The
.BR fseek ()
function sets the file position indicator for the stream pointed to by
.IR stream .
The new position, measured in bytes, is obtained by adding
.I offset
bytes to the position specified by
.IR whence .
If
.I whence
is set to
.BR SEEK_SET ,
.BR SEEK_CUR ,
or
.BR 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
.BR fseek ()
function clears the end-of-file indicator for the stream and undoes
any effects of the
.BR ungetc (3)
function on the same stream.
.PP
The
.BR ftell ()
function obtains the current value of the file position indicator for the
stream pointed to by
.IR stream .
.PP
The
.BR rewind ()
function sets the file position indicator for the stream pointed to by
.I stream
to the beginning of the file.
It is equivalent to:
.PP
.RS
(void) fseek(stream, 0L, SEEK_SET)
.RE
.PP
except that the error indicator for the stream is also cleared (see
.BR clearerr (3)).
.PP
The
.BR fgetpos ()
and
.BR fsetpos ()
functions are alternate interfaces equivalent to
.BR ftell ()
and
.BR fseek ()
(with
.I whence
set to
.BR SEEK_SET ),
setting and storing the current value of the file offset into or from the
object referenced by
.IR pos .
On some non-UNIX systems, an
.I fpos_t
object may be a complex object and these routines may be the only way to
portably reposition a text stream.
.SH RETURN VALUE
The
.BR rewind ()
function returns no value.
Upon successful completion,
.BR fgetpos (),
.BR fseek (),
.BR fsetpos ()
return 0,
and
.BR ftell ()
returns the current offset.
Otherwise, \-1 is returned and
.I errno
is set to indicate the error.
.SH ERRORS
.TP
.B EINVAL
The
.I whence
argument to
.BR fseek ()
was not
.BR SEEK_SET ,
.BR SEEK_END ,
or
.BR SEEK_CUR .
Or: the resulting file offset would be negative.
.TP
.B ESPIPE
The file descriptor underlying
.I stream
is not seekable (e.g., it refers to a pipe, FIFO, or socket).
.PP
The functions
.BR fgetpos (),
.BR fseek (),
.BR fsetpos (),
and
.BR ftell ()
may also fail and set
.I errno
for any of the errors specified for the routines
.BR fflush (3),
.BR fstat (2),
.BR lseek (2),
and
.BR malloc (3).
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.ad l
.nh
.TS
allbox;
lbx lb lb
l l l.
Interface	Attribute	Value
T{
.BR fseek (),
.BR ftell (),
.BR rewind (),
.BR fgetpos (),
.BR fsetpos ()
T}	Thread safety	MT-Safe
.TE
.hy
.ad
.sp 1
.SH CONFORMING TO
POSIX.1-2001, POSIX.1-2008, C89, C99.
.SH SEE ALSO
.BR lseek (2),
.BR fseeko (3)