PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT

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

PROLOG         top

       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         top

       lseek — move the read/write file offset

SYNOPSIS         top

       #include <unistd.h>

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

DESCRIPTION         top

       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         top

       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         top

       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         top

       None.

APPLICATION USAGE         top

       None.

RATIONALE         top

       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         top

       None.

SEE ALSO         top

       open(3p)

       The Base Definitions volume of POSIX.1‐2008, sys_types.h(0p),
       unistd.h(0p)

COPYRIGHT         top

       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 .

IEEE/The Open Group                 2013                           LSEEK(3P)