lseek() - Unix, Linux System Call

NAME

lseek - reposition read/write file offset

SYNOPSIS

#include <sys/types.h>
#include <unistd.h>

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

DESCRIPTION

The lseek() function repositions the offset of the open file associated with the file descriptor fildes to the argument offset according to the directive whence as follows:

Tag	Description
SEEK_SET
	The offset is set to offset bytes.
SEEK_CUR
	The offset is set to its current location plus offset bytes.
SEEK_END
	The offset is set to the size of the file plus offset bytes.

The lseek() function allows the file offset to be set beyond the end of the file (but this does not change the size of the file). If data is later written at this point, subsequent reads of the data in the gap (a "hole") return null bytes (’\0’) until data is actually written into the gap.

RETURN VALUE

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

ERRORS

Tag	Description
EBADF	fildes is not an open file descriptor.
EINVAL	whence is not one of SEEK_SET, SEEK_CUR, SEEK_END; or the resulting file offset would be negative, or beyond the end of a seekable device.
EOVERFLOW
	The resulting file offset cannot be represented in an off_t.
ESPIPE	fildes is associated with a pipe, socket, or FIFO.

CONFORMING TO

SVr4, 4.3BSD, POSIX.1-2001.

RESTRICTIONS

Some devices are incapable of seeking and POSIX does not specify which devices must support lseek().

Linux specific restrictions: using lseek() on a tty device returns ESPIPE.

NOTES

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

When converting old code, substitute values for whence with the following macros:

old	new
0	SEEK_SET
1	SEEK_CUR
2	SEEK_END
L_SET	SEEK_SET
L_INCR	SEEK_CUR
L_XTND	SEEK_END

SVr1-3 returns long instead of off_t, BSD returns int.

Note that file descriptors created by dup(2) or fork(2) share the current file position pointer, so seeking on such files may be subject to race conditions.