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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 (consult 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 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 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, -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 rep-
              resented 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 POSIX.1-2008 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 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  indica-
       tion  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 POSIX.1-2008, <sys_types.h>, <unistd.h>
COPYRIGHT
       Portions of this text are reprinted and reproduced in  electronic  form
       from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
       -- Portable Operating System Interface (POSIX),  The  Open  Group  Base
       Specifications Issue 7, Copyright (C) 2013 by the Institute of Electri-
       cal and Electronics Engineers,  Inc  and  The  Open  Group.   (This  is
       POSIX.1-2008  with  the  2013  Technical Corrigendum 1 applied.) 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.unix.org/online.html .
       Any  typographical  or  formatting  errors that appear in this page are
       most likely to have been introduced during the conversion of the source
       files  to  man page format. To report such errors, see https://www.ker-
       nel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group                  2013                            LSEEK(3P)