lseek -- reposition read/write file offset
Standard C Library (libc, -lc)
lseek(int fildes, off_t offset, int whence);
The lseek() system call repositions the offset of the file descriptor
fildes to the argument offset according to the directive whence. The
argument fildes must be an open file descriptor. The lseek() system call
repositions the file position pointer associated with the file descriptor
fildes as follows:
If whence is SEEK_SET, the offset is set to offset bytes.
If whence is SEEK_CUR, the offset is set to its current location
plus offset bytes.
If whence is SEEK_END, the offset is set to the size of the file
plus offset bytes.
The lseek() system call allows the file offset to be set beyond the end
of the existing end-of-file 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).
Some devices are incapable of seeking. The value of the pointer associated
with such a device is undefined.
Upon successful completion, lseek() returns the resulting offset location
as measured in bytes from the beginning of the file. Otherwise, a value
of -1 is returned and errno is set to indicate the error.
The lseek() system call will fail and the file position pointer will
remain unchanged if:
[EBADF] The fildes argument is not an open file descriptor.
[EINVAL] The whence argument is not a proper value or the
resulting file offset would be negative for a noncharacter
[EOVERFLOW] The resulting file offset would be a value which cannot
be represented correctly in an object of type
[ESPIPE] The fildes argument is associated with a pipe, socket,
This document's use of whence is incorrect English, but is maintained for
The lseek() system call is expected to conform to ISO/IEC 9945-1:1990
The lseek() function appeared in Version 7 AT&T UNIX.
FreeBSD 5.2.1 April 19, 1994 FreeBSD 5.2.1 [ Back ]