C sizeof

• sizeof is a unary operator in C.
  
  int main() { printf("%d", sizeof(3,3.0999999)); }

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  int main() {    printf("%d", sizeof(3,3.0999999)); }

  
  Output:
  
  8

  1

  8

  
  The terms in sizeof() are stored in stack. Firstly the 3 is inserted then 3.0999999. Thus as we know stack is a LIFO (last in first out) data structure. Therefore sizeof operator will give a size of last item i.e, 3.0999999. As 3.0999999 is of double data type, it returns size of double i.e, 8.
• Here in the first printf, address array elements are always an integer. So that it return value of integer data type i.e, 4 bytes. In the second printf statement, it returns the number of elements in arr and ‘\0’
  
  #include <stdio.h> int main() { char arr[] = "johnjohny"; printf(" %d \n ", sizeof( &arr[0] ) ); //8 printf(" %d\n ", sizeof( arr ) ); //10 }

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  #include <stdio.h> int main() { char arr[] = "johnjohny"; printf(" %d \n ", sizeof( &arr[0] ) );    //8 printf(" %d\n ", sizeof( arr ) );         //10 }

• In first statement, null consider as a string. In the second statement, nothing means zero. So there are two elements zero and \0. Therefore it return number of elements.
  
  int main() { printf(" %d\n ", sizeof( " null " )); //5 printf(" %d\n ", sizeof(" ")); //2 }

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  int main() {    printf(" %d\n ", sizeof( " null " ));    //5    printf(" %d\n ", sizeof(" "));           //2 }

Difference between sizeof and strlen