What does the author mean about "extern linkage" and "C language linkage" in the following paragraph, extracted from [1].
"There are two different forms of the extern C declaration: extern C as used above, and extern C { … } with the declarations between the braces. The first (inline) form is a declaration with extern linkage and with C language linkage; the second only affects language linkage. The following two declarations are thus equivalent:"
Can you further elaborate what he's trying to explain with this example?
[1] http://tldp.org/HOWTO/C++-dlopen/thesolution.html
What the author is saying relates to these two lines:
extern "C" int foo;
extern "C" { int bar; }
foo is a variable declared but not defined. It exists elsewhere. On the other hand, bar is both declared and defined.
Think of a declaration as just stating that something exists somewhere but not actually creating it. Definition is therefore declaration plus bringing that thing into existence.
The latter one is exactly the same as int bar; but will "publish" the variable with C linkage. For example, a function int max (int a, int b); may be published as _max in C language linkage and _max$$int$int in C++ language linkage (to allow more than one function with the same name).
Note that "publishing" in this context is how the function looks to the linker, so that your code can link to it. Without C language linkage, it's usually rather difficult to link C code with C++ libraries.
Clumsy wording, yeah. What he's trying to get at is,
extern "C" int foo();
is the same as
extern "C" { extern int foo(); }
but not the same as
extern "C" { int foo(); }
... except that there is no practical difference between "extern int foo();" and "int foo();" at file scope in C++, so you are entirely forgiven for scratching your head. Here's a case where it actually makes a difference:
extern "C" const int x = 12;
extern "C" { const int y = 12; }
x will be visible outside the translation unit, y won't.
Related
I wants to export a global variable from dll I define the global variable like below. Assume this variable is defined in A.cpp
extern "C" __declspec(dllexport) int A;
Meanwhile, within the dll, another source file B.cpp wants to use and modify this value. I am wonderring how to declare the variable within B.cpp before using it.
extern "C" int A; ?
If in this case, how does compile recognize between declare and definition?
extern extern "C" int A; ?
It's definitely ill-formed.
Per [basic.def]/2.2, a declaration of an object at namespace scope is a definition unless:
it contains the extern specifier (9.2.1) or a linkage-specification19 (9.11) and neither an initializer nor a function-body,
...
19 Appearing inside the brace-enclosed declaration-seq in a linkage-specification does not affect whether a declaration is a definition.
Thus:
extern "C" int A; is a declaration.
extern "C" int A = 0; is a definition.
The below defines A and B, and declares C: the effect is the same as it would be without the extern "C" block, except that the entities declared have C linkage instead of C++ linkage.
extern "C" {
int A;
extern int B = 0;
extern int C;
}
extern extern "C" int A; ?
extern "C" can be used as a block:
extern "C"
{
extern int A;
}
The declaration needs to be in a header file like A.h or other code files cannot use it. The dllexport is especially useless without a header file. You probably want some kind of DLLEXPORT macro so you can define it to dllexport and dllimport as necessary. See pretty much any Windows DLL code.
Then in a cpp file you include the header. That allows your code to use the variable which is declared extern.
In one of your cpp files you include the header AND then define the variable using the same type and name and no extern. The linker will then put the data storage for it into the same module as the rest of that cpp file and all other uses of the name get linked to that definition.
However, just like private member variables in C++ it is a bad idea to expose global variables in a DLL. It is much better to hide access to them behind function calls.
Let's say, I have the header file:
#ifdef __cplusplus
extern "C" {
#endif
struct S
{
int i;
double d;
// etc
};
// Or start the 'extern "C"' block here?..
void f(A* a);
#ifdef __cplusplus
}
#endif
It should be used by C/C++ sources.
My question is: if I carry the struct A declaration out of the extern "C", whether this code will be compatible with a library which was built with the struct A declared as extern "C"?
That is, we built a library with the header as it is presented above, then we moved the extern "C" to the position marked by the corresponding comment line and, finally, we are trying to build a C++ application using the resulting header.
If all of this will be successful? In other words, if the extern "C" influence on a struct declaration?
Whether it depends what language (C/C++) the library is written on?
extern by itself declares external linkage, which is a default state for C++ functions.
Specification "C" gives C language linkage.
Only function names and variable names with external linkage have a language linkage, classes and class members are not affected. struct declaration is a class. If that file meant to be used with C code, there might be problem that older C standard do not support declaration in that format.
Specification "C" makes name conventions of externally linked entities compatible with said language, even while compatibility is implementation-dependent. E.g. for gcc your function would have C++ mangled name _Z3fooP1A, but with C it might be just _foo.
I'm writing a C++ shared library for a C program to use. However, I have a question about extern and extern "C".
Consider the following code
My header file is like this:
#ifdef __cplusplus
extern "C" int global;
extern "C" int addnumbers(int a, int b);
#else
extern int global;
#endif
This works perfectly fine; I just have to declare
int global;
in either my .cpp or my .c file. However, what I don't understand is:
What is the difference between extern "C" and extern here? I tried commenting out extern "C" int global and it works! Why?
I know that extern "C" is used for making C linkage. That's why I have extern "C" int addnumbers(int,int). In other words, if I want to write a C++ function that is to be used in a C program, I write extern "C". Now, what about global variables - the situation is different here I guess? I want the C program to use a C++ variable named global, but I can use extern not extern "C". Why is that? This is not intuitive to me.
Comment: I don't think this is a duplicate, because I'm asking what the difference is when you use it for variables versus functions.
By attaching extern "C" to your C++ declarations (objects and functions alike) you give them "C linkage" - make them accessible from C code. If you omit this "language linkage" specification, the compiler doesn't do any effort to do proper linkage. In the case of functions, this results in failed linkage, because of mangling. In the case of global variables, everything might work fine, because variables don't need mangling.
However, on my system (MS Visual Studio), linkage between C and C++ doesn't work if I "forget" to specify the extern "C" linkage specification in the C++ header file. Example error message:
error LNK2001: unresolved external symbol "int global" (?_global)
While, when I examine a compiled C++ source code that contains the definition of global with the dumpbin utility, I see
00B 00000014 SECT4 notype External | ?global##3HA (int global)
So MS Visual Studio mangles the names of global variables, unless they have C linkage - this makes C linkage specifications mandatory.
In addition, consider the following example:
namespace example {
int global;
}
If the global variable is inside a namespace, C code will not get access to it. In this case, all compilers will require the proper linkage specification on the C++ declaration:
namespace example {
extern "C" int global;
}
Conclusion:
Use extern "C" when you want C linkage - doesn't matter if it's a function or a global variable. If it's a global variable, it may work regardless, but it's not guaranteed (and may be dangerous).
extern simply tells the compiler that the next variable(global) may not have been declared yet, but it is declared as global in a different translation unit, and during the linking stage the symbol "global" will be associated with an area in the memory.
while extern "C" is, as a few people commented, is used to solve the issue of name mangling in C++, this function will be known as addnumbers_i_i (or something similar) by the linker, while in c its symbol is addnumbers
"C++ has a special keyword to declare a function with C bindings: extern "C". A function declared as extern "C" uses the function name as symbol name, just as a C function. For that reason, only non-member functions can be declared as extern "C", and they cannot be overloaded."
There is no standard in C++ for function names generated by compiler. Keyword extern "C" instructs compiler to generate function name in C standard.
I find that extern "C" is used to make the C++ function compiled in C standard and it don't do with variables, for the solutions of function name in C and C++ are different. Such as "void foo( int x, int y )", C compiler will translate it into "_foo", while C++ compiler will translate it into "_foo_int_int".
After reading about extern and static, I am confused to come across the code which has the line below:
extern "C" static void* foo(int* a){
return foo1(a);
}
Why does this not generate any error?
The following also compiles and does the same thing as your line:
extern "C" {
static void* foo(int* a){
return foo1(a);
}
}
The static means that foo() will only be available in file scope, and it overrides the extern "C" when it comes to linkage. Normally, extern "C" effects the name of the function used by the linker if/when it is exported, so that the function could be called from other object files when the entire program is linked. Usually, this is used when either you want to link to an object file built from C source where foo() was defined, or where you want to define foo() so that C code can call it. However, the static causes foo to simply not be exported at all, so it basically doesn't even get a name (it has internal, not external linkage).
extern "C" also has a secondary effect. It also becomes part of the type of foo. That is, C++ code will see foo as having type extern "C" void(int*). Basically, this controls calling convention. A C++ compiler may e.g. arrange arguments differently in registers/on the stack than a C compiler might. Making foo be a C function means that it will use the C conventions and not the C++ conventions. This makes it safe to e.g. pass a function pointer to foo to a C function that expects a pointer to a C function. For example, the standard library has
extern "C" typedef int C_CMP(void const*, void const*);
extern "C++" typedef int CXX_CMP(void const*, void const*);
void std::qsort(void *, std::size_t, std::size_t, C_CMP);
void std::qsort(void *, std::size_t, std::size_t, CXX_CMP);
With the extern "C", &foo is passed to the first overload, but without it/with extern "C++", it is passed to the second. It would not be safe to declare foo without extern "C" and then try to pass it into a C function that expects a pointer to a C function. It would probably work, but it might also break horribly. Add the extern "C" and it becomes correct—you're safe.
I'm writing a C++ shared library for a C program to use. However, I have a question about extern and extern "C".
Consider the following code
My header file is like this:
#ifdef __cplusplus
extern "C" int global;
extern "C" int addnumbers(int a, int b);
#else
extern int global;
#endif
This works perfectly fine; I just have to declare
int global;
in either my .cpp or my .c file. However, what I don't understand is:
What is the difference between extern "C" and extern here? I tried commenting out extern "C" int global and it works! Why?
I know that extern "C" is used for making C linkage. That's why I have extern "C" int addnumbers(int,int). In other words, if I want to write a C++ function that is to be used in a C program, I write extern "C". Now, what about global variables - the situation is different here I guess? I want the C program to use a C++ variable named global, but I can use extern not extern "C". Why is that? This is not intuitive to me.
Comment: I don't think this is a duplicate, because I'm asking what the difference is when you use it for variables versus functions.
By attaching extern "C" to your C++ declarations (objects and functions alike) you give them "C linkage" - make them accessible from C code. If you omit this "language linkage" specification, the compiler doesn't do any effort to do proper linkage. In the case of functions, this results in failed linkage, because of mangling. In the case of global variables, everything might work fine, because variables don't need mangling.
However, on my system (MS Visual Studio), linkage between C and C++ doesn't work if I "forget" to specify the extern "C" linkage specification in the C++ header file. Example error message:
error LNK2001: unresolved external symbol "int global" (?_global)
While, when I examine a compiled C++ source code that contains the definition of global with the dumpbin utility, I see
00B 00000014 SECT4 notype External | ?global##3HA (int global)
So MS Visual Studio mangles the names of global variables, unless they have C linkage - this makes C linkage specifications mandatory.
In addition, consider the following example:
namespace example {
int global;
}
If the global variable is inside a namespace, C code will not get access to it. In this case, all compilers will require the proper linkage specification on the C++ declaration:
namespace example {
extern "C" int global;
}
Conclusion:
Use extern "C" when you want C linkage - doesn't matter if it's a function or a global variable. If it's a global variable, it may work regardless, but it's not guaranteed (and may be dangerous).
extern simply tells the compiler that the next variable(global) may not have been declared yet, but it is declared as global in a different translation unit, and during the linking stage the symbol "global" will be associated with an area in the memory.
while extern "C" is, as a few people commented, is used to solve the issue of name mangling in C++, this function will be known as addnumbers_i_i (or something similar) by the linker, while in c its symbol is addnumbers
"C++ has a special keyword to declare a function with C bindings: extern "C". A function declared as extern "C" uses the function name as symbol name, just as a C function. For that reason, only non-member functions can be declared as extern "C", and they cannot be overloaded."
There is no standard in C++ for function names generated by compiler. Keyword extern "C" instructs compiler to generate function name in C standard.
I find that extern "C" is used to make the C++ function compiled in C standard and it don't do with variables, for the solutions of function name in C and C++ are different. Such as "void foo( int x, int y )", C compiler will translate it into "_foo", while C++ compiler will translate it into "_foo_int_int".