Why does a negative NSInteger (long) value become garbage when sent through variadic arguments?

Question

I've created a C function with variadic arguments (NSIntegers) like below:

NSInteger test(NSInteger arg, ...)
{
    va_list args;
    va_start(args, arg);
    NSInteger arg2 = va_arg(args, NSInteger);
    return arg + arg2;
}

On calling this function with negative second value, it somehow becomes a very large number.

test( 2,  2); // result = 4
test(-2,  2); // result = 0
test( 2, -2); // result = 4294967296

However, if I use int instead of NSInteger for the variadic argument, everything works as expected.

NSInteger test2(NSInteger arg, ...)
{
    va_list args;
    va_start(args, arg);
    int arg2 = va_arg(args, int);
    return arg + arg2;
}

Tests:

test2( 2,  2); // result = 4
test2(-2,  2); // result = 0
test2( 2, -2); // result = 0

Anything I'm probably missing here or doing something wrong? Would be great if someone can point me in the right direction here.

Thanks!

Answer 1

Because you're passing integer literals as the arguments, so in the case of 2 and -2 , these will be passed as int s. So you're invoking undefined behaviour by trying to read an NSInteger .

To solve this, use an explicit cast:

test(2, (NSInteger)-2);