LSEEK(3P)                                           POSIX Programmer's Manual                                          LSEEK(3P)



PROLOG
       This  manual  page is part of the POSIX Programmer's Manual.  The Linux implementation of this interface may differ (con-
       sult the corresponding Linux manual page for details of Linux behavior), or the  interface  may  not  be  implemented  on
       Linux.

NAME
       lseek - move the read/write file offset

SYNOPSIS
       #include <unistd.h>

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


DESCRIPTION
       The  lseek() function shall set the file offset for the open file description associated with the file descriptor fildes,
       as follows:

        * If whence is SEEK_SET, the file offset shall be set to offset bytes.

        * If whence is SEEK_CUR, the file offset shall be set to its current location plus offset.

        * If whence is SEEK_END, the file offset shall be set to the size of the file plus offset.

       The symbolic constants SEEK_SET, SEEK_CUR, and SEEK_END are defined in <unistd.h>.

       The behavior of lseek() on devices which are incapable of seeking is implementation-defined. The value of the file offset
       associated with such a device is undefined.

       The  lseek()  function  shall allow the file offset to be set beyond the end of the existing data in the file. If data is
       later written at this point, subsequent reads of data in the gap shall return bytes with the value 0 until data is  actu-
       ally written into the gap.

       The lseek() function shall not, by itself, extend the size of a file.

       If fildes refers to a shared memory object, the result of the lseek() function is unspecified.

       If fildes refers to a typed memory object, the result of the lseek() function is unspecified.

RETURN VALUE
       Upon successful completion, the resulting offset, as measured in bytes from the beginning of the file, shall be returned.
       Otherwise, (off_t)-1 shall be returned, errno shall be set to indicate the  error,  and  the  file  offset  shall  remain
       unchanged.

ERRORS
       The lseek() function shall fail if:

       EBADF  The fildes argument is not an open file descriptor.

       EINVAL The  whence  argument  is  not  a proper value, or the resulting file offset would be negative for a regular file,
              block special file, or directory.

       EOVERFLOW
              The resulting file offset would be a value which cannot be represented correctly in an object of type off_t.

       ESPIPE The fildes argument is associated with a pipe, FIFO, or socket.


       The following sections are informative.

EXAMPLES
       None.

APPLICATION USAGE
       None.

RATIONALE
       The ISO C standard includes the functions fgetpos() and fsetpos(), which work on very large files by  use  of  a  special
       positioning type.

       Although  lseek()  may position the file offset beyond the end of the file, this function does not itself extend the size
       of the file. While the only function in IEEE Std 1003.1-2001 that may directly extend the size of the  file  is  write(),
       truncate(),  and  ftruncate(), several functions originally derived from the ISO C standard, such as fwrite(), fprintf(),
       and so on, may do so (by causing calls on write()).

       An invalid file offset that would cause [EINVAL] to be returned may be both implementation-defined  and  device-dependent
       (for example, memory may have few invalid values). A negative file offset may be valid for some devices in some implemen-
       tations.

       The POSIX.1-1990 standard did not specifically prohibit lseek() from returning a negative offset. Therefore, an  applica-
       tion  was  required to clear errno prior to the call and check errno upon return to determine whether a return value of (
       off_t)-1 is a negative offset or an indication of an error condition. The standard developers did  not  wish  to  require
       this  action on the part of a conforming application, and chose to require that errno be set to [EINVAL] when the result-
       ing file offset would be negative for a regular file, block special file, or directory.

FUTURE DIRECTIONS
       None.

SEE ALSO
       open(), the Base Definitions volume of IEEE Std 1003.1-2001, <sys/types.h>, <unistd.h>

COPYRIGHT
       Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003  Edition,  Standard  for
       Information  Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 6, Copy-
       right (C) 2001-2003 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. In the event of any
       discrepancy  between this version and the original IEEE and The Open Group Standard, the original IEEE and The Open Group
       Standard  is  the  referee   document.   The   original   Standard   can   be   obtained   online   at   http://www.open-
       group.org/unix/online.html .



IEEE/The Open Group                                           2003                                                     LSEEK(3P)