如何在 C 中使用 #define 中的指针和定义

Question

I was wondering exactly how the gattCharCfg_t within this defined snippet is being used.

Here is the #define snipped:

    // Client Characteristic Configuration table (from CCC attribute value pointer)
      #define GATT_CCC_TBL( pValue )           ( (gattCharCfg_t *)(*((PTR_TYPE)(pValue))) )

And here is how it is being accessed in a couple spots in the given code:

    // Characteristic Configuration: voltage
    static gattCharCfg_t *voltDataConfig;

      // Allocate Client Characteristic Configuration table
  voltDataConfig = (gattCharCfg_t *)ICall_malloc(sizeof(gattCharCfg_t) *
                                                    linkDBNumConns);
    if (voltDataConfig == NULL)
    {
      return (bleMemAllocError);
    }

I guess I am not understanding the mechanics of how it is being accessed. I would definitely appreciate a more thorough explanation as I am just diving back into c.

Answer 1

( (gattCharCfg_t *)(*((PTR_TYPE)(pValue))) )

1. (PTR_TYPE)(pValue) - cast pValue as a PTR_TYPE (pointer)
2. *((PTR_TYPE)(pValue)) - look at the memory location pointed to by the pointer
3. (gattCharCfg_t *)(*((PTR_TYPE)(pValue))) - for a pointer pointing to a gattCharCfg_t structure (could be an array of gattCharCfg_t ) instead of void *

GATT_CCC_TBL is a macro to shortcut having to write out the conversion. With respect to the other code in your post,

  voltDataConfig = (gattCharCfg_t *)ICall_malloc(sizeof(gattCharCfg_t) *
                                                    linkDBNumConns);

1. sizeof(gattCharCfg_t) - take the size of gattCharCfg_t , eg, 23 bytes

2. * linkDBNumConns - multiply it by linkDBNumConns, eg, 100 making 2300 bytes

3. ICall_malloc(..) - allocate that much memory

4. (gattCharCfg_t *) - cast that location in memory as a pointer to gattCharCfg_t structures

Update. Looks like this code is from the Bluetooth stack, in which case

typedef struct
{
  uint16 connHandle; //!< Client connection handle
  uint8  value;      //!< Characteristic configuration value for this client
} gattCharCfg_t;

So GATT_CCC_TBL( pValue ) will take a pointer value pValue and turn what it points to to one or more gattCharCfg_t structures - most likely an array of these since _TBL is likely "table".

Answer 2

In the code there is no use of the macro GATT_CCC_TBL ¹ anywhere. The code is casting the return value of ICall_malloc() which presumably returns void * ² to a pointer of type gattCharCfg_t ³ . If this is really c code that is actually not necessary and

voltDataConfig = ICall_malloc(sizeof(gattCharCfg_t) * linkDBNumConns)

and the better version would be

voltDataConfig = ICall_malloc(sizeof(*voltDataConfig) * linkDBNumConns)

would probably be correct.

---

¹ _{Which is what #define defines actually.}
² _{You never need to cast void * to another pointer type in c .}
³ _{Note that gattCharCfg_t is actually a type defined somewhere with a typedef .}