LSEEK(2)							      LSEEK(2)

NAME
       lseek - move read/write file pointer

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

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

DESCRIPTION
       The  lseek()  function  sets  the file pointer associated with the open
       file descriptor specified by fildes as follows:

	   o	  If whence is SEEK_SET, the pointer is set to offset bytes.

	   o	  If whence is SEEK_CUR, the pointer is	 set  to  its  current
		  location plus offset.

	   o	  If whence is SEEK_END, the pointer is set to the size of the
		  file plus offset.

	   o	  If whence is SEEK_HOLE, the offset of the start of the  next
		  hole	greater	 than  or  equal  to  the  supplied  offset is
		  returned. The definition of a hole is provided near the  end
		  of the DESCRIPTION.

	   o	  If whence is SEEK_DATA, the file pointer is set to the start
		  of the next non-hole file region greater than	 or  equal  to
		  the supplied offset.

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

       Some devices are incapable of seeking. The value of  the	 file  pointer
       associated with such a device is undefined.

       The  lseek()  function  allows  the  file  pointer to be set beyond the
       existing data in the file. If data are later  written  at  this	point,
       subsequent  reads  in  the gap between the previous end of data and the
       newly written data will return bytes of value 0 until data are  written
       into the gap.

       If  fildes  is a remote file descriptor and offset is negative, lseek()
       returns the file pointer	 even if it is negative. The lseek()  function
       will not, by itself, extend the size of a file.

       If  fildes  refers  to  a  shared  memory object, lseek() behaves as if
       fildes referred to a regular file.

       A "hole" is defined as a contiguous range of bytes in a file, all  hav‐
       ing the value of zero, but not all zeros in a file are guaranteed to be
       represented as holes returned with SEEK_HOLE. Filesystems  are  allowed
       to expose ranges of zeros with SEEK_HOLE, but not required to. Applica‐
       tions can use SEEK_HOLE to optimise their behavior for ranges of zeros,
       but  must not depend on it to find all such ranges in a file. The exis‐
       tence of a hole at the end of every data region allows  for  easy  pro‐
       gramming and implies that a virtual hole exists at the end of the file.
       Applications   should   use   fpathconf(_PC_MIN_HOLE_SIZE)   or	 path‐
       conf(_PC_MIN_HOLE_SIZE)	 to   determine	  if   a  filesystem  supports
       SEEK_HOLE. See fpathconf(2).

       For filesystems that do not supply information about  holes,  the  file
       will be represented as one entire data region.

RETURN VALUES
       Upon  successful completion, the resulting offset, as measured in bytes
       from the beginning of the file, is returned.  Otherwise,	 (off_t)−1  is
       returned,  the file offset remains unchanged, and errno is set to indi‐
       cate the error.

ERRORS
       The lseek() function will fail if:

       EBADF
		    The fildes argument is not an open file descriptor.

       EINVAL
		    The	 whence	 argument  is  not  SEEK_SET,	SEEK_CUR,   or
		    SEEK_END;  or  the	fildes	argument  is not a remote file
		    descriptor and the resulting file pointer would  be	 nega‐
		    tive.

       ENXIO
		    For	 SEEK_DATA,  there  are	 no more data regions past the
		    supplied offset.  For SEEK_HOLE, there are no  more	 holes
		    past the supplied offset.

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

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

USAGE
       The lseek() function has a transitional interface for 64-bit file  off‐
       sets.  See lf64(5).

       In  multithreaded  applications,	 using	lseek()	 in conjunction with a
       read(2) or write(2) call on a file descriptor shared by more  than  one
       thread is not an atomic operation.  To ensure atomicity, use pread() or
       pwrite().

ATTRIBUTES
       See attributes(5) for descriptions of the following attributes:

       ┌────────────────────┬───────────────────┐
       │  ATTRIBUTE TYPE    │  ATTRIBUTE VALUE	│
       ├────────────────────┼───────────────────┤
       │Interface Stability │ Standard		│
       ├────────────────────┼───────────────────┤
       │MT-Level	    │ Async-Signal-Safe │
       └────────────────────┴───────────────────┘

SEE ALSO
       creat(2), dup(2), fcntl(2), fpathconf(2), open(2),  read(2),  write(2),
       attributes(5), lf64(5), standards(5)

				  May 4, 2005			      LSEEK(2)

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