lseek

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
              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>

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 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)