path: root/doc/unload.n

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'\" Copyright (c) 2003 George Petasis <petasis@iit.demokritos.gr>.
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.TH unload n 8.5 Tcl "Tcl Built-In Commands"
.so man.macros
.BS
'\" Note:  do not modify the .SH NAME line immediately below!
.SH NAME
unload \- Unload machine code
.SH SYNOPSIS
\fBunload \fR?\fIswitches\fR? \fIfileName\fR
.br
\fBunload \fR?\fIswitches\fR? \fIfileName prefix\fR
.br
\fBunload \fR?\fIswitches\fR? \fIfileName prefix interp\fR
.BE
.SH DESCRIPTION
.PP
This command tries to unload shared libraries previously loaded
with \fBload\fR from the application's address space.  \fIfileName\fR
is the name of the file containing the library file to be unload;  it
must be the same as the filename provided to \fBload\fR for
loading the library.
The \fIprefix\fR argument is the prefix (as
determined by or passed to \fBload\fR), and is used to
compute the name of the unload procedure; if not supplied, it is
computed from \fIfileName\fR in the same manner as \fBload\fR.
The \fIinterp\fR argument is the path name of the interpreter from
which to unload the package (see the \fBinterp\fR manual entry for
details); if \fIinterp\fR is omitted, it defaults to the
interpreter in which the \fBunload\fR command was invoked.
.PP
If the initial arguments to \fBunload\fR start with \fB\-\fR then
they are treated as switches.  The following switches are
currently supported:
.TP
\fB\-nocomplain\fR
.
Suppresses all error messages. If this switch is given, \fBunload\fR will
never report an error.
.TP
\fB\-keeplibrary\fR
.
This switch will prevent \fBunload\fR from issuing the operating system call
that will unload the library from the process.
.TP
\fB\-\|\-\fR
.
Marks the end of switches.  The argument following this one will
be treated as a \fIfileName\fR even if it starts with a \fB\-\fR.
.SS "UNLOAD OPERATION"
.PP
When a file containing a shared library is loaded through the
\fBload\fR 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 \fBload\fR
call on the file), these counters track how many interpreters use the library.
Each subsequent call to \fBload\fR after the first simply increments the
proper reference count.
.PP
\fBunload\fR works in the opposite direction. As a first step, \fBunload\fR
will check whether the library is unloadable: an unloadable library exports
a special unload procedure. The name of the unload procedure is determined by
\fIprefix\fR and whether or not the target interpreter
is a safe one.  For normal interpreters the name of the initialization
procedure will have the form \fIpfx\fB_Unload\fR, where \fIpfx\fR
is the same as \fIprefix\fR except that the first letter is
converted to upper case and all other letters
are converted to lower case.  For example, if \fIprefix\fR is
\fBfoo\fR or \fBFOo\fR, the initialization procedure's name will
be \fBFoo_Unload\fR.
If the target interpreter is a safe interpreter, then the name
of the initialization procedure will be \fIpkg\fB_SafeUnload\fR
instead of \fIpkg\fB_Unload\fR.
.PP
If \fBunload\fR 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 \fBunload\fR will call the unload
procedure. If the unload procedure returns \fBTCL_OK\fR, \fBunload\fR 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.
.SS "UNLOAD HOOK PROTOTYPE"
.PP
The unload procedure must match the following prototype:
.PP
.CS
typedef int \fBTcl_LibraryUnloadProc\fR(
        Tcl_Interp *\fIinterp\fR,
        int \fIflags\fR);
.CE
.PP
The \fIinterp\fR argument identifies the interpreter from which the
library is to be unloaded.  The unload procedure must return
\fBTCL_OK\fR or \fBTCL_ERROR\fR 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
\fBunload\fR command will be the result returned by the unload procedure.
.PP
The \fIflags\fR argument can be either \fBTCL_UNLOAD_DETACH_FROM_INTERPRETER\fR
or \fBTCL_UNLOAD_DETACH_FROM_PROCESS\fR. 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),
\fBTCL_UNLOAD_DETACH_FROM_INTERPRETER\fR 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 \fIflags\fR argument will be set to \fBTCL_UNLOAD_DETACH_FROM_PROCESS\fR.
.SS NOTES
.PP
The \fBunload\fR command cannot unload libraries that are statically
linked with the application.
If \fIfileName\fR is an empty string, then the \fIprefix\fR argument must
be specified.
.PP
If \fIprefix\fR 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
\fIfileName\fR, strip off the first three characters if they
are \fBlib\fR, then strip off the next three characters if they
are \fBtcl9\fR, and use any following wordchars but not digits,
converted to titlecase as the prefix.
For example, the command \fBunload libxyz4.2.so\fR uses the prefix
\fBXyz\fR and the command \fBunload bin/last.so {}\fR uses the
prefix \fBLast\fR.
.SH "PORTABILITY ISSUES"
.TP
\fBUnix\fR\0\0\0\0\0
.
Not all unix operating systems support library unloading. Under such
an operating system \fBunload\fR returns an error (unless \fB\-nocomplain\fR
has been specified).
.SH BUGS
.PP
If the same file is \fBload\fRed by different \fIfileName\fRs, 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
\fBunload\fR on such a library (as the library will be completely detached
from the application while some interpreters will continue to use it).
.SH EXAMPLE
.PP
If an unloadable module in the file \fBfoobar.dll\fR had been loaded
using the \fBload\fR command like this (on Windows):
.PP
.CS
load c:/some/dir/foobar.dll
.CE
.PP
then it would be unloaded like this:
.PP
.CS
\fBunload\fR c:/some/dir/foobar.dll
.CE
.PP
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.
.SH "SEE ALSO"
info sharedlibextension, load(n), safe(n)
.SH KEYWORDS
binary code, unloading, safe interpreter, shared library
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