Skip site navigation (1)Skip section navigation (2)

FreeBSD Manual Pages


home | help
LSEEK(2)		    BSD	System Calls Manual		      LSEEK(2)

     lseek -- reposition read/write file offset

     Standard C	Library	(libc, -lc)

     #include <unistd.h>

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

     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 of the start of the next hole
	   greater than	or equal to the	supplied offset	is returned.  The def-
	   inition 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).

     Some devices are incapable	of seeking.  The value of the pointer associ-
     ated with such a device is	undefined.

     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.  The existence
     of	a hole at the end of every data	region allows for easy programming and
     implies that a virtual hole exists	at the end of the file.	 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.

     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.

     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.  For SEEK_HOLE, there are no more
			holes past the supplied	offset.

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

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

     dup(2), open(2), pathconf(2)

     The lseek() system	call is	expected to conform to ISO/IEC 9945-1:1990

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

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

BSD				 April 5, 2007				   BSD


Want to link to this manual page? Use this URL:

home | help