Introduction

GCC can automatically determine which compiler to use based on the file suffix. It uses the 'Objective-C' compiler for example on files with an '.m' suffix, or the FORTRAN compiler on files with a '.FOR' suffix.

But this is only when it is left up to GCC to choose which compiler to use.

A basic C test file

Most vendor BSP's (board support packages) are advertised as C and C++. When put to the test, it is often evident that the latter has never been tested.
Hence, it can be usefull to verify that some C sources can be compiled as C++. So let's see how that can be done.

Save the simple test code below as "Hello.c".

#include <stdlib.h>
#include <stdio.h>

#ifdef __cplusplus
extern "C"
{
#endif

int main()
{
    printf( "Hello world!" );

    return EXIT_SUCCESS;
}

#ifdef __cplusplus
}
#endif

Compile the source

With the source file in the current directory, it can be compiled using:

gcc -S Hello.c

This will invoke gcc and lets it select the compiler based on the file's .c suffix. The -S option tells gcc to stop before the assembly stage of compilation and dump what it has generated from the compiler backend in the file Hello.s in the current directory.

Or:

gcc -x c -S Hello.c

Which will invoke gcc and tells it to ignore the .c suffix and use the C compiler regardless.

Compile the source as C++

To test if the code in our "Hello.c" file is also syntactilly correct C++ code it can be compiled with the C++ compiler as follows:

gcc -x c++ -S Hello.c

The -x c++ option passed to gcc tells it to ignore the .c suffix and use the C++ compiler regardless.

Use a make file

It is easy to forget the correct option to select the compiler. That is why there are make files.

#Compile a source file either as C or as C++

Sources = Hello.c

#C++ compilation flags
CXXFLAGS += -Wall -Wextra -Wpedantic
CXXFLAGS += -std=c++11

#C compilation flags
CFLAGS += -Wall -Wextra -Wpedantic
CFLAGS += -std=c99

.PHONY: default
default:
	@echo "supported targets are 'build-c' and 'build-cpp'"

build-c:
	@echo "Compile " $(Sources) " as C"
	$(CC) $(CFLAGS) -x c -S $(Sources)

build-cpp:
	@echo "Compile " $(Sources) " as C++"
	$(CXX) $(CXXFLAGS) -x c++ -S $(Sources)