#include <unistd.h> off_t 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 po- sition 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() 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 (un- til 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 position 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.
This document's use of whence is incorrect English, but is maintained for historical reasons.
The lseek() function call is expected to conform to IEEE Std1003.1-1990 (``POSIX'').