LSEEK(2) BSD System Calls Manual LSEEK(2)
lseek -- reposition read/write file offset
lseek(int fildes, off_t offset, int whence);
The lseek() function 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. lseek() repositions the file
pointer 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() function 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 associ-
ated 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.
lseek() will fail and the file pointer will remain unchanged if:
[EBADF] fildes is not an open file descriptor.
[ESPIPE] fildes is associated with a pipe, socket, or FIFO.
[EINVAL] whence is not a proper value or the resulting offset
would be negative on a file system or special device
that does not allow negative offsets to be used.
The lseek() function conforms to IEEE Std 1003.1-2008 (``POSIX.1'').
A seek() system call first appeared in Version 1 AT&T UNIX. In Version 7
AT&T UNIX it was renamed to lseek() for ``long seek'' due to a larger
offset argument type.
This document's use of whence is incorrect English, but is maintained for
BSD March 30, 2017 BSD