关于空隙长度的困惑*

Question

I have a question about void*. I have a function which captures blocks of 0.2 sec of audio by microphone. I have to process these blocks, in concrete a convolution.

This function returns these blocks of audio as a void * . To process this information I can't use void * because I can't access to them, so I have to convert in other kind of data, for example double but I don't know which length is assigned to this new pointer or how can I do it.

My code:

void Pre_proc_mono::PreProcess(void *data, int lenbytes, float t_max){
    double * aux = (double*) data;
}

Now, aux's length is lenbytes too? Or I have to do something like:

int size = lenbytes/sizeof(double);

How can I make this work?

Answer 1

A pointer is an address in memory. This is the address of the first byte of data. The type of the pointer tells you how long is the data. So if we have

int *p

the value of p tells you where the data starts, and the type of the pointer, in this case int * tells you that from that address you need to take 4 bytes (on most architectures).

A void * pointer has only the starting address, but not the length of the data, so that's why you can't dereference a void * pointer.

sizeof(p) where p is a pointer (of any type) is the size of the pointer, and has nothing to do with the kind of data you find where the pointer points to

for instance:

sizeof(char) == 1
sizeof(char *) == 4
sizeof(void *) == 4

In your function:

void *data, int lenbytes, float t_max

data is a pointer to where the data starts, lenbytes is how many bytes the data has.

So you can have something like:

uint8_t *aux =  (uint8_t*) data;

and you have a vector of lenbytes elements of type uint8_t (uint8_t is guaranteed to have 1 byte).

Or something like this:

double * aux = (double*) data;

and you have a vector of lenbutes/sizeof(double) elements of type double . But you need to be careful so that lenbytes is a multiple of sizeof(double) .

Edit

And as regarding to what you should convert to, the answer depends on only the format of your blocks of data. Read the documentation, or search for an example.