Provided by: tcl9.0-doc_9.0.1+dfsg-2_all 
NAME
load - Load machine code and initialize new commands
SYNOPSIS
load ?-global? ?-lazy? ?--? fileName
load ?-global? ?-lazy? ?--? fileName prefix
load ?-global? ?-lazy? ?--? fileName prefix interp
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DESCRIPTION
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. fileName 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 .so file under Solaris or a DLL under Windows. prefix is used to compute the
name of an initialization procedure. interp is the path name of the interpreter into which to load the
library (see the interp manual entry for details); if interp is omitted, it defaults to the interpreter
in which the load command was invoked.
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 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 prefix_Init. For example, if prefix is Foo, the initialization procedure's name will be
Foo_Init.
If the target interpreter is a safe interpreter, then the name of the initialization procedure will be
prefix_SafeInit instead of prefix_Init. The prefix_SafeInit 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 safe manual entry.
The initialization procedure must match the following prototype:
typedef int Tcl_LibraryInitProc(
Tcl_Interp *interp);
The interp argument identifies the interpreter in which the library is to be loaded. The initialization
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. The result of the
load command will be the result returned by the initialization procedure.
The actual loading of a file will only be done once for each fileName in an application. If a given
fileName is loaded into multiple interpreters, then the first load will load the code and call the
initialization procedure; subsequent loads 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 unload command allows the unloading of libraries loaded with load, for libraries
that are aware of the Tcl's unloading mechanism.
The load command also supports libraries that are statically linked with the application, if those
libraries have been registered by calling the Tcl_StaticLibrary procedure. If fileName is an empty
string, then prefix must be specified.
If prefix is omitted or specified as an empty string, Tcl tries to guess the prefix by taking the last
element of fileName, strip off the first three characters if they are lib, then strip off the next four
characters if they are tcl9, and use any following wordchars but not digits, converted to titlecase as
the prefix. For example, the command load libxyz4.2.so uses the prefix Xyz and the command load
bin/last.so {} uses the prefix Last.
If fileName is an empty string, then prefix must be specified. The load command first searches for a
statically loaded library (one that has been registered by calling the Tcl_StaticLibrary procedure) by
that name; if one is found, it is used. Otherwise, the load command searches for a dynamically loaded
library by that name, and uses it if it is found. If several different files have been loaded with
different versions of the library, Tcl picks the file that was loaded first.
If -global is specified preceding the filename, all symbols found in the shared library are exported for
global use by other libraries. The option -lazy delays the actual loading of symbols until their first
actual use. The options may be abbreviated. The option -- indicates the end of the options, and should
be used if you wish to use a filename which starts with - and you provide a prefix to the load command.
On platforms which do not support the -global or -lazy options, the options still exist but have no
effect. Note that use of the -global or -lazy option may lead to crashes in your application later (in
case of symbol conflicts resp. missing symbols), which cannot be detected during the load. 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.
PORTABILITY ISSUES
Windows
When a load fails with “library not found” error, it is also possible that a dependent library was
not found. To see the dependent libraries, type “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 “./” as a path specifier and use a search heuristic to find the DLL instead. To avoid
this, load the DLL with:
load [file join [pwd] mylib.DLL]
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).
EXAMPLE
The following is a minimal extension:
#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\n", 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;
}
When built into a shared/dynamic library with a suitable name (e.g. foo.dll on Windows, libfoo.so on
Solaris and Linux) it can then be loaded into Tcl with the following:
# Load the extension
switch $tcl_platform(platform) {
windows {
load [file join [pwd] foo.dll]
}
unix {
load [file join [pwd] libfoo[info sharedlibextension]]
}
}
# Now execute the command defined by the extension
foo
SEE ALSO
info sharedlibextension, package(3tcl), Tcl_StaticLibrary(3tcl), safe(3tcl)
KEYWORDS
binary code, dynamic library, load, safe interpreter, shared library
Tcl 7.5 load(3tcl)