memory management - 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;  //Using another space that I didn't allocate for. 

printf("%d", *(p+1)); //Correctly prints 20
printf("%d", *(p+2)); //Also, correctly prints 30
                      //although I didn't allocate space for it

With the line malloc(2 * sizeof *p) I am allocating space for two integers, right ? But if I add an int to the third position, I still gets allocated correctly and retrievable.

So my question is, why do you specify a size when you use malloc ?

Answer 1

Simple logic: If you do not park in a legal parking space, nothing might happen but occasionally your car might get towed and you might get stuck with a huge fine. And, sometimes, as you try to find your way to the pound where your car was towed, you might get run over by a truck.

malloc gives you as many legal parking spots as you asked. You can try to park elsewhere, it might seem to work, but sometimes it won't.

For questions such as this, the Memory Allocation section of the C FAQ is a useful reference to consult. See 7.3b.

On a related (humorous) note, see also a list of bloopers by ART.