从ObjC传递char数组到C

Question

I've got a project with a pure c code and c code handled by the ObjC compiler [ .m file ]. The one handled by ObjC compiler has a following class:

unsigned long getMessage(char ** message) {
 *message = (char*)calloc(1, [dMessage bytes], [dMessage length]);
 memcpy(message, [dMessage bytes], [dMessage length])
return [dMessage length];
}

dMessage is an NSData object filled with text.

On C side, I do:

char* msg = NULL
unsigned long length = getMessage(&msg)

After the call, msg is empty, but the length variable is set to correct size.

What should I do to pass char* between objc and c?

Thank you

Answer 1

You're passing the wrong arguments to calloc() . It takes two arguments, but you're passing three. The compiler should be screaming at you about that.

Since the second argument is the bytes pointer from the NSData , you're effectively requesting some huge allocation. It's probably failing. There would usually be a message logged to the console about that failure.

You want:

*message = (char*)calloc([dMessage length], 1);

Answer 2

It was a mistake in my code.

I should havecalled

memcpy(*message, [dMessage bytes], [dMessage length])

instead of

memcpy(message, [dMessage bytes], [dMessage length])