c - integer downcast

Question

About integer numbers downcasts in C, e.g.

An int value 000F'E000 downcast to short or unsigned short will become E000.
short -> -8192,
unsigned short -> 57344,

So does it simply cut the bits?

And what about upcasts? E.g. int -10 is ffffff81, what is the rule to cast to long long?

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@Update

About upcasting, according to the answers I did some tests and found that with 2's complement it has the following rules:

    signed:
        positive -> positive: add 0 as prefix bits,
        negative -> negative: add 1 as prefix bits,
    unsigned:
        add 0 as prefix bits,

code:

// integer numbers, downcast & upcast,
#include <stdio.h>

void downcastTest() {
    int i = 127<<13;
    printf("%X, %hX, %hi\n", i, i, i);
}

void upcastTest() {
    int i = 127;
    int j = -127;
    printf("%x, %llx\n", i, (long long)i);
    printf("%x, %llx\n", j, (long long)j);
}

int main(int argc, char * argv[]) {
    downcastTest();
    upcastTest();
    return 0;
}

Answer 1

Downcasts

A cast to a smaller integer type discards the most significant (left-most as you'd write the full binary integer on paper) bits that are not present in the destination type.

Upcasts

An upcast to a larger integer is more complex:

• For unsigned to unsigned types, it adds sufficient zero most-significant bytes; this always preserves the value.
• For signed to signed types, it sign-extends the the source type (i.e. packs the new byte(s) with bits equal to the sign bit of the source integer); this always preserves the value, positive or negative
• For unsigned to signed types, it effectively adds sufficient zero-most significant bytes; this always preserves the value, as in the nature of an upcast, there are always more bits in the destination, so there is always space for an extra sign 'bit'
• For signed to unsigned types, it sign-extends, then casts; this cannot always preserve the value, as a negative value cannot be represented.

Answer 2

Downcast cuts the bits, up-cast depends on "signedness". Up-cast on unsigned types adds zero bits to the value, up-cast on signed types replicates the sign bit. In this way, the expression has the same value before and after an upcast.