07-15-12 - libpng DLL Delay Load

cblib for a long time has used libpng , which creates a damn DLL dependency. Annoyingly, that happens at app startup, which means even if you are trying to load a .BMP it will fail to start the app if it can't find the PNG DLL (or, you know, using cblib on totally non-image related stuff). What I've wanted for a long time is to put the PNG DLL load into the PNG code so that it's only done if you actually try to load a PNG.

MS could have very easily put DLL load-on-first use into the thunk libs they make. That would have been nice. (ADDENDUM : I guess they did!)

Anyhoo, you can do it yourself, and it looks like this :

We need CALL_IMPORT from before :

template <typename t_func_type>
t_func_type GetImport( t_func_type * pFunc , const char * funcName, HMODULE hm )
{
    if ( *pFunc == 0 )
    {
        ASSERT( hm != 0 );
        t_func_type fp = (t_func_type) GetProcAddress( hm, funcName );
        // not optional :
        ASSERT_RELEASE_THROW( fp != 0 );
        *pFunc = fp;
    }
    return (*pFunc); 
}

#define CALL_IMPORT(name,hm) (*GetImport(&STRING_JOIN(fp_,name),STRINGIZE(name),hm))

and I'm going to just explicitly load the PNG DLL when I get to LoadPNG :

static HMODULE hm_png = 0;
#define HMODULE_FAILED  (HMODULE) 1

static bool my_png_init()
{
    if ( hm_png ) 
    {
        return ( hm_png == HMODULE_FAILED ) ? false : true;
    }
    
    HMODULE hm_zl = LoadLibrary(ZLIB_NAME);
    if ( hm_zl == 0 )
    {
        lprintf("Couldn't load Zlib (%s)\n",ZLIB_NAME);
        hm_png = HMODULE_FAILED;
        return false;
    }
    
    hm_png = LoadLibrary(PNG_NAME);
    if ( hm_png == 0 )
    {
        lprintf("Couldn't load PNG lib (%s)\n",PNG_NAME);
        hm_png = HMODULE_FAILED;
        return false;
    }
    
    lprintf_v2("Using libpng.\n");
                
    return true;
}

#define CALL_PNG(name)      CALL_IMPORT(name,hm_png)

so now we just have to replace all our png calls with CALL_PNG() calls, like :

    png_ptr = png_create_read_struct(PNG_LIBPNG_VER_STRING, NULL,NULL,NULL);

    ->

    png_ptr = CALL_PNG(png_create_read_struct)(PNG_LIBPNG_VER_STRING, NULL,NULL,NULL);

(obviously you could #define png_create_read_struct CALL_PNG(png_create_read_struct) to make it totally transparent)

Lastly we need to define all the fp_ func pointers with the right signature. This is particularly easy because png.h wraps its function externs with macros (nice, thanks guys), so we can just abuse those macros. png.h contains things like :

PNG_EXPORT(png_voidp,png_get_io_ptr) PNGARG((png_structp png_ptr));

So we can just do :

#define PNG_EXPORT(ret,func)    static ret (PNGAPI * STRING_JOIN(fp_,func))
#define PNGARG(args)            args = 0

and then the png proto is defining our fp for us. (unfortunately png.h sticks "extern" in front of all the protos, so you have to copy out the protos and take off the extern).

So anyhoo there you go, libpng use with delay-load.

BTW this also suggests a way that you can make your DLL very easy to use with delay-load and manual imports. What you should do is provide a header with only your function protos in it, and wrap them with a macro like :

DECLFUNC( ret, name , args );

eg.

DECLFUNC( png_voidp, png_get_io_ptr, (png_structp png_ptr) );

Then a client could just include that header multiple times and change DECLFUNC to various things. For example if you had a header like that, you can look up all the func names at LoadLibrary time, instead of doing each one on first use (this removes a branch from each function call site). eg :

#define DECLFUNC( ret, name , args )    ret (CALLBACK * STRING_JOIN(fp_,name)) args = 0
#include "allfuncs.inc"
#undef DECLFUNC

void ImportAllFuncs()
{
HMODULE hm = LoadLibrary;
#define DECLFUNC( ret, name , args )    STRING_JOIN(fp_,name) = ImportFunc(name,hm)
#include "allfuncs.inc"
#undef DECLFUNC
}

Unfortunately you can't do a #define inside a #define or this could be used to alias the names transparently with something like

#define DECLFUNC(ret,name,args) #define name (* STRING_JOIN(fp_,name) )
#include "allfuncs.inc"
#undef DECLFUNC

(ADDENDUM : or, you know, just use the /DELAYLOAD option)