Understanding structures and pointers..?

[1530dean]

Hi
I am a newby to C trying to get my head around some basic structure syntax within an embedded ARM FM4 environment.
I am trying to understand the meaning of the * character in the following statement..?

#define FM4_GPIO ((FM4_GPIO_TypeDef *)FM4_GPIO_BASE)

Can anyone help please..?

[weaknessforcats]

#define FM4_GPIO ((FM4_GPIO_TypeDef *)FM4_GPIO_BASE)

The is a pre-processor directive (the line starts with a #). The pre-processor scans the code and does a global search and replace. In this case anytime FM4_GP10 is located, it is changed to (FM4_GPIO_TypeDef *)FM4_GPIO_BASE.

What this does is change all FM4_GP10 into a type cast of FM4_GP10_BASE to a pointer to FM4_GP10_Typedef.

The exact meaning can only be determined from the code. In any case, this is not good stuff for a newbie to C. This is kind of like learning a foreign language and you have encountered a rarely used big word. I suggest you accept this on face value and move on to understand such parts of the code as you can.

If the original author is still around you can get a complete explanation there.

[donbock]

Generally in C, pointers are assigned to the address of some variable. For instance:

int a;
int *pa = &a;

Something different is needed if you want to point at a memory-mapped I/O location. Suppose the hardware/software interface manual for your system tells you that a memory-mapped I/O device is at address 0xDD800000 and that the device must be accessed with 32-bit bus transaction. A common idiom in C is:

#define DEVICE_ADDR 0xDD800000
int32_t * const pDevice = ((int32_t*)DEVICE_ADDR);

The #define associates a macro name with the address of the device. Thus, there is only one place in your code (the #define itself) where the specific hex address of the device appears. Everywhere else reference that value with the descriptive macro name.

The next line defines a pointer to 32-bit memory and sets the pointer to the address of the device. The cast is needed to tell the compiler (and more importantly, the next person to read the code) that you meant to assign an integer value to a pointer. You can use this pointer to read from or write to the device.

The const is positioned so that the value of the pointer cannot be changed - it always points to the device.

I'm only guessing this is what your code is doing; but it seems likely. GPIO might stand for "general purpose I/O".

[1530dean]

Hi Everyone - Thank you for your help with this I guess I will go back to writing in assembler for now until I can get my head around some of these convoluted C structures..!