lseek - reposition read/write file offset
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 as follows:
The offset is set to offset bytes.
The offset is set to its current location plus off
The offset is set to the size of the file plus off
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 actu
ally written into the gap).
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.
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.
SVr4, POSIX, BSD 4.3
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. Other systems return the number of writ
return a pointer which value is undefined.
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:
SVR1-3 returns long instead of off_t, BSD returns int.
dup(2), open(2), fseek(3)