Pkg_mkIndex is a utility procedure that is part of the standard Tcl library. It is used to create index files that allow packages to be loaded automatically when package require commands are executed. To use pkg_mkIndex, follow these steps:
The package management facilities overlap somewhat with the auto-loader, in that both arrange for files to be loaded on-demand. However, package management is a higher-level mechanism that uses the auto-loader for the last step in the loading process. It is generally better to index a package with pkg_mkIndex rather than auto_mkindex because the package mechanism provides version control: several versions of a package can be made available in the index files, with different applications using different versions based on package require commands. In contrast, auto_mkindex does not understand versions so it can only handle a single version of each package. It is probably not a good idea to index a given package with both pkg_mkIndex and auto_mkindex. If you use pkg_mkIndex to index a package, its commands cannot be invoked until package require has been used to select a version; in contrast, packages indexed with auto_mkindex can be used immediately since there is no version control.
Pkg_mkIndex depends on the package unknown command, the package ifneeded command, and the auto-loader. The first time a package require command is invoked, the package unknown script is invoked. This is set by Tcl initialization to a script that evaluates all of the pkgIndex.tcl files in the auto_path. The pkgIndex.tcl files contain package ifneeded commands for each version of each available package; these commands invoke package provide commands to announce the availability of the package, and they setup auto-loader information to load the files of the package. If the -lazy flag was provided when the pkgIndex.tcl was generated, a given file of a given version of a given package isn't actually loaded until the first time one of its commands is invoked. Thus, after invoking package require you may not see the package's commands in the interpreter, but you will be able to invoke the commands and they will be auto-loaded.
Some packages, for instance packages which use namespaces and export commands or those which require special initialization, might select that their package files be loaded immediately upon package require instead of delaying the actual loading to the first use of one of the package's command. This is the default mode when generating the package index. It can be overridden by specifying the -lazy argument.
If each script or file contains one package, and packages are only contained in one file, then things are easy. You simply specify all files to be indexed in any order with some glob patterns.
In general, it is OK for scripts to have dependencies on other packages. If scripts contain package require commands, these are stubbed out in the interpreter used to process the scripts, so these do not cause problems. If scripts call into other packages in global code, these calls are handled by a stub unknown command. However, if scripts make variable references to other package's variables in global code, these will cause errors. That is also bad coding style.
If binary files have dependencies on other packages, things can become tricky because it is not possible to stub out C-level APIs such as Tcl_PkgRequire API when loading a binary file. For example, suppose the BLT package requires Tk, and expresses this with a call to Tcl_PkgRequire in its Blt_Init routine. To support this, you must run pkg_mkIndex in an interpreter that has Tk loaded. You can achieve this with the -load pkgPat option. If you specify this option, pkg_mkIndex will load any packages listed by info loaded and that match pkgPat into the interpreter used to process files. In most cases this will satisfy the Tcl_PkgRequire calls made by binary files.
If you are indexing two binary files and one depends on the other, you should specify the one that has dependencies last. This way the one without dependencies will get loaded and indexed, and then the package it provides will be available when the second file is processed. You may also need to load the first package into the temporary interpreter used to create the index by using the -load flag; it won't hurt to specify package patterns that are not yet loaded.
If you have a package that is split across scripts and a binary file, then you should avoid the -load flag. The problem is that if you load a package before computing the index it masks any other files that provide part of the same package. If you must use -load, then you must specify the scripts first; otherwise the package loaded from the binary file may mask the package defined by the scripts.