c - Is it legal to compare a pointer to the beginning of an array with a pointer of the same type pointing before the beginning of the array? -

is program legal c? if so, please support claim (either way) references 1 of language standards.

void f(char *p) {     char *q = p - 1;     (void)( q < p ); };  int main(void) {     char arr[] = "hello";     f( arr ); } 

in particular, i'm interested in whether q < p comparison legal or not.

no, isn't. using pointer doesn't point element of array or 1 past end (i. e. isn't in range [&arr[0], &arr[size]]) invokes undefined behavior.

c11 standard, 6.5.6.8 ("additive operators"):

if both pointer operand , result [of p + n] point elements of same array object, or 1 past last element of array object, evaluation shall not produce overflow; otherwise, behavior undefined.

(emphasis mine)