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 are asking for space for two integers. p[3] assumes that you have space for 4 integers!

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You need to tell malloc how much you need because it can't guess how much memory you need.

malloc can do whatever it wants as long as it returns at least the amount of memory you ask for.

It's like asking for a seat in a restaurant. You might be given a bigger table than you need. Or you might be given a seat at a table with other people. Or you might be given a table with one seat. Malloc is free to do anything it wants as long as you get your single seat.

As part of the "contract" for the use of malloc, you are required to never reference memory beyond what you have asked for because you are only guaranteed to get the amount you asked for.