lseek - reposition read/write file offset
The lseek(2) function repositions the offset of the file descriptor fildes to the argument offset according to the directive whence as follows:
The lseek(2) 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 and POSIX does not specify which devices must support it.
Linux specific restrictions: using lseek(2) on a tty device returns ESPIPE. Other systems return the number of written characters, using SEEK_SET to set the counter. Some devices, e.g. /dev/null do not cause the error ESPIPE, but return a pointer which value is undefined.
This document's use of whence is incorrect English, but maintained for historical (hysterical?) reasons.
When converting old code, substitute values for whence with the following macros:
SVR1-3 returns long instead of off_t, BSD returns int.