unload(n) Tcl Built-In Commands unload(n)______________________________________________________________________________NAMEunload - Unload machine code
SYNOPSISunload ?switches? fileName
unload ?switches? fileName packageName
unload ?switches? fileName packageName interp
_________________________________________________________________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 packageName
argument is the name of the package (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 sys‐
tem 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 com‐
mand, 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 contain‐
ing 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 deter‐
mined by packageName and whether or not the target interpreter is a
safe one. For normal interpreters the name of the initialization pro‐
cedure will have the form pkg_Unload, where pkg is the same as package‐
Name except that the first letter is converted to upper case and all
other letters are converted to lower case. For example, if packageName
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 func‐
tionality 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 tar‐
get 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 packa‐
geName argument must be specified.
If packageName is omitted or specified as an empty string, Tcl tries to
guess the name of the package. This may be done differently on differ‐
ent platforms. The default guess, which is used on most UNIX plat‐
forms, 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 as the module name. For example, the command
unload libxyz4.2.so uses the module name xyz and the command unload
bin/last.so {} uses the module name last.
PORTABILITY ISSUES
Unix
Not all unix operating systems support library unloading. Under
such an operating system unload returns an error (unless -nocom‐
plain 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) with‐
out having to shut down the overall Tcl process.
SEE ALSO
info sharedlibextension, load(n), safe(n)KEYWORDS
binary code, unloading, safe interpreter, shared library
Tcl 8.5 unload(n)
