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LSEEK(2)		       System Calls Manual			LSEEK(2)

NAME
     lseek -- reposition read/write file offset

LIBRARY
     Standard C Library (libc, -lc)

SYNOPSIS
     #include <unistd.h>

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

DESCRIPTION
     The  lseek()  system  call  repositions  the  offset of the file descriptor
     fildes to the argument offset according to the directive whence.  The argu-
     ment fildes must be an open file descriptor.  The lseek() system call repo-
     sitions the file position	pointer  associated  with  the	file  descriptor
     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.

	   If  whence  is  SEEK_HOLE, the offset is set to the start of the next
	   hole greater than or equal to the supplied offset.  The definition of
	   a hole is provided below.

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

     The  lseek() system call 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).	However, the lseek() system call
     does not, by itself, extend the size of a file.

     A "hole" is defined as a contiguous range of bytes in a  file,  all  having
     the  value of zero, but not all zeros in a file are guaranteed to be repre-
     sented as holes returned with SEEK_HOLE.  File systems are allowed  to  ex-
     pose ranges of zeros with SEEK_HOLE, but not required to.	Applications 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.  Each file is presented  as
     having a zero-size virtual hole at the very end of the file.  The existence
     of  a  hole at the end of every data region allows for easy programming and
     also provides compatibility to the original implementation in Solaris.   It
     also causes the current file size (i.e., end-of-file offset) to be returned
     to  indicate that there are no more holes past the supplied offset.  Appli-
     cations	should	   use	   fpathconf(_PC_MIN_HOLE_SIZE)     or	   path-
     conf(_PC_MIN_HOLE_SIZE)  to  determine if a file system supports SEEK_HOLE.
     See pathconf(2).

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

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
     The lseek() system call will fail and the file position pointer will remain
     unchanged if:

     [EBADF]		The fildes argument is not an open file descriptor.

     [EINVAL]		The whence argument is not a proper value or the result-
			ing file offset would be negative  for	a  non-character
			special file.

     [ENXIO]		For  SEEK_DATA,  there are no more data regions past the
			supplied offset.  Due to existence of the  hole  at  the
			end  of  the  file, for SEEK_HOLE this error is only re-
			turned when the offset already points to the end-of-file
			position.

     [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, socket,
			or FIFO.

SEE ALSO
     dup(2), open(2), pathconf(2)

STANDARDS
     The lseek() system call is expected to  conform  to  IEEE	Std  1003.1-2008
     ("POSIX.1").

     The SEEK_HOLE and SEEK_DATA directives, along with the ENXIO error, are ex-
     tensions to that specification.

HISTORY
     The lseek() function appeared in Version 7 AT&T UNIX.

BUGS
     If  the  lseek() system call is operating on a device which is incapable of
     seeking, it will request the seek operation and return  successfully,  even
     though  no  seek was performed.  Because the offset argument will be stored
     unconditionally in the file descriptor of that device, there is no  way  to
     confirm  if  the  seek  operation	succeeded or not (e.g. using the ftell()
     function).  Device types which are known to be incapable of seeking include
     tape drives.

     The lseek() system call will  not	detect	whether  media	are  present  in
     changeable  media devices such as DVD or Blu-ray devices.	A requested seek
     operation will therefore return sucessfully when no medium is present.

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

FreeBSD ports 15.1		  July 13, 2020 			LSEEK(2)

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