Pointer to pointer arithmetic operations

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#include <stdio.h>
    int main(){ 

int a=95;
int *ptrint=&a;
printf("%u\n",ptrint);
int  b=85;
int *ptri2=&b;

    printf("%u\n",ptri2);


printf("%u and %u\n",ptrint ,ptri2);
ptrint--;
printf("%u\n", ptri2 - ptrint ); //why its illegal 
printf("%u\n", ptrint -ptri2  ); // whys its legal 

return 0;

}

What I have tried:

As per logic say the way I am getting this error is because Its address not vairable . PLease do let me know if my logic is right here . I tried to subtract one address from another

Posted 4-Jun-22 8:39am

Aayush Jain 2022

Updated 4-Jun-22 9:57am

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OriginalGriff · Answer 1 · 2022-06-04T09:57:00

Both are legal - if I run your code through an online C compiler (GDB) I get what I expect:

1396230176
1396230180
1396230176 and 1396230180
2
4294967294

The last number is an unsigned integer - it's actually -2 if you use a signed printf format code like "%d" instead of "%u"
However, pointer arithmetic is rarely of any real use, just like arithmetic performed on mobile phone numbers: you could subtract my number from yours and get a valid value, but it is not of any real use to anyone for anything!