bionic (3) lseek.3posix.gz

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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.1‐2008, <sys_types.h>, <unistd.h>


       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 Electrical 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.kernel.org/doc/man-pages/reporting_bugs.html .