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_SETThe offset is set to offset bytes.
SEEK_CURThe offset is set to its current location plus
offset
bytes.
SEEK_ENDThe 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)
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