Question

I don’t know what the compilar is doing with ++*p;

Can anyone explain me pictorically what is going on inside the memory in this code?

int main()
{
    int arr[]={1,2,3,4};
    int *p;
    p=arr;
    ++*p;
    printf("%d",*p);
}

Answer 1

Making up the actual memory addresses and using “ma” for memory address

at memory address starting at 1000 we have 4 continuous 4-byte (sizeof(int) = 4) slots.
each slot contains the integer value given in the array initializer:

  arr
  ma1000 ... ma1015
  _____________________
  |   1|   2|   3|   4|   
  _____________________

arr gives the starting address of the 4 int slots and how many there are.
p holds the address of an integer and refers to one 8-byte slot in memory (assuming we are on a 64-bit system where pointers are 8 bytes – 64 address bits/8bits-per-byte) at location 2000.
After the statement p = arr, p holds the address 1000

  p                     *p or arr[0]
  ma2000 .. ma2007      ma1000 .. ma1003
  __________            ________
  |    1000|            |    1 |
  __________            ________

*p gives the value at the memory address pointed to by p. p holds memory address 1000 and memory address 1000 contains 1, thus *p results in 1.

++*p says to increment the value of the int “pointed to” by p. p holds memory address 1000 which holds the value 1. The value at address 1000 then goes from 1 to 2

 arr
 ma1000 ... ma1015
 _____________________
 |   2|   2|   3|   4|   
 _____________________

printf then prints the int value at the address “pointed to” by p, which is 2.

  p                     *p or arr[0]

  ma2000 .. ma2007       ma1000 .. ma1003
  ___________           ___________
  |     1000|           |        2|
  ___________           ___________