ObjC blocks & openssl C callbacks

Question

Have a simple one-off tasks which needs a progress bar. OpenSSL has a useful callback which one can use for that:

rsa=RSA_generate_key(bits,RSA_F4,progressCallback,NULL);

with

static void callback(int p, int n, void *arg) { .. stuff

However I want to call this from ObjectiveC without too much ado:

 MBProgressHUD *hud = [MBProgressHUD showHUDAddedTo:self.view animated:YES]; hud.mode = MBProgressHUDModeAnnularDeterminate; hud.labelText = @"Generating CSR"; [self genReq:^(int p,int n,void *arg) { hud.progress = --heuristic to guess where we are -- } completionCallback:^{ [MBProgressHUD hideHUDForView:self.view animated:YES]; }];

With Genrec: as an objC method:

-(void)genReq:(void (^)(int,int,void *arg))progressCallback completionCallback:(void (^)())completionCallback { ..... rsa=RSA_generate_key(bits,RSA_F4,progressCallback,NULL); assert(EVP_PKEY_assign_RSA(pkey,rsa)); rsa=NULL; .... completionCallback(); }

Now completionCallback(); works splendidly and as expected. But I get a compiler warning/error which I cannot quell for the progress callback:

 Passing 'void (^__strong)(int, int, void *)' to parameter of incompatible type 'void (*)(int, int, void *)'

So am curious -- what is the appropriate way to do this ?

Thanks,

Dw.

CRD · Accepted Answer · 2012-05-04 09:33:08Z

All code is just typed into this answer, test carefully before using!

Function pointers and blocks are not the same thing; the former is just a reference to code, the latter is a closure containing both code and an environment; they are not trivially interchangeable.

You can of course use function pointers in Objective-C, so that is your first option.

If you wish to use blocks then you need to find a way to wrap a block and pass it as a function reference...

The definition of RSA_generate_key is:

RSA *RSA_generate_key(int num, unsigned long e, void (*callback)(int,int,void *), void *cb_arg);

The fourth argument can be anything and is passed as the third argument to the callback; this suggests we could pass the block along with a pointer to a C function which calls it:

typedef void (^BlockCallback)(int,int); static void callback(int p, int n, void *anon) { BlockCallback theBlock = (BlockCallback)anon; // cast the void * back to a block theBlock(p, n); // and call the block } - (void) genReq:(BlockCallback)progressCallback completionCallback:(void (^)())completionCallback { ..... // pass the C wrapper as the function pointer and the block as the callback argument rsa = RSA_generate_key(bits, RSA_F4, callback, (void *)progressCallback); assert(EVP_PKEY_assign_RSA(pkey,rsa)); rsa = NULL; .... completionCallback(); }

And to invoke:

[self genReq:^(int p, int n) { hud.progress = --heuristic to guess where we are -- } completionCallback:^{ [MBProgressHUD hideHUDForView:self.view animated:YES]; } ];

Whether you need any bridge casts (for ARC) is left as an exercise!

Most Lovely ! And __bridge is all that is needed for ARC. Shame that one needs to have the callback() in between - i.e. that one cannot pass the progressCallback as a (void (*)(int, int, void *)) from its void (^__strong)(int, int, void *) directly.
@Dirk-WillemvanGulik - the only reasonable way to support automatic passing of a block, i.e. closure (code pointer + environment), as a function pointer (just a code pointer) would be to dynamically generate code stubs; and that has various drawbacks. The common pattern in C is to specify a function pointer taking an extra value, usually of type void *, that can be used to pass a user-defined environment and thus manually construct a closure - and openssl follows that pattern and the solution above just uses it.
Clear. Thanks (again!). Have dug into NSStackBlock and makes total sense.

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