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 there are no libraries linked against, 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 linked in library will require. To help with this situation qmake will follow a library's dependencies when it feels appropriate, however this behaviour must be enabled in qmake. To enable requires two steps. First, you must enable it in the library - to do this you must tell qmake to save information about this 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 (called a .prl file) which will save some meta information about the library. This metafile is itself just a qmake project file, but with all internal variables. You are free to view this file, and if deleted qmake will know to recreate it when necesary (either when the .pro file is later read, or if a dependent library (described below) has changed). When installing this library (by using target in INSTALLS, above) qmake will automatically copy the .prl file to your install path.

The second step to enabling this processing is to turn on reading of the meta information created above:

    CONFIG += link_prl

When this is turned on qmake will process all libraries linked to, and find their meta information. With this meta information qmake will figure out what is relevant to linking, specifically it will add to your list of DEFINES as well as LIBS. Once qmake has processed this file, it will then look through the newly introduced LIBS and find their dependent .prl files, and continue until all libraries have been resolved. At this point the Makefile is created as usual, and the libraries are linked explicity against your program.

The internals of the .prl file are left closed so they can easily change later. It is not designed to be changed by hand however, and should only be created by qmake - these .prl files should also not be transfered from operating system to operating system as they may be platform dependent (like a Makefile).

    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