I'm trying to understand how it's working.
#include<stdio.h> int main() { int a = 110; double d = 10.21; printf("sum d: %d \t\t size d: %d \n", a+d, sizeof(a+d)); printf("sum lf: %lf \t size lf: %lf \n", a+d, sizeof(a+d)); printf("sum lf: %lf\t size d: %d \n", a+d, sizeof(a+d)); printf("sum d: %d \t\t size lf: %lf \n", a+d, sizeof(a+d)); return 0; } The output is:
sum d: 8 size d: 1343288280 sum lf: 120.210000 size lf: 0.000000 sum lf: 120.210000 size d: 8 sum d: 8 size lf: 120.210000 printf reads a certain number of bytes off the stack for each of the format specifiers you provide. The format specifiers must match the actual arguments or you can end up with arguments being partially read or reads going past the boundaries of the argument.
In your first statement, the first argument is a double so %f is the correct format specifier. Using %d may result in printf attempting to read more bytes than were provided for that argument, leading to undefined behavior. The second argument is of type size_t which requires %zu or another valid specifier for that type.
a + d must be %f.%lf or %f (both have the same definition)%d in the answer.
sizeofneeds a%zuformat specifier.printf()that are mismatched with their corresponding formatting directives. There is nothing to understand here, beyond that C makes no guarantees about what the program will do.