What is ppcc?

You might have noticed on some systems nmake runs ppcc instead of running the compiler directly. ppcc is a shell script which allows us to compile code using the nmake preprocessor with compilers that don't provide a means to access an external preprocessor. ppcc first calls nmake cpp to generate a preprocessed .i file. The .i file is then passed to the compiler for compilation. Since the source code is already preprocessed we do not have to rely on the native preprocessor.

Why is ppcc used?

nmake has its own C preprocessor (cpp) which is used to faciliate viewpathing. It treats the -I arguments a bit differently than standard cpp implementations to guarantee correct include files are picked up when viewpathing. (For more information see the cpp FAQ page.)

There are three cases concerning nmake cpp and the compiler:

1. A compiler will have some option to reference an external cpp instead of the native one. In this case the compiler is run directly with the appropriate command line argument to pick up nmake cpp.
2. The compiler does not have any way to use an external preprocessor. In this case ppcc is used. ppcc runs nmake cpp to do the preprocessing, and then passes the preprocessed file (a .i file) to the compiler. The compiler doesn't have to do any preprocessing since it is already done.
3. In some cases the native preprocessor can treat the -I list the same as nmake cpp. In these cases the native cpp will pick up the correct include files, so nmake cpp is not used. This is indicated by the native preprocessor accepting the -I- argument. Examples are the Gnu C compiler, EDG based C++ 4.x compilers, HP aCC, and later release of Sun Forte C++.

When is ppcc used?

When nmake is first run with a given compiler it "probes" the compiler to learn the compiler's capibilities and what kind of arguments are supported. At this time "probe" files are created, and from then on these probe files are referenced by nmake rather than re-probing every time.

The make probe file (located in (nmake_root)/lib/probe/C/make/ indicates which of the above three cases is relevant for the compiler, and determines if and how the nmake cpp is used. You can see the make probe file by running: 'probe -l C make /path/to/cc'.

• If CC.DIALECT contains -I- then use the native cpp, it supports -I- (case number 3 above).
• If if both CC.ALTPP.FLAGS and CC.ALTPP.ENV are empty, and there is no -I- in CC.DIALECT, then use ppcc (case number 2 above).
• Otherwise, either CC.ALTPP.FLAGS or CC.ALTPP.ENV tells how the compiler can use an external cpp (case number 1 above).