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 follows:

       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.

       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  cur-
       rent  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)