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Q: I type GCC PROG.C to compile a C program which I already remember to pass compilation without a single warning, and suddenly it gives all kinds of strange error messages and unresolved externals.

.C

.cxx

.cpp

C++ source (passed through cpp).

.c

C source that must be passed through cpp first.

.i

Raw C source (no cpp pass).

.ii

Raw C++ source (not to be preprocessed).

.m

Objective-C source.

.S

Assembler that must be passed through cpp first.

.s

Raw assembler source (no cpp pass).

In the examples above, PROG.C is taken as a C++ program, not a C one, and PROG.CC is passed to the linker as if it were an object file. You can see what GCC does by adding the -v switch to the GCC command line; if you see that it's invoking cc1plus.exe (the C++ compiler) instead of cc1.exe (the C compiler), or calling ld.exe (the linker) on a source file, then you'd know this is your problem. If you have problems keeping up with the verbose GCC output caused by -v, see how to capture GCC output, earlier in this FAQ.

You can override the default rules gcc uses to decide how each input file should be treated, using the -x LANGUAGE switch. For instance, the command

      gcc -x c++ prog.c

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