How to use struct and malloc in C99

Question

I need to translate my C++ code into C99:

struct TransferData  {
    int data[256];
} *readData;    

readData = (TransferData  *) malloc(sizeof(struct TransferData));       

I nearly tried everything but get compiler errors each time. Can anyone help?

Answer 1

For C you need to cast to struct TransferData *, not to TransferData *. You don't have a type named TransferData * (or drop the cast altogether, but it's another holy war, pointless IMHO).

Answer 2

You can either do as @icepack said, or change your struct definition like so:

typedef struct  {
    int data[256];
} TransferData;

TransferData *readData;
readData = malloc(sizeof(TransferData));

Edit: Note that it's preferable to use sizeof with variables instead of types to avoid repetition:

readData = malloc(sizeof(*readData));

But be careful not to accidentally pass the size of a pointer when using this style.

Without the typedef, you need to write struct StructType in C, which you don't need in C++. As said in the comments, casting the result of malloc is unnecessary in C, and just clutters your code. It is necessary in C++, but you shouldn't be using malloc there.

Answer 3

If you are using C, you should not cast the result of malloc. It's a bad style to repeat type information. Also, the type of the structure is struct TransferData and not TransferData as in C++. I suggest

readData = malloc(sizeof(*readData));