Question

I’m trying to write a function that converts a base 10 integer to a binary number.

I am constructing the function like this:

void dec2bin(int dec, char * bin);

The function will store the binary result by means of the pointer in the argument. It is passed a variable declared elsewhere as follows:

char *r1 = (char *)malloc(24);

The first thing the function has to do is fill the “24 bit result” with all zeros. It works great now since I’ve coded it based on me knowing the “size,” but I can’t figure out how to determine the “size” of an arbitrary “r1”. Every way I can think to use the sizeof function does not give me 24.

I am confused at this point, and not connecting the relationship between the respective sizes of a char, char*, etc and how I can use those sizes to get what I’m looking for with respect to “r1”.

Would someone mind giving me a pointer? Pun intended.

Thanks!

Answer 1

I can’t figure out how to determine the “size” of an arbitrary “r1”.

You cannot do that: the information about the size that you passed to malloc is irreversibly lost after the call. All you have at that point is a pointer to the chunk of at least 24 bytes of memory. If you need the size, you need to store it somewhere else – for example, in a separate variable that you carry around.

Two typical work-arounds to this issue include (1) storing a zero-terminated string in the allocated space, and using the length of that string as the proxy to the size of the allocated chunk, and (2) defining your own structure that contains both the size and the allocated pointer. Neither solution is ideal, but at least the choice is with you.