LSEEK(3)                   POSIX Programmer's Manual                  LSEEK(3)



PROLOG
       This manual page is part of the POSIX Programmer's Manual.  The OS
       implementation of this interface may differ (consult the corresponding
       OS manual page for details of OS behavior), or the interface may not be
       implemented on OS.

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 .



IEEE/The Open Group                  2017                             LSEEK(3)
