load(3tcl) Tcl Built-In Commands load(3tcl)
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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 ad-
dress space and calls an initialization procedure in the library to in-
corporate 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 So-
laris 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 de-
tails); 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. Typ-
ically the initialization procedure will add new commands to a Tcl in-
terpreter. 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 pfx_Init, where pfx is the same as prefix except that the
first letter is converted to upper case and all other letters are con-
verted to lower case. For example, if prefix is foo or FOo, the ini-
tialization procedure's name will be Foo_Init.
If the target interpreter is a safe interpreter, then the name of the
initialization procedure will be pfx_SafeInit instead of pfx_Init. The
pfx_SafeInit function should be written carefully, so that it initial-
izes the safe interpreter only with partial functionality provided by
the library that is safe for use by untrusted code. For more informa-
tion on Safe-Tcl, see the safe manual entry.
The initialization procedure must match the following prototype:
typedef int Tcl_PackageInitProc(
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 re-
turned 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 inter-
preters, then the first load will load the code and call the initial-
ization procedure; subsequent loads will call the initialization pro-
cedure 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 mecha-
nism.
The load command also supports libraries that are statically linked
with the application, if those libraries have been registered by call-
ing the Tcl_StaticPackage 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. 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 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_StaticPackage 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 differ-
ent 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 ac-
tual 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 op-
tions 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 dur-
ing 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 de-
pendent 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_StaticPackage(3tcl),
safe(3tcl)
KEYWORDS
binary code, dynamic library, load, safe interpreter, shared library
Tcl 7.5 load(3tcl)
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