Why do you specify the size when using malloc in C?

Question

Take the following code :

int *p = malloc(2 * sizeof *p);

p[0] = 10;  //Using the two spaces I
p[1] = 20;  //allocated with malloc before.

p[2] = 30;  //U         


        

Answer 1

The reason for the size given to malloc() is for the memory manager to keep track of how much space has been given out to each process on your system. These tables help the system to know who allocated how much space, and what addresses are free()able.

Second, c allows you to write to any part of ram at any time. Kernel's may prevent you from writing to certain sections, causing protection faults, but there is nothing preventing the programmer from attempting.

Third, in all likelyhood, malloc()ing the first time probably doesn't simply allocate 8 bytes to your process. This is implementation dependent, but it is more likely for the memory manager to allocate a full page for your use just because it is easier to allocate page size chunks....then subsequent malloc()'s would further divide the previously malloc()ed page.