LSEEK(2)                         System Calls                         LSEEK(2)

NAME

       lseek - reposition read/write file offset

SYNOPSIS

       #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.  The argument
       fildes  must  be  an  open file descriptor.  Lseek repositions the file
       pointer 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
       (until data is actually written into the gap).

       Some devices are incapable of seeking.  The value of the pointer  asso‐
       ciated with such a device is undefined.

RETURN VALUES

       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.

ERRORS

       Lseek will fail and the file 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.

SEE ALSO

       dup(2), open(2)

BUGS

       In  the GNO implementation, doing an lseek past the end of file is per‐
       mitted, but the created gap (as mentioned above) is not  gauranteed  to
       contain bytes of zeros.

       This  document's  use of whence is incorrect English, but is maintained
       for historical reasons.

STANDARDS

       The lseek function conforms to IEEE Std 1003.1-1988 (POSIX).



GNO                             22 January 1997                       LSEEK(2)