LSEEK(2) Linux Programmer's Manual LSEEK(2)
NAME
lseek - reposition read(2,n,1 builtins)/write(1,2) file(1,n) 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 file(1,n) descriptor fildes
to the argument offset according to the directive whence as follows:
SEEK_SET
The offset is set(7,n,1 builtins) to offset bytes.
SEEK_CUR
The offset is set(7,n,1 builtins) to its current location plus offset bytes.
SEEK_END
The offset is set(7,n,1 builtins) to the size of the file(1,n) plus offset bytes.
The lseek function allows the file(1,n) offset to be set(7,n,1 builtins) beyond the end of
the existing end-of-file of the file(1,n) (but this does not change the size
of the file(1,n)). If data is later written at this point, subsequent reads
of the data in(1,8) the gap return bytes of zeros (until data is actually
written into the gap).
RETURN VALUE
Upon successful completion, lseek returns the resulting offset location
as measured in(1,8) bytes from the beginning of the file. Otherwise, a
value of (off_t)-1 is returned and errno is set(7,n,1 builtins) to indicate the error.
ERRORS
EBADF fildes is not an open(2,3,n) file(1,n) descriptor.
EINVAL whence is not one of SEEK_SET, SEEK_CUR, SEEK_END, or the
resulting file(1,n) offset would be negative.
EOVERFLOW
The resulting file(1,n) offset cannot be represented in(1,8) an off_t.
ESPIPE fildes is associated with a pipe(2,8), socket(2,7,n), or FIFO.
CONFORMING TO
SVr4, POSIX, BSD 4.3
RESTRICTIONS
Some devices are incapable of seeking and POSIX does not specify which
devices must support it.
Linux specific restrictions: using lseek on a tty(1,4) 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 follow-
ing 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(1,n) descriptors created by dup(2) or fork(2) share the cur-
rent file(1,n) position pointer, so seeking on such files may be subject to
race conditions.
SEE ALSO
dup(2), fork(2), open(2,3,n)(2), fseek(3)
Linux 2001-09-24 LSEEK(2)