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 offset
The offset is set to the size of the file plus offset
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 writ-
ten 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.
Fildes is not an open file descriptor.
Fildes is associated with a pipe, socket, or FIFO.
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 written
characters, using SEEK_SET to set the counter. Some dev-
ices, 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 reasons.
When converting old code, substitute values for whence with
the following macros:
c c l l. old new 0 SEEK_SET 1 SEEK_CUR 2 SEEK_END
L_SET SEEK_SET L_INCR SEEK_CUR L_XTND SEEK_END
SVR1-3 returns long instead of off_t, BSD returns int.
dup(2), open(2), fseek(3)