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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:
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 existing end-of-file 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 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 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
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.
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, POSIX, 4.3BSD
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 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.
SEE ALSO
dup(2), fork(2), open(2), fseek(3), lseek64(3)