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

You got (un)lucky. Accessing p[3] is undefined, since you haven't allocated that memory for yourself. Reading/writing off the end of an array is one of the ways that C programs can crash in mysterious ways.

For example, this might change some value in some other variable that was allocated via malloc. That means it might crash later, and it'll be very hard to find the piece of (unrelated) code that overwrote your data.

Worse yet, you might overwrite some other data and might not notice. Imagine this accidentally overwrites the amount of money you owe someone ;-)