redhat man page for lseek

LSEEK(2)							   System calls 							  LSEEK(2)

NAME
       lseek - reposition read/write file offset

SYNOPSIS
       #include <sys/types.h>
       #include <unistd.h>

       off_t lseek(int fildes, off_t offset, int whence);

DESCRIPTION
       The  lseek  function  repositions the offset of the file descriptor fildes to the argument offset according to the directive whence as fol-
       lows:

       SEEK_SET
	      The offset is set to offset bytes.

       SEEK_CUR
	      The offset is set to its current location plus offset bytes.

       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).

RETURN VALUE
       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.

ERRORS
       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.

CONFORMING TO
       SVr4, POSIX, BSD 4.3

RESTRICTIONS
       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 devices, e.g. /dev/null do not cause the error ESPIPE, but return a pointer which value is undefined.

NOTES
       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:

	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.

       Note that file descriptors created by dup(2) or fork(2) share the current file position pointer, so seeking on such files may be subject to
       race conditions.

SEE ALSO
       dup(2), fork(2), open(2), fseek(3)

Linux								    2001-09-24								  LSEEK(2)