Question

I am new to C and i have this question. why does the following code crash:

int *a = 10; *a = 100; 

Answer 1

Because you are trying to write 100 to the memory location 0x0000000A which is probably not allocated to your program. That is,

int *a = 10; 

does not mean that the pointer ‘a’ will point to a location in memory having the value of 10. It means it is pointing to address 10 (0x0000000A) in the memory. Then, you want to write something into that address, but you don’t have the ‘rights’ to do so, since it is not allocated

You can try the following:

int *a = malloc(sizeof(int)); *a = 100; 

This would work, although horribly inefficient. If you only need a single int, you should just put it into the stack, not the heap. On a 32-bit architecure, a pointer is 32 bits long, and an int is 32 bits long too, so your pointer-to-an-int structure takes up (at least) 8 bytes of memory space this way instead of 4. And we haven’t even mentioned caching issues.