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man pages section 1: User Commands

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Updated: Wednesday, February 9, 2022

load (1t)


load - Load machine code and initialize new commands


load ?-global? ?-lazy? ?--? fileName
load ?-global? ?-lazy? ?--? fileName packageName
load ?-global? ?-lazy? ?--? fileName packageName interp


load(1t)                     Tcl Built-In Commands                    load(1t)


       load - Load machine code and initialize new commands

       load ?-global? ?-lazy? ?--? fileName
       load ?-global? ?-lazy? ?--? fileName packageName
       load ?-global? ?-lazy? ?--? fileName packageName interp

       This  command  loads  binary  code  from  a file into the application's
       address space and calls an initialization procedure in the  package  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.  packageName is the name of  the  pack-
       age,  and  is  used to compute the name of an initialization procedure.
       interp is the path name of the interpreter into which to load the pack-
       age (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.  Typ-
       ically the initialization procedure will add  new  commands  to  a  Tcl
       interpreter.  The name of the initialization procedure is determined by
       packageName and whether or not the target interpreter is  a  safe  one.
       For  normal  interpreters the name of the initialization procedure will
       have the form pkg_Init, where pkg is the  same  as  packageName  except
       that  the first letter is converted to upper case and all other letters
       are converted to lower case.  For example, if  packageName  is  foo  or
       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 pkg_SafeInit instead of pkg_Init.  The
       pkg_SafeInit  function should be written carefully, so that it initial-
       izes the safe interpreter only with partial functionality  provided  by
       the  package  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 package  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  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 package. 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-

       The load command also supports packages that are statically linked with
       the application, if those packages have been registered by calling  the
       Tcl_StaticPackage  procedure.   If  fileName  is  an empty string, then
       packageName must be specified.

       If packageName is omitted or specified as an empty string, Tcl tries to
       guess the name of the package.  This may be done differently on differ-
       ent platforms.  The default guess, which is used  on  most  UNIX  plat-
       forms,  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 as the module name.  For example, the command load
       libxyz4.2.so uses the module name xyz and the command load  bin/last.so
       {} uses the module name last.

       If  fileName  is  an  empty string, then packageName must be specified.
       The load command first searches for a statically  loaded  package  (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 package by that name, and uses it if
       it is found.  If several different files have been loaded with  differ-
       ent versions of the package, 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 file-
       name which starts with - and you provide a packageName to the load com-

       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  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.

              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]

       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).

       The following is a minimal extension:

              #include <tcl.h>
              #include <stdio.h>
              static int fooCmd(ClientData 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

       See attributes(7) for descriptions of the following attributes:

       |Availability   | runtime/tcl-8    |
       |Stability      | Uncommitted      |

       info sharedlibextension, package(n), Tcl_StaticPackage(3), safe(n)

       binary code, dynamic library, load, safe interpreter, shared library

       Source code for open source software components in Oracle  Solaris  can
       be found at https://www.oracle.com/downloads/opensource/solaris-source-

       This    software    was    built    from    source     available     at
       https://github.com/oracle/solaris-userland.    The  original  community
       source was downloaded from  http://prdownloads.sourceforge.net/tcl/tcl-

       Further information about this software can be found on the open source
       community website at https://www.tcl.tk/.

Tcl                                   7.5                             load(1t)