what is the meaning of restrict in the function signature?

It’s something introduced in C99 which lets the compiler know that the pointer passed in there isn’t pointing to the same place as any other pointers in the arguments. If you give this hint to the compiler, it can do some more aggressive optimizations without breaking code.

As an example, consider this function:

int add(int *a, int *b) {
    return *a + *b;
}

Obviously, it adds two numbers from pointers. We can use it like this if we want:

// includes excluded for brevity
int main(int argc, char **argv) {
    int number=4;
    printf("%d\n", add(&number, &number));
    return 0;
}

Obviously, it will output 8; it’s adding 4 to itself. However, if we add restrict to add like so:

int add(int *restrict a, int *restrict b) {
    return *a + *b;
}

Then the previous main is now invalid; it’s passing &number as both arguments. You may, however, pass in two pointers pointing to different places.

int main(int argc, char **argv) {
    int numberA=4;
    int numberB=4;
    printf("%d\n", add(&numberA, &numberB));
    return 0;
}