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LSEEK(2)		  FreeBSD System Calls Manual		      LSEEK(2)

NAME
     lseek -- reposition read/write file offset

LIBRARY
     Standard C	Library	(libc, -lc)

SYNOPSIS
     #include <unistd.h>

     off_t
     lseek(int fildes, off_t offset, int whence);

DESCRIPTION
     The lseek() system	call repositions the offset of the file	descriptor
     fildes to the argument offset according to	the directive whence.  The ar-
     gument fildes must	be an open file	descriptor.  The lseek() system	call
     repositions the file position pointer associated with the file descriptor
     fildes as follows:

	   If whence is	SEEK_SET, the offset is	set to offset bytes.

	   If whence is	SEEK_CUR, the offset is	set to its current location
	   plus	offset bytes.

	   If whence is	SEEK_END, the offset is	set to the size	of the file
	   plus	offset bytes.

	   If whence is	SEEK_HOLE, the offset is set to	the start of the next
	   hole	greater	than or	equal to the supplied offset.  The definition
	   of a	hole is	provided below.

	   If whence is	SEEK_DATA, the offset is set to	the start of the next
	   non-hole file region	greater	than or	equal to the supplied offset.

     The lseek() system	call allows the	file offset to be set beyond the end
     of	the existing end-of-file of the	file.  If data is later	written	at
     this point, subsequent reads of the data in the gap return	bytes of zeros
     (until data is actually written into the gap).  However, the lseek() sys-
     tem call does not,	by itself, extend the size of a	file.

     A "hole" is defined as a contiguous range of bytes	in a file, all having
     the value of zero,	but not	all zeros in a file are	guaranteed to be rep-
     resented as holes returned	with SEEK_HOLE.	 File systems are allowed to
     expose ranges of zeros with SEEK_HOLE, but	not required to.  Applications
     can use SEEK_HOLE to optimise their behavior for ranges of	zeros, but
     must not depend on	it to find all such ranges in a	file.  Each file is
     presented as having a zero-size virtual hole at the very end of the file.
     The existence of a	hole at	the end	of every data region allows for	easy
     programming and also provides compatibility to the	original implementa-
     tion in Solaris.  It also causes the current file size (i.e., end-of-file
     offset) to	be returned to indicate	that there are no more holes past the
     supplied offset.  Applications should use fpathconf(_PC_MIN_HOLE_SIZE) or
     pathconf(_PC_MIN_HOLE_SIZE) to determine if a file	system supports
     SEEK_HOLE.	 See pathconf(2).

     For file systems that do not supply information about holes, the file
     will be represented as one	entire data region.

RETURN VALUES
     Upon successful completion, lseek() returns the resulting offset location
     as	measured in bytes from the beginning of	the file.  Otherwise, a	value
     of	-1 is returned and errno is set	to indicate the	error.

ERRORS
     The lseek() system	call will fail and the file position pointer will re-
     main unchanged if:

     [EBADF]		The fildes argument is not an open file	descriptor.

     [EINVAL]		The whence argument is not a proper value or the re-
			sulting	file offset would be negative for a non-char-
			acter special file.

     [ENXIO]		For SEEK_DATA, there are no more data regions past the
			supplied offset.  Due to existence of the hole at the
			end of the file, for SEEK_HOLE this error is only re-
			turned when the	offset already points to the end-of-
			file position.

     [EOVERFLOW]	The resulting file offset would	be a value which can-
			not be represented correctly in	an object of type
			off_t.

     [ESPIPE]		The fildes argument is associated with a pipe, socket,
			or FIFO.

SEE ALSO
     dup(2), open(2), pathconf(2)

STANDARDS
     The lseek() system	call is	expected to conform to IEEE Std	1003.1-2008
     ("POSIX.1").

     The SEEK_HOLE and SEEK_DATA directives, along with	the ENXIO error, are
     extensions	to that	specification.

HISTORY
     The lseek() function appeared in Version 7	AT&T UNIX.

BUGS
     If	the lseek() system call	is operating on	a device which is incapable of
     seeking, it will request the seek operation and return successfully, even
     though no seek was	performed.  Because the	offset argument	will be	stored
     unconditionally in	the file descriptor of that device, there is no	way to
     confirm if	the seek operation succeeded or	not (e.g. using	the ftell()
     function).	 Device	types which are	known to be incapable of seeking in-
     clude tape	drives.

     The lseek() system	call will not detect whether media are present in
     changeable	media devices such as DVD or Blu-ray devices.  A requested
     seek operation will therefore return sucessfully when no medium is
     present.

     This document's use of whence is incorrect	English, but is	maintained for
     historical	reasons.

FreeBSD	13.0			 July 13, 2020			  FreeBSD 13.0

NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUES | ERRORS | SEE ALSO | STANDARDS | HISTORY | BUGS

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