Question

Given: SInt16 *samples = NULL;

What does (char **)(&samples) declare?

Edit to show actual use case:

CMBlockBufferGetDataPointer(audioBlockBuffer, audioBlockBufferOffset, &lengthAtOffset, &totalLength, (char **)(&samples));
//  CMBlockBufferGetDataPointer(<CMBlockBufferRef theBuffer>, <size_t offset>, <size_t *lengthAtOffset>, <size_t *totalLength>, <char **dataPointer>)

Answer 1

Update: The original question was asking simply What does (char **)(&samples) declare? As such, in my original answer I tried to outline the possible uses of that syntax in the abstract. But subsequently it was clarified that this was for a parameter to CMBlockBufferGetDataPointer. But I’ll leave my original answer for the sake of completeness.

Original answer:

This syntax has one of two possible interpretations. First, this syntax can be used as a pointer to a pointer of a SInt16 which has been cast to a pointer to a pointer of a char. By the way, Apple uses the term indirect reference for these pointers to pointers. Or, second, this could be a pointer to an array of SInt16 which has been cast to a pointer to an array of char. It depends upon how this (char **)&samples is being used.

Focusing on the first interpretation, in your example the variable samples is defined to be (SInt16 *), a pointer (or direct reference) to a SInt16 (a signed short integer). Thus &samples is the address of that that samples pointer, which could properly be used in any situation where you need to pass a pointer to a pointer (an indirect reference) of a SInt16, namely (SInt16 **).

In terms of where you’d use a pointer to a pointer, an indirect reference, it’s frequently used when you have a method that will allocate some memory for some object and it needs to update one of your pointers with a reference to this new object. The most common Cocoa example of this construct is the frequent use of (NSError **). You can see examples of this in Apple’s Error Handling Programming Guide.

What’s strange about your example, is that you’re casting &samples, your indirect reference to a SInt16 (or a pointer to an array of SInt16), to be a (char **), a char indirect reference (or an array of char). That sends a shudder down the spine of all of us reformed C programmers. If you do this, you should be very, very comfortable with the code that is using this construct as this is a little dangerous. But I realize that we’re sometimes constrained by legacy code.