CODEWITHSUNDEEP
cpointerssyntaxparametersconstants
snuff
snuff

Reputation: 83

Troubles with c Syntax - const * const * definition

I have troubles using a function that is defined like this:

some_function(address_t const * const * my_addrs, uint8_t length)

wheareas address_t is defined as:

typedef struct
{
  uint8_t addr_id   : 1;       
  uint8_t addr_type : 7;       
  uint8_t addr[6]; 
} address_t;

How am I supposed to call this function?

The code is from a bluetooth library and is supposed to set a whitelist of bluetooth addresses. So the idea is to define multible address_t structs with different addr[6] information.

Thank you very much for any help

edit: here are some more information

I have several addres_t structs. They are defined like this:

address_t addr1 = {.addr_id= 1, .addr_type = 3, .addr = {0x12,0x34,0x56,0x78,0x90,0xAB}};
address_t addr2 = ...

I can combine then to an array like:

address_t my_whitelist[6];
my_whitelist[0] = addr1;
my_whitelist[1] = addr2;
...

I'm not sure if this is needed or not. Now I have to pass this some how to this function. I hope this further information helps.

Upvotes: 4

Views: 105

Answers (1)

chux
chux

Reputation: 153457

How am I supposed to call this function?

Example call

typedef struct {
  uint8_t addr_id :1;
  uint8_t addr_type :7;
  uint8_t addr[6];
} address_t;

//                      1st const,  2nd const
//                          v---v   v---v
int some_function(address_t const * const * my_addrs, uint8_t length) {
  (void) my_addrs;
  (void) length;
  return 0;
}

int foo() {
  const address_t addr1 = { .addr_id = 1, .addr_type = 3, .addr = { 1,2,3,4,5,6 } };
  const address_t addr2 = { .addr_id = 1, .addr_type = 3, .addr = { 1,2,3,4,5,6 } };
  const address_t addr3 = { .addr_id = 1, .addr_type = 3, .addr = { 1,2,3,4,5,6 } };

Notice the type change of my_whitelist[]. This need to be an array of pointers. Those pointers need to point to const data due to 1st const above.

  // address_t my_whitelist[6];
  const address_t *my_whitelist[6];
  my_whitelist[0] = &addr1;
  my_whitelist[1] = &addr2;
  my_whitelist[2] = &addr3;
  my_whitelist[3] = &addr1;
  my_whitelist[4] = &addr2;
  my_whitelist[5] = &addr1;
  uint8_t len = sizeof my_whitelist / sizeof my_whitelist[0];

Notice my_whitelist[] does not need to be const due to the 2nd const above as with const address_t * const my_whitelist[6];. This 2nd const above informs the calling code that some_function() will not modify the array elements of my_whitelist[].

  return some_function(my_whitelist, len);
}

Note: If my_whitelist[] was a const array, its values cannot be assigned yet can be initialized.

// Example usage with a `const my_whitelist[]`
const address_t * const my_whitelist[] = { &addr1, &addr2, &addr3 };

Note: address_t const * is like const address_t *. Leading with const matches the style of the C spec.

address_t const * const * my_addrs;
// same as 
const address_t * const * my_addrs;  // More common

Upvotes: 2

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