Libsvm code reading: About extern C

When reading svm. h, we found the following code:

#ifndef _LIBSVM_H#define _LIBSVM_H#define LIBSVM_VERSION 317#ifdef __cplusplusextern "C" {#endif

Apparently, the first three lines of code are intended to prevent repeated references to header files. What do 5-6 lines mean?

An enterprise once gave the following interview question: why do standard header files have a structure similar to the following?

#ifndef __INCvxWorksh#define __INCvxWorksh #ifdef __cplusplusextern "C" {#endif /*...*/ #ifdef __cplusplus}#endif #endif /* __INCvxWorksh */

Analysis
Obviously, the compilation macro "# ifndef _ INCvxWorksh, # define _ INCvxWorksh, and # endif" in the header file is used to prevent the header file from being repeatedly referenced.

So

#ifdef __cplusplusextern "C" {#endif #ifdef __cplusplus}#endif

What is the role?

Extern "C" contains a double meaning, which can be obtained literally: first, the target is "extern", and second, the target is "C. Let's explain these two meanings in detail.

The function or variable specified by extern "C" is of the extern type, and extern is a keyword in C/C ++ that indicates the range (visibility) of the function and global variable, this keyword tells the compiler that the declared functions and variables can be used in this module or other modules. Remember, the following statement: extern int a; is just a declaration of a variable. It is not defining variable a and does not allocate memory space for. Variable a can only be defined once as a global variable in all modules. Otherwise, a connection error occurs. In general, the function and global variables referenced by this module to other modules are declared with the keyword extern in the module header file. For example, if Module B wants to reference the global variables and functions defined in module A, it only needs to include the header file of module. In this way, when Module B calls A function in module A, although Module B cannot find the function in the compilation phase, no error is reported; it will find this function from the target Code Compiled by module A in the connection phase.

The keyword corresponding to extern is static. The global variables and functions modified by it can only be used in this module. Therefore, a function or variable can only be used by this module and cannot be modified by extern "C.

Variables and functions modified by extern "C" are compiled and connected in C language;

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 "mangled name "). 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_Hint 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_Hextern "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.

Therefore, we can summarize the true purpose of the statement "extern" C "in one sentence: to implement mixed programming of C ++ and C and other languages.
　　
Usage of extern "C"

(1) To reference functions and variables in C language in C ++, the following processing must be performed when the C Language header file (assumed as cExample. h) is included:

extern "C"{#include "cExample.h"}

In the header file of the C language, the external function can only be specified as the extern type. The C language does not support the extern "C" declaration. when the c file contains extern "C", a compilation syntax error occurs. The source code of the three files in the example project of C ++ referenced by the author is as follows:

/* C header file: cExample. h */# ifndef C_EXAMPLE_H # define C_EXAMPLE_Hextern int add (int x, int y); # endif/* C implementation file: cExample. c */# include "cExample. h "int add (int x, int y) {return x + y;} // c ++ implementation file, call add: cppFile. cppextern "C" {# include "cExample. h "} int main (int argc, char * argv []) {add (2, 3); return 0 ;}

If C ++ calls a. DLL written in C language, when it includes the header file of. DLL or the declared interface function, it should add extern "C "{}.

(2) When referencing functions and variables in C ++ in C, the header file of C ++ needs to add extern "C ", however, you cannot directly reference this header file that declares extern "C" in C. You should only declare the extern "C" function defined in C ++ as the extern type.
The source code of the three files contained in the example project of C ++ is as follows:

// C ++ header file cppExample. h # ifndef CPP_EXAMPLE_H # define CPP_EXAMPLE_Hextern "C" int add (int x, int y); # endif // C ++ implementation file cppExample. cpp # include "cppExample. h "int add (int x, int y) {return x + y;}/* C implementation file cFile. c/* compilation errors: # include "cExample. h "*/extern int add (int x, int y); int main (int argc, char * argv []) {add (2, 3); return 0 ;}