Why should we learn the basics of computer systems?
Half-understanding
You have already learned the basics of programming and have a certain foundation in C and C++ programming. But you will find that you may still not understand the results of the following program:
Array sum
int sum(int a[ ], unsigned len) {
int i, sum = 0;
for (i = 0; i <= len-1; i++)
sum += a[i];
return sum;
}When len = 0, Access Violation will occur when executing the for loop of the sum function. However, when the parameter len is specified as int type, the sum function can be executed correctly. Why?
Explanation
When len is 0 and the type is unsigned int, the for loop of the sum function has an Access Violation because of the arithmetic overflow behavior of unsigned int.
For unsigned integers, underflow results wrap around to the maximum value of the type. For example, if unsigned int is 32 bits, the result of 0 - 1 will be 2^32 - 1 (which is UINT_MAX).
Therefore, the loop condition becomes i <= UINT_MAX.
The loop will start executing from i = 0. In the first iteration, sum += a[i] (i.e. sum += a[0]) will try to access the 0th element of array a.
However, since len is 0, array a is actually empty or should not be accessed. Accessing a[0] is beyond the valid range of the array (out-of-bounds access), resulting in an "Access Violation" runtime error. The loop will continue to try to access a[1], a[2], ..., until UINT_MAX is reached, which will continue to access invalid memory.
Tuntun Yuchiha