Unix/Linux Go Back    


Linux 2.6 - man page for lseek (linux section 3posix)

Linux & Unix Commands - Search Man Pages
Man Page or Keyword Search:   man
Select Man Page Set:       apropos Keyword Search (sections above)


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

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  off-
	  set.

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

       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 actually 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 unspeci-
       fied.

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

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  implementa-
       tion-defined  and  device-dependent  (for  example, memory may have few invalid values). A
       negative file offset may be valid for some devices in some implementations.

       The POSIX.1-1990 standard did not specifically prohibit lseek() from returning a  negative
       offset.	Therefore, an application 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 resulting 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, Copyright (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 orig-
       inal Standard can be obtained online at http://www.opengroup.org/unix/online.html .

IEEE/The Open Group			       2003					 LSEEK(P)
Unix & Linux Commands & Man Pages : ©2000 - 2018 Unix and Linux Forums


All times are GMT -4. The time now is 02:21 AM.