Question

I encountered this problem while solving a practice test

Consider this C code to swap two integers and these five statements:

void swap (int *px, int *py) {

*px = *px – *py;

*py = *px + *py;

*px = *py – *px;

}

S1: will generate a compilation error

S2: may generate a segmentation fault at runtime depending on the arguments passed

S3: correctly implements the swap procedure for all input pointers referring to integers stored in memory locations accessible to the process

S4: implements the swap procedure correctly for some but not all valid input pointers

S5: may add or subtract integers and pointers.

Which of the above statement(s) is/are correct?

I think S2 and S3. Could anyone please confirm

Answer 1

I don’t believe S3 holds, since if you call it with px == py, it will just set the integer to 0 in the first line (*px = *px - *py is then equivalent to *px = *px - *px which obviously stores a 0 in *px). With all input data set to 0, it’s unable to recover and re-generate the value.