What does mean for a name or type to have a certain language linkage?

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Language linkage is the term used for linkage between C++ and non-C++ code fragments. Typically, in a C++ program, all function names, function types and even variable names have the default C++ language linkage.

A C++ object code can be linked to another object code which is produced using some other source language (like C) using a predefined linkage specifier.

As you must be aware of the concept of name mangling, which encodes function names, function types and variable names so as to generate a unique name for them. This allows the linker to differentiate between common names (as in the case of function overloading). Name mangling is not desirable when linking C modules with libraries or object files compiled with a C++ compiler. To prevent name mangling for such cases, linkage specifiers are used. In this case, extern "C" is the linkage specifier. Let’s take an example (c++ code mentioned here):

typedef int (*pfun)(int);  // line 1
extern "C" void foo(pfun); // line 2
extern "C" int g(int)      // line 3
...
foo( g ); // Error!        // line 5

Line 1 declares pfun to point to a C++ function, because it lacks a linkage specifier.

Line 2 therefore declares foo to be a C function that takes a pointer to a C++ function.

Line 5 attempts to call foo with a pointer to g, a C function, a type mis-match.

Diff in function name linkage:

Let’s take two different files:

One with extern "c" linkage (file1.cpp):

#include <iostream>
using namespace std;

extern "C"
{
void foo (int a, int b)
{
    cout << "here";
}
}

int main ()
{
    foo (10,20);
    return 0;
}

One without extern "c" linkage (file2.cpp):

#include <iostream>
using namespace std;

void foo (int a, int b)
{
    cout << "here";
}

int main ()
{
    foo (10,20);
    return 0;
}

Now compile these two and check the objdump.

# g++ file1.cpp -o file1
# objdump -Dx file1

# g++ file2.cpp -o file2
# objdump -Dx file2

With extern “C” linkage, there is no name mangling for the function foo. So any program that is using it (assuming we make a shared lib out of it) can directly call foo (with helper functions like dlsym and dlopen) with out considering any name mangling effects.

0000000000400774 <foo>:
  400774:   55                      push   %rbp
  400775:   48 89 e5                mov    %rsp,%rbp
....
....
  400791:   c9                      leaveq 
  400792:   c3                      retq   

0000000000400793 <main>:
  400793:   55                      push   %rbp
  400794:   48 89 e5                mov    %rsp,%rbp
  400797:   be 14 00 00 00          mov    $0x14,%esi
  40079c:   bf 0a 00 00 00          mov    $0xa,%edi
  4007a1:   e8 ce ff ff ff          callq  400774 <foo>
  4007a6:   b8 00 00 00 00          mov    $0x0,%eax
  4007ab:   c9                      leaveq

On the other hand, when no extern "C" is being used, func: foo is mangled with some predefined rules (known to compiler/linker being used) and so an application can not directly call it from it specifying the name as foo. You can however call it with the mangled name (_Z3fooii in this case) if you want, but nobody use it for the obvious reason.

0000000000400774 <_Z3fooii>:
  400774:   55                      push   %rbp
  400775:   48 89 e5                mov    %rsp,%rbp
 ...
...
  400791:   c9                      leaveq 
  400792:   c3                      retq   

0000000000400793 <main>:
  400793:   55                      push   %rbp
  400794:   48 89 e5                mov    %rsp,%rbp
  400797:   be 14 00 00 00          mov    $0x14,%esi
  40079c:   bf 0a 00 00 00          mov    $0xa,%edi
  4007a1:   e8 ce ff ff ff          callq  400774 <_Z3fooii>
  4007a6:   b8 00 00 00 00          mov    $0x0,%eax
  4007ab:   c9                      leaveq 
  4007ac:   c3                      retq

This page is also a good read for this particular topic.

A nice and clearly explained article about calling convention: http://www.codeproject.com/KB/cpp/calling_conventions_demystified.aspx

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