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.