unload(3tcl) Tcl Built-In Commands unload(3tcl)
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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 ar-
gument 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 de-
faults 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 in-
crements 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 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 in-
terpreter 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 re-
turned. 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 inter-
preter 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_ER-
ROR 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 at-
tached to the process after the unload procedure returns (i.e. because
the library is used by other interpreters), TCL_UNLOAD_DETACH_FROM_IN-
TERPRETER will be defined. However, if the library is used only by the
target interpreter and the library will be detached from the applica-
tion 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 -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 in-
terpreters 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(3tcl), safe(3tcl)
KEYWORDS
binary code, unloading, safe interpreter, shared library
Tcl 8.5 unload(3tcl)
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