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

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.Dd April 2, 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 LIBRARY
.Lb libc
.Sh SYNOPSIS
.In 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 * restrict stream" "fpos_t * restrict pos"
.Ft int
.Fn fsetpos "FILE *stream" "const fpos_t *pos"
.In sys/types.h
.Ft int
.Fn fseeko "FILE *stream" "off_t offset" "int whence"
.Ft off_t
.Fn ftello "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
and
.Xr ungetwc 3
functions 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
Since
.Fn rewind
does not return a value,
an application wishing to detect errors should clear
.Va errno ,
then call
.Fn rewind ,
and if
.Va errno
is non-zero, assume an error has occurred.
.Pp
The
.Fn fseeko
function is identical to
.Fn fseek ,
except it takes an
.Fa off_t
argument
instead of a
.Fa long .
Likewise, the
.Fn ftello
function is identical to
.Fn ftell ,
except it returns an
.Fa off_t .
.Pp
The
.Fn fgetpos
and
.Fn fsetpos
functions
are alternate interfaces for retrieving and setting the current position in
the file, similar to
.Fn ftell
and
.Fn fseek ,
except that the current position is stored in an opaque object of
type
.Vt fpos_t
pointed to by
.Fa pos .
These functions provide a portable way to seek to offsets larger than
those that can be represented by a
.Vt long int .
They may also store additional state information in the
.Vt fpos_t
object to facilitate seeking within files containing multibyte
characters with state-dependent encodings.
Although
.Vt fpos_t
has traditionally been an integral type,
applications cannot assume that it is;
in particular, they must not perform arithmetic on objects
of this type.
.Pp
If the stream is a wide character stream (see
.Xr fwide 3 ) ,
the position specified by the combination of
.Fa offset
and
.Fa whence
must contain the first byte of a multibyte sequence.
.Sh RETURN VALUES
The
.Fn rewind
function
returns no value.
.Pp
.Rv -std fgetpos fseek fseeko fsetpos
.Pp
Upon successful completion,
.Fn ftell
and
.Fn ftello
return the current offset.
Otherwise, \-1 is returned and the global variable
.Va errno
is set to indicate the error.
.Sh ERRORS
.Bl -tag -width Er
.It Bq Er EBADF
The
.Fa stream
argument
is not a seekable stream.
.It Bq Er EINVAL
The
.Fa whence
argument is invalid or
the resulting file-position
indicator would be set to a negative value.
.It Bq Er EOVERFLOW
The resulting file offset would be a value which
cannot be represented correctly in an object of type
.Fa off_t
for
.Fn fseeko
and
.Fn ftello
or
.Fa long
for
.Fn fseek
and
.Fn ftell .
.It Bq Er ESPIPE
The file descriptor underlying stream is associated with a pipe or FIFO
or file-position indicator value is unspecified
(see
.Xr ungetc 3 ) .
.El
.Pp
The functions
.Fn fgetpos ,
.Fn fseek ,
.Fn fseeko ,
.Fn fsetpos ,
.Fn ftell ,
.Fn ftello ,
and
.Fn rewind
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 ,
.Xr clearerr 3 ,
.Xr fwide 3 ,
.Xr ungetc 3 ,
.Xr ungetwc 3
.Sh STANDARDS
The
.Fn fgetpos ,
.Fn fsetpos ,
.Fn fseek ,
.Fn ftell ,
and
.Fn rewind
functions
conform to
.St -isoC .
.Pp
The
.Fn fseeko
and
.Fn ftello
functions conform to
.St -p1003.1-2001 .
.Sh HISTORY
The functions
.Fn fseek ,
.Fn ftell ,
and
.Fn rewind
first appeared in
.At v7 .