Explain this C array syntax

Question

There is a section of code from a learning resource that I am studying. I've come across a good bit of syntax pertaining to arrays and I would be grateful if somebody could explain them to me.

1)
AudioBufferList *bufferList;
 bufferList = (AudioBufferList *) malloc (
 sizeof (AudioBufferList) + sizeof (AudioBuffer) * (channelCount - 1)
 );

Line 1 here I understand. What I don't understand is everything from sizeof on. The only value that I know for sure what it is here is channelcount. What is sizeof doing here and what size values could AudioBufferList and AudioBuffer be passing to it? Are there constant values or something?

2)
 bufferList->mNumberBuffers = channelCount;
 // initialize the mBuffers member to 0
 AudioBuffer emptyBuffer = {0};
 size_t arrayIndex;
 for (arrayIndex = 0; arrayIndex < channelCount; arrayIndex++) {
 bufferList->mBuffers[arrayIndex] = emptyBuffer;
 }
 // set up the AudioBuffer structs in the buffer list
 bufferList->mBuffers[0].mNumberChannels = 1;
 bufferList->mBuffers[0].mDataByteSize = totalFramesInFile * sizeof (AudioUnitSampleType);
 bufferList->mBuffers[0].mData = soundStructArray[audioFile].audioDataLeft;
 if (2 == channelCount) {
 bufferList->mBuffers[1].mNumberChannels = 1;
 bufferList->mBuffers[1].mDataByteSize = totalFramesInFile * sizeof (AudioUnitSampleType);
 bufferList->mBuffers[1].mData = soundStructArray[audioFile].audioDataRight;
 }

What does -> do?

What does size_t mean?

Also is mBuffers some sort of system constant?

Answer 1

It is not objectiveC but just C

sizeof return the size of the datatype (static by the compilator)

wikipedia reference:

• sizeof
• size_t
• pointer (->)

Answer 2

This is almost exclusively c syntax and if you are confused about this code, then you should get a good reference on c. Instead of explaining the various bits, I can recommend an excellent reference: C A Reference Manual.

Answer 3

As a few other posters noted, this is more C than Objective-C. Grabbing a good C primer (K&R's C book is always a good choice) might be helpful. However, to answer a few of your questions:

What I don't understand is everything from "sizeof" on. The only value that I know for sure what it is here is "channelcount". What is sizeof doing here and what size values could AudioBufferList and AudioBuffer be passing to it? Are there constant values or something?

sizeof is a C operator (despite the fact that it looks like a function) that returns the size in bytes of a given data type. malloc is a function that allocates memory, so that line is basically saying "allocate enough memory for an AudioBufferList, an AudioBuffer, and the channels I need, and return a pointer to the allocated memory region".

What does "->" do?

It gets a member of a struct via a pointer. In C, a struct is sort of like an Objective-C object: it has members which are basically like instance variables in Objective-C. In Objective-C, if you want to call a method myValue, you'd do [obj myValue]; in C, if you have a pointer to a struct and want to get the value my_value, you'd do obj->my_value, which is basically the same as (*obj).my_value, which is saying "get the object pointed to by obj, and find the member my_value in it".

What does "size_t" mean?

size_t is a common C datatype. It is "typedef'ed" on most platforms to be an unsigned int or unsigned long, and it is used to represent the sizes of objects in a platform-independent way.

Also is "mBuffers" some sort of system constant?

No; it's a member (remember, kind of like an instance variable) of bufferList, which is probably an AudioBufferList data type.

• dubbeat: thanks for the detailed explanations. I really do need a book so the k & r one is on the way!

Answer 4

Starting with the great comments and readings suggested above, which are a must to learn, perhaps it whold not be unworth mentioning that the code you're trying to understand is a CoreAudio code, an API which is entirely written in plain-C, although in itself far from being plain.

An additional piece of literature you may find helpful for reading may be:

Adamson/Avila: Learning Core Audio A Hands-On Guide to Audio Programming for Mac and iOS

ISBN-13: 978-0321636843

ISBN-10: 0321636848

Hope this can facilitate your learning curve.