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.TH load n 7.5 Tcl "Tcl Built-In Commands"
.so man.macros
.BS
'\" Note:  do not modify the .SH NAME line immediately below!
.SH NAME
load \- Load machine code and initialize new commands
.SH SYNOPSIS
\fBload\fR ?\fB\-global\fR? ?\fB\-lazy\fR? ?\fB\-\-\fR? \fIfileName\fR
.br
\fBload\fR ?\fB\-global\fR? ?\fB\-lazy\fR? ?\fB\-\-\fR? \fIfileName prefix\fR
.br
\fBload\fR ?\fB\-global\fR? ?\fB\-lazy\fR? ?\fB\-\-\fR? \fIfileName prefix interp\fR
.BE
.SH DESCRIPTION
.PP
This command loads binary code from a file into the
application's address space and calls an initialization procedure
in the library to incorporate it into an interpreter.  \fIfileName\fR
is the name of the file containing the code;  its exact form varies
from system to system but on most systems it is a shared library,
such as a \fB.so\fR file under Solaris or a DLL under Windows.
\fIprefix\fR is used to compute the name of an initialization procedure.
\fIinterp\fR is the path name of the interpreter into which to load
the library (see the \fBinterp\fR manual entry for details);
if \fIinterp\fR is omitted, it defaults to the
interpreter in which the \fBload\fR command was invoked.
.PP
Once the file has been loaded into the application's address space,
one of two initialization procedures will be invoked in the new code.
Typically the initialization procedure will add new commands to a
Tcl interpreter.
The name of the initialization 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_Init\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_Init\fR.
.PP
If the target interpreter is a safe interpreter, then the name
of the initialization procedure will be \fIpfx\fB_SafeInit\fR
instead of \fIpfx\fB_Init\fR.
The \fIpfx\fB_SafeInit\fR function should be written carefully, so that it
initializes the safe interpreter only with partial functionality provided
by the library that is safe for use by untrusted code. For more information
on Safe\-Tcl, see the \fBsafe\fR manual entry.
.PP
The initialization procedure must match the following prototype:
.PP
.CS
typedef int \fBTcl_PackageInitProc\fR(
        Tcl_Interp *\fIinterp\fR);
.CE
.PP
The \fIinterp\fR argument identifies the interpreter in which the
library is to be loaded.  The initialization 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.  The result of the \fBload\fR command
will be the result returned by the initialization procedure.
.PP
The actual loading of a file will only be done once for each \fIfileName\fR
in an application.  If a given \fIfileName\fR is loaded into multiple
interpreters, then the first \fBload\fR will load the code and
call the initialization procedure;  subsequent \fBload\fRs will
call the initialization procedure without loading the code again.
For Tcl versions lower than 8.5, it is not possible to unload or reload a
library. From version 8.5 however, the \fBunload\fR command allows the unloading
of libraries loaded with \fBload\fR, for libraries that are aware of the
Tcl's unloading mechanism.
.PP
The \fBload\fR command also supports libraries that are statically
linked with the application, if those libraries have been registered
by calling the \fBTcl_StaticPackage\fR procedure.
If \fIfileName\fR is an empty string, then \fIprefix\fR 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, and use any following alphabetic and
underline characters, converted to titlecase as the prefix.
For example, the command \fBload libxyz4.2.so\fR uses the prefix
\fBXyz\fR and the command \fBload bin/last.so {}\fR uses the
prefix \fBLast\fR.
.PP
If \fIfileName\fR is an empty string, then \fIprefix\fR must
be specified.
The \fBload\fR command first searches for a statically loaded library
(one that has been registered by calling the \fBTcl_StaticPackage\fR
procedure) by that name; if one is found, it is used.
Otherwise, the \fBload\fR command searches for a dynamically loaded
library by that name, and uses it if it is found.  If several
different files have been \fBload\fRed with different versions of
the library, Tcl picks the file that was loaded first.
.PP
If \fB\-global\fR is specified preceding the filename, all symbols
found in the shared library are exported for global use by other
libraries. The option \fB\-lazy\fR delays the actual loading of
symbols until their first actual use. The options may be abbreviated.
The option \fB\-\-\fR indicates the end of the options, and should
be used if you wish to use a filename which starts with \fB\-\fR
and you provide a prefix to the \fBload\fR command.
.PP
On platforms which do not support the \fB\-global\fR or \fB\-lazy\fR
options, the options still exist but have no effect. Note that use
of the \fB\-global\fR or \fB\-lazy\fR option may lead to crashes
in your application later (in case of symbol conflicts resp. missing
symbols), which cannot be detected during the \fBload\fR. So, only
use this when you know what you are doing, you will not get a nice
error message when something is wrong with the loaded library.
.SH "PORTABILITY ISSUES"
.TP
\fBWindows\fR\0\0\0\0\0
.
When a load fails with
.QW "library not found"
error, it is also possible
that a dependent library was not found.  To see the dependent libraries,
type
.QW "dumpbin -imports <dllname>"
in a DOS console to see what the library must import.
When loading a DLL in the current directory, Windows will ignore
.QW ./
as a path specifier and use a search heuristic to find the DLL instead.
To avoid this, load the DLL with:
.RS
.PP
.CS
\fBload\fR [file join [pwd] mylib.DLL]
.CE
.RE
.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).
.SH EXAMPLE
.PP
The following is a minimal extension:
.PP
.CS
#include <tcl.h>
#include <stdio.h>
static int fooCmd(void *clientData,
        Tcl_Interp *interp, int objc, Tcl_Obj *const objv[]) {
    printf("called with %d arguments\en", objc);
    return TCL_OK;
}
int Foo_Init(Tcl_Interp *interp) {
    if (Tcl_InitStubs(interp, "8.1", 0) == NULL) {
	return TCL_ERROR;
    }
    printf("creating foo command");
    Tcl_CreateObjCommand(interp, "foo", fooCmd, NULL, NULL);
    return TCL_OK;
}
.CE
.PP
When built into a shared/dynamic library with a suitable name
(e.g. \fBfoo.dll\fR on Windows, \fBlibfoo.so\fR on Solaris and Linux)
it can then be loaded into Tcl with the following:
.PP
.CS
# Load the extension
switch $tcl_platform(platform) {
    windows {
        \fBload\fR [file join [pwd] foo.dll]
    }
    unix {
        \fBload\fR [file join [pwd] libfoo[info sharedlibextension]]
    }
}

# Now execute the command defined by the extension
foo
.CE
.SH "SEE ALSO"
info sharedlibextension, package(n), Tcl_StaticPackage(3), safe(n)
.SH KEYWORDS
binary code, dynamic library, load, safe interpreter, shared library
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