Introduction to QDoc

Currently, the Qt5 build system does not use QDoc's single execution mode for generating the Qt5 documentation. It runs QDoc in the standard mode. The standard mode was came about because it was the easiest way to get convert the Qt4 QDoc to handle the modularization of Qt in Qt5. In Qt4, QDoc ran once over all the Qt4 sources to generate the HTML documentation for Qt. While generating the Qt documentation, Qt4 QDoc also generated an index file for Qt. That index file was meant to be used as input to subsequent QDoc runs for generating HTML documentation for other software libraries/products that were based on Qt. The Qt index file allowed QDoc to link documentation written for those other libraries/products to the Qt4 documentation.

When Qt5 came along, Qt was divided into modules. Since then, many new modules have been added to Qt. As of version 5.5, there are over 40 separate modules in Qt5, each with its own documentation that links to (depends on) the documentation of other Qt modules.

In standard mode, QDoc runs twice for each module. The first QDoc run for a particular Qt module, parses all the module's source files and then uses the information to generate the module's index file. It is called the prepare phase because it prepares the module's index file. The second QDoc run for the module also parses all the module's source files and then generates the module's documentation pages. This is called the generate phase because it generates the module's documentation.

The module's documentation will likely contain HTML links to the documentation of one or more of the other Qt modules. For example, most Qt5 modules contain links to documentation in QtCore. When a Qt module contains links into the documentation of other Qt module's, that module is said to depend on those other Qt modules. Hence when QDoc runs the generate phase for that module, it must also load the index files for those modules so it can create those links.

Hence, when the Qt build system generates the Qt documentation, it first runs QDoc once for each module to perform the prepare phase to generate all the index files. Then it runs QDoc once for each module to perform the generate phase, where it uses the dependent index files to generate the module's documentation, including any cross-module links it finds. Each execution of QDoc, both prepare phase and generate phase, parses all the source files that are included in the module, and in the generate phase also parses the index files for the dependent modules. Nothing is retained or retainable between QDoc runs.

As the name implies, single execution mode uses a single QDoc process to generate all the Qt5 documentation. The single QDoc process still performs a prepare phase for each module and then a generate phase for each module, but there are a few differences. It begins by reading the master qdocconf file. Then it reads each qdocconf file in the master list and performs the prepare phase for each module. During the prepare phase, all the source files for the module are parsed to build a syntax tree for the module. The module's index file is then generated, although QDoc will not re-read the index files in the generate phase. The important difference here is that the module's syntax tree is retained after the index file is generated, so that after the prepare phase has been run for all the modules, QDoc still has all the syntax trees it built.

QDoc then processes each module again for the generate phase. But now QDoc doesn't need to re-parse each module's source files, because the module's syntax tree is still in memory. Nor does QDoc need to re-read the index files for the dependent modules, again because it still has the syntax trees for those modules in memory. It remains only to traverse each module's syntax tree to generate the documentation pages.

Hence, QDoc parses each source file once and only once and doesn't need to read index files. This is what makes single execution mode much faster than the standard mode. It is anticipated that the Qt build system will eventually run QDoc in single execution mode. However, changes to the master qdocconf file might be required, so the method described above for running QDoc in single execution mode might have to change, watch this space for updates.