Question

When compiling a c file that uses old style function definition like

int foo(a)
   int a;
{
  ...
}

g++ will give and error: ‘a’ was not declared in this scope.
gcc can parse this.
Is there a way to let g++ recognize this?

This comes up as an issue to me because I’m compiling a mix of c and c++ files.
A related question is what’s the standard practice
of building this type of mixed source? Running g++ on all files or only the cc files? The former is convenient but keeps getting me some trouble because of the inconsistencies between c and c++ specification(for example, char[4]=”four”;)

Answer 1

Is there a way to let g++ recognize this?

This syntax is not supported in C++.

Running g++ on all files or only the cc files?

See e.g. Compiling C++ programs from the GCC docs:

C++ source files conventionally use one of the suffixes .C’, .cc, .cpp, .CPP, .c++, .cp, or .cxx; C++ header files often use .hh, .hpp, .H, or (for shared template code) .tcc; and preprocessed C++ files use the suffix .ii. GCC recognizes files with these names and compiles them as C++ programs even if you call the compiler the same way as for compiling C programs (usually with the name gcc).

However, the use of gcc does not add the C++ library. g++ is a program that calls GCC and treats .c, .h and .i files as C++ source files instead of C source files unless -x is used, and automatically specifies linking against the C++ library. This program is also useful when precompiling a C header file with a .h extension for use in C++ compilations.

So two possibilities:

1. Run gcc on C files, and g++ on C++ files.
2. Run gcc on all files.

In both cases you will need to link with g++ (or gcc -lstdc++).