Extern "C"

Compilation Method without the extern "C" declaration

First, let's take a look at how C-like functions are compiled in C ++.

As an object-oriented language, C ++ supports function overloading, while Procedural Language C does not. The name of the function in the symbol library after being compiled by C ++ is different from that in the C language. For example, assume that the prototype of a function is:

Void Foo (int x, int y );

After the function is compiled by the C compiler, its name in the symbol library is _ Foo, while the C ++ compiler generates names such as _ foo_int_int (different compilers may generate different names, but all adopt the same mechanism, and the new name is called "mangledname "). A name such as _ foo_int_int contains the function name, number of function parameters, and type information. c ++ relies on this mechanism to implement function overloading.

For example, in C ++, the void Foo (int x, int y) and void Foo (int x, float y) functions generate different symbols, the latter is _ foo_int_float.

Similarly, variables in C ++ support both local variables and class member variables and global variables. The class member variables of the program written by the user may have the same name as the global variables, which are distinguished. In essence, the compiler uses a unique name for the variables in the class when compiling, similar to the function processing. This name is different from the global variable name with the same name in the user program.

Connection method when extern "C" is not added

Suppose in C ++, the header file of module A is as follows:

// Module A header file modulea. h

# Ifndef module_a_h

# Define module_a_h

Int Foo (int x, int y );

# Endif

Reference this function in Module B:

// Module B implements the file moduleb. cpp

# Include "modulea. H"

Foo (2, 3 );

In fact, in the connection phase, the connector looks for symbols such as _ foo_int_int from the target file modulea. OBJ generated by module!

Compilation and Connection Methods After the extern "C" clause is added

After the extern "C" statement is added, the header file of module A is changed:

// Module A header file modulea. h

# Ifndef module_a_h

# Define module_a_h

Extern "C" int Foo (int x, int y );

# Endif

In Module B's implementation file, Foo (2, 3) is still called. The result is:

(1) When module A compiles and generates the foo target code, it does not perform special processing on its name and uses the C language;

(2) When the connector looks for the Foo (2, 3) call for the target code of Module B, it looks for the unmodified symbol name _ Foo.

If the function in module A declares that foo is of the extern "C" type, and Module B contains the extern int Foo (INT X, int y ), module B cannot find the function in module A, and vice versa.