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I am trying to make a function, that lets me look in an array of structs, and return the one whose value of a certain member matches the input. I found another thread, where a guy had a familiar issue, but it doesn't seem to work for me. The other thread can be found here: Get a struct based on member values

I have the struct:

typedef struct
{
    uint8_t register_id;
    uint8_t size; //In bytes
    char * name;
    char * access;
} Register;

I've made an array of these:

Register VN100_registers[4] = {
                                 {9,  16, "Attitude Quaternion",      "read only"},
                                {17, 12, "Magnetic measurements",     "read only"},
                                {18, 12, "Acceleration measurements", "read only"},
                                {19, 12, "Angular rate measurements", "read only"},

                                };

I use this function:


static Register* GetVN100Register(const void* value_ptr, const size_t offset, const size_t field_size)
{
    for (uint32_t i = 0; i < N_ELEMS(VN100_registers); i++)
    {
        if (0 == memcmp(value_ptr, ((unsigned char*)&VN100_registers[i])+offset, field_size))
        {
            return &VN100_registers[i];
        }
    }
    return NULL;
}

It is supposed to point to the struct in the array VN100_registers, where a member value matches function input.

I initialize the variables:

Register *test_ptr;
uint32_t value_to_find;

And call the function:


  value_to_find = 18;

  test_ptr = GetVN100Register(&value_to_find, offsetof(Register, register_id), sizeof(value_to_find));

I should be pointed to the third struct of the array, but when I debug, I get this:

Debug view

victorsoeby
  • 1
  • 2
  • It should work if you change the declaration to `uint8_t value_to_find;` so that it matches the type of the `register_id` member. This technique is unlikely to be suitable for finding your `char *` members, because you probably want to compare the contents of two strings rather than compare two pointer values. – Ian Abbott Dec 09 '20 at 14:58
  • I think you would be better off with a function that finds the Register by ID (using `==` rather than `memcmp`) and another function that finds the Register by name (using `strcmp`) if required. – Ian Abbott Dec 09 '20 at 15:03

0 Answers0