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 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 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  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 POSIX.1‐2017, <sys_types.h>, <unistd.h>

COPYRIGHT

       Portions of this text are reprinted and  reproduced  in  electronic  form  from  IEEE  Std
       1003.1-2017,  Standard  for  Information Technology -- Portable Operating System Interface
       (POSIX), The Open Group Base Specifications Issue 7, 2018 Edition, Copyright (C)  2018  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.opengroup.org/unix/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.kernel.org/doc/man-pages/reporting_bugs.html .