Why do I see a cast to signed integer on my unsigned array index?

Question

MathWorks Support Team 2012-7-17

0
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/101992-why-do-i-see-a-cast-to-signed-integer-on-my-unsigned-array-index

We explain here why even if the index of an array is unsigned, Polyspace says that there is conversion to signed integer

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Answer 1

MathWorks Support Team 2012-7-17

0
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/101992-why-do-i-see-a-cast-to-signed-integer-on-my-unsigned-array-index#answer_111339

Example of implicit cast from unsigned to signed:

unsigned int r;
int tab[3] = {1,2,3};
void f(void) {
    int x;
    x = tab[r]; 
    // check details on 'r' says :
    // global variable 'r' (unsigned int 32): full-range [0 .. 2^32 -1]
    // conversion from unsigned int 32 to int 32
}

The C Standard says that array indexes are (signed) integers. The reason behind that is that pointers and arrays are close in C.

For example, tab[index] is strictly equivalent to *(tab + index).

You can use pointer arithmetic with negative values, hence an array index can be negative.

Exemple of valid array access with a negative index:

void f(void) {
   int x;
   int *p;
   int tab[3] = {1,2,3};
   p = &tab[2];  // points to the 3rd element
   x = p[-1];  // x is assigned to the second element, '2'
}