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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 dif-
       fer (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 descrip-
       tor 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, (off_t)-1 shall be returned, errno shall be set to indicate the error, and the file  off-
       set 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 IEEE Std 1003.1-2001 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 IEEE Std 1003.1-2001, <sys/types.h>, <unistd.h>

COPYRIGHT
       Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003 Edition, Stan-
       dard  for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifica-
       tions Issue 6, Copyright (C) 2001-2003 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 Stan-
       dard, 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 .



IEEE/The Open Group                  2003                            LSEEK(3P)