lseek — reposition read/write file offset
#include <sys/types.h> #include <unistd.h>
off_t
lseek( |
int | fildes, |
off_t | offset, | |
int | whence) ; |
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:
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.
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.
fildes
is
not an open file descriptor.
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.
The resulting file offset cannot be represented in
an off_t
.
fildes
is
associated with a pipe, socket, or FIFO.
This document's use of whence
is incorrect English,
but maintained for historical reasons.
Some devices are incapable of seeking and POSIX does not
specify which devices must support lseek
().
On Linux, using lseek
() on a
tty device returns ESPIPE.
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.
dup(2), fork(2), open(2), fseek(3), lseek64(3), posix_fallocate(3)
|