NAME

       unload - Unload machine code

SYNOPSIS

       unload ?switches? fileName
       unload ?switches? fileName prefix
       unload ?switches? fileName prefix interp
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DESCRIPTION

       This  command tries to unload shared libraries previously loaded with load from the application's address
       space.  fileName is the name of the file containing the library file to be unload;  it must be  the  same
       as  the  filename  provided  to  load  for  loading  the  library.  The prefix argument is the prefix (as
       determined by or passed to load), and is used to compute  the  name  of  the  unload  procedure;  if  not
       supplied,  it is computed from fileName in the same manner as load.  The interp argument is the path name
       of the interpreter from which to unload the package (see the interp manual entry for details); if  interp
       is omitted, it defaults to the interpreter in which the unload command was invoked.

       If  the  initial  arguments  to  unload  start  with  - then they are treated as switches.  The following
       switches are currently supported:

       -nocomplain
              Suppresses all error messages. If this switch is given, unload will never report an error.

       -keeplibrary
              This switch will prevent unload from issuing the  operating  system  call  that  will  unload  the
              library from the process.

       --     Marks  the end of switches.  The argument following this one will be treated as a fileName even if
              it starts with a -.

   UNLOAD OPERATION
       When a file containing a shared library is loaded through the load command, Tcl associates two  reference
       counts  to  the  library  file.  The  first counter shows how many times the library has been loaded into
       normal (trusted) interpreters while the second describes how many times the library has been loaded  into
       safe  interpreters.  As a file containing a shared library can be loaded only once by Tcl (with the first
       load call on the file), these counters track how many interpreters use the library.  Each subsequent call
       to load after the first simply increments the proper reference count.

       unload works in the opposite direction. As a first  step,  unload  will  check  whether  the  library  is
       unloadable: an unloadable library exports a special unload procedure. The name of the unload procedure is
       determined  by  prefix  and whether or not the target interpreter is a safe one.  For normal interpreters
       the name of the initialization procedure will have the form pfx_Unload, where pfx is the same  as  prefix
       except  that  the  first  letter  is converted to upper case and all other letters are converted to lower
       case.  For example, if prefix is foo or FOo, the initialization procedure's name will be Foo_Unload.   If
       the  target  interpreter  is  a  safe  interpreter, then the name of the initialization procedure will be
       pkg_SafeUnload instead of pkg_Unload.

       If unload determines that a library is not unloadable (or unload functionality has been  disabled  during
       compilation),  an error will be returned.  If the library is unloadable, then unload will call the unload
       procedure. If the unload procedure returns TCL_OK, unload will proceed and decrease the proper  reference
       count  (depending on the target interpreter type). When both reference counts have reached 0, the library
       will be detached from the process.

   UNLOAD HOOK PROTOTYPE
       The unload procedure must match the following prototype:

              typedef int Tcl_PackageUnloadProc(
                      Tcl_Interp *interp,
                      int flags);

       The interp argument identifies the interpreter from which the library is  to  be  unloaded.   The  unload
       procedure  must  return TCL_OK or TCL_ERROR to indicate whether or not it completed successfully;  in the
       event of an error it should set the interpreter's result to point to an error message.  In this case, the
       result of the unload command will be the result returned by the unload procedure.

       The flags argument can be either TCL_UNLOAD_DETACH_FROM_INTERPRETER or TCL_UNLOAD_DETACH_FROM_PROCESS. In
       case the library will remain attached to the process after the unload procedure returns (i.e. because the
       library is used by other interpreters), TCL_UNLOAD_DETACH_FROM_INTERPRETER will be defined.  However,  if
       the  library is used only by the target interpreter and the library will be detached from the application
       as   soon   as   the   unload   procedure   returns,   the   flags    argument    will    be    set    to
       TCL_UNLOAD_DETACH_FROM_PROCESS.

   NOTES
       The  unload command cannot unload libraries that are statically linked with the application.  If fileName
       is an empty string, then the prefix argument must be specified.

       If prefix is omitted or specified as an empty string, Tcl tries to guess the prefix.  This  may  be  done
       differently  on  different platforms. The default guess, which is used on most UNIX platforms, is to take
       the last element of fileName, strip off the first three characters if they are lib, and use any following
       alphabetic and underline characters, converted to titlecase as the  prefix.   For  example,  the  command
       unload libxyz4.2.so uses the prefix Xyz and the command unload bin/last.so {} uses the prefix Last.

PORTABILITY ISSUES

       Unix
              Not  all  unix  operating systems support library unloading. Under such an operating system unload
              returns an error (unless -nocomplain has been specified).

BUGS

       If the same file is loaded by different fileNames, it will be loaded into  the  process's  address  space
       multiple times.  The behavior of this varies from system to system (some systems may detect the redundant
       loads,  others  may  not). In case a library has been silently detached by the operating system (and as a
       result Tcl thinks the library is still loaded), it may be dangerous to use unload on such a  library  (as
       the library will be completely detached from the application while some interpreters will continue to use
       it).

EXAMPLE

       If  an  unloadable  module  in  the  file foobar.dll had been loaded using the load command like this (on
       Windows):

              load c:/some/dir/foobar.dll

       then it would be unloaded like this:

              unload c:/some/dir/foobar.dll

       This allows a C code module to be installed temporarily into a long-running Tcl program and then  removed
       again  (either  because it is no longer needed or because it is being updated with a new version) without
       having to shut down the overall Tcl process.

SEE ALSO

       info sharedlibextension, load(3tcl), safe(3tcl)

KEYWORDS

       binary code, unloading, safe interpreter, shared library

Tcl                                                    8.5                                          unload(3tcl)