Need a refresher, the difference between static and dynamic.

This is a discussion on Need a refresher, the difference between static and dynamic. within the C++ Programming forums, part of the General Programming Boards category; I haven't been using as much C++ these days as I would like to and am getting back into installing ...

  1. #1
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    Need a refresher, the difference between static and dynamic.

    I haven't been using as much C++ these days as I would like to and am getting back into installing some libraries. I was just wondering, when creating projects using the libraries, there are the static libraries that are the standard .a files and the .a filles that are postfix with dll and come complimentary with dlls. If I remember the dll enabled libraries allow the compilation of the source code and allow the program to execute functions during runtime, while the .a files compile all the functions entirely. Am I correct?

  2. #2
    Cat without Hat CornedBee's Avatar
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    .a/.lib are static libraries. When you link them against your program, the linker pulls the code from the library and adds it to your executable.

    .so/.dll are dynamic libraries. When you link them against your program, the linker marks the dependency in the header and creates indirect calls in the code. The program loader loads the required dynamic libraries and resolves the indirect calls.

    Dynamic libraries can also be loaded at runtime with the appropriate functions (dlopen()/LoadLibrary()). This is not possible with static libraries.
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  3. #3
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    So why i I build a .dll library it generates the .a which I have to include in the project options.

  4. #4
    Cat without Hat CornedBee's Avatar
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    The .a contains stubs that jump into the .dll. It's the way Windows DLLs work. They're weird.
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    "There is not now, nor has there ever been, nor will there ever be, any programming language in which it is the least bit difficult to write bad code."
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  5. #5
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    And those stubs are compiled within the executables?

    If I wanted to create the functionality seen in something like C# where you can invoke the methods and classes from just a dll file without the .a, is it possible?

  6. #6
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    Quote Originally Posted by indigo0086 View Post
    And those stubs are compiled within the executables?

    If I wanted to create the functionality seen in something like C# where you can invoke the methods and classes from just a dll file without the .a, is it possible?
    It is certainly possible to load a library at runtime. I'm a bit blurry on the details of how you achieve that, because you essentially need to have a connection to get the vtable from the DLL. I'm sure someone else can explain, or perhaps you can find the answer on the web somewhere.

    --
    Mats
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  7. #7
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    Example:

    DLL:

    Code:
    #include <stdio.h>
    #include <Windows.h>
    
    void sampleputs(const char *msg)
    {
    	if(msg)
    	{
    		puts(msg);
    	}
    	else puts("<NULL Pointer>");
    }
    
    BOOL WINAPI DllMain(HINSTANCE hInst, DWORD dwReason, LPVOID lpReserved)
    {
    	return TRUE;
    }
    Test program:

    Code:
    #include <stdio.h>
    #include <Windows.h>
    
    typedef void (*FN_OUTPUT)(const char *);
    
    int main(void)
    {
    	FN_OUTPUT fnOutput = NULL;
    	HMODULE hModule = LoadLibrary("sampledll.dll");
    	
    	if(hModule)
    	{
    		if((fnOutput = (FN_OUTPUT)GetProcAddress(hModule,"sampleputs")))
    		{
    			fnOutput("Success!");
    			fnOutput(NULL);
    			fnOutput = NULL;
    			FreeLibrary(hModule);
    		}
    		else fputs("Invalid proc.", stderr);
    	}
    	else fputs("Invalid dll.", stderr);
    	return 0;
    }
    Output:

    Code:
    Success!
    <NULL Pointer>
    Compilation for MinGW:

    Code:
    gcc -Wall -ansi -pedantic -O3 -shared sampledll.c -osampledll.dll
    
    gcc -Wall -ansi -pedantic -O3 sampledyn.c
    Edit: Apologies for C code, but this was the quickest example I had.
    Last edited by MacGyver; 06-27-2008 at 02:33 AM.

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