I have a program question, here is the code.
int main() { int *p,*q; p=(int*)1000; printf("%d ",p); q=(int*)2000; printf("%d",q); printf("%d",(p-q)); return 0; } But answer coming as
1000 2000 -250 I am unable to understand what happen with p-q and why answer came as -250?
Correct but probably useless answer: p - q is equal to (1000 - 2000) / (sizeof int). For most C compiles, sizeof int is 4.
Potentially more useful answer: the effect of typecasts like (int*) 1000 is undefined. That code creates a pointer to an int at address 1000. That address is probably invalid. To create a pointer to an int with value 1000, write this:
int i = 1000; int *p = &i; Now p points to i, and *p, the value pointed to by p, is 1000.
Here is some correct code that may say what you meant:
int main() { int i = 1000; int j = 2000; int *p = &i; int *q = &j; printf("i = %d *p = %d\n", i, *p); printf("j = %d *q = %d\n", j, *q); printf("*p - *q = %d\n", *p - *q); } When you subtract two pointers, as long as they point into the same array, the result is the number of elements separating them.
On your platform, an int is 4 bytes. There are -250 elements between address 2000 and address 1000.
Since p and q don't both point to the same array, the result is undefined. You could get any result, including the one you expect.
p is a pointer variable, which can store only address of a int variable. But you are storing 1000 as an address, which is invalid. And then you are storing 2000 to the variable q.
Now you are doing pointer arithmatic p-q. Always pointer arithmatic will gives the output based on the size of the type of address. Which will work like (p - q)/sizeof(type).
Consider if p and q are char * variable, then p-q will gives you the output as -1000
In your case p and q are int * variable, then p-q will gives you the output as 250 if size of int is 4 bytes. Execute this on the compiler where size of int is 2 bytes, you will get a result as -500.
%dformat specifier invokes undefined behaviour. You can printvoid*with the%pformat specifier,printf("%p ", (void*)p);, or you can cast the pointer to an integer type before printing,printf("%" PRIdPTR " ", (intptr_t)p);. For the pointer difference,printf("%td ", (p-q));.