Configuring qmake's Environment

Library Dependencies

Often when linking against a library, qmake relies on the underlying platform to know what other libraries this library links against, and lets the platform pull them in. In many cases, however, this is not sufficent. For example, when statically linking a library, no other libraries are linked to, and therefore no dependencies to those libraries are created. However, an application that later links against this library will need to know where to find the symbols that the static library will require. To help with this situation, qmake attempts to follow a library's dependencies where appropriate, but this behavior must be explicitly enabled by following two steps.

The first step is to enable dependency tracking in the library itself. To do this you must tell qmake to save information about the library:

 CONFIG += create_prl

This is only relevant to the lib template, and will be ignored for all others. When this option is enabled, qmake will create a file ending in .prl which will save some meta-information about the library. This metafile is just like an ordinary project file, but only contains internal variable declarations. You are free to view this file and, if it is deleted, qmake will know to recreate it when necessary, either when the project file is later read, or if a dependent library (described below) has changed. When installing this library, by specifying it as a target in an INSTALLS declaration, qmake will automatically copy the .prl file to the installation path.

The second step in this process is to enable reading of this meta information in the applications that use the static library:

 CONFIG += link_prl

When this is enabled, qmake will process all libraries linked to by the application and find their meta-information. qmake will use this to determine the relevant linking information, specifically adding values to the application project file's list of DEFINES as well as LIBS. Once qmake has processed this file, it will then look through the newly introduced libraries in the LIBS variable, and find their dependent .prl files, continuing until all libraries have been resolved. At this point, the Makefile is created as usual, and the libraries are linked explicitlyy against the application.

The internals of the .prl file are left closed so they can easily change later. They are not designed to be changed by hand, should only be created by qmake, and should not be transferred between operating systems as they may contain platform-dependent information.

 QMAKE_EXT_MOC = .mymoc

 mytarget.target = .buildfile
 mytarget.commands = touch $$mytarget.target
 mytarget.depends = mytarget2

 mytarget2.commands = @echo Building $$mytarget.target

 QMAKE_EXTRA_TARGETS += mytarget mytarget2

 new_moc.output  = moc_${QMAKE_FILE_BASE}.cpp
 new_moc.commands = moc ${QMAKE_FILE_NAME} -o ${QMAKE_FILE_OUT}
 new_moc.depend_command = g++ -E -M ${QMAKE_FILE_NAME} | sed "s,^.*: ,,"
 new_moc.input = NEW_HEADERS
 QMAKE_EXTRA_COMPILERS += new_moc