Concatenating hex values in C++

Question

I am having trouble concatenating two hex values in C++;

int virtAddr = (machine->mainMemory[ptrPhysicalAddr + 1] << 8) | (machine->mainMemory[ptrPhysicalAddr]);
int physAddr = currentThread->space->GetPhysicalAddress(virtAddr);

For machine->mainMemory[ptrPhysicalAddr + 1], this yields 0x5. For machine->mainMemory[ptrPhysicalAddr], this yields 0x84. I expect the result 0x584. However, I am getting 0xffffff84. I followed this question Concatenate hex numbers in C.

size_t and ptrdiff_t are the types you want – Oct 29 '15 at 20:38 — , Oct 29 '15 at 20:38
What is the type of `mainMemory`? – Alan Stokes Oct 29 '15 at 20:47 — Alan Stokes, Oct 29 '15 at 20:47
@AlanStokes it is a char array – mrQWERTY Oct 29 '15 at 20:48 — mrQWERTY, Oct 29 '15 at 20:48

score 4 · Accepted Answer · answered Oct 29 '15 at 20:40

4

0x84 is -124. It gets widened to (int)-124 before the bitwise-OR operation (integer promotion). And 0x00000500 | 0xFFFFFF84 is the result you got. Use an unsigned type to prevent sign-extension when widening.

intptr_t virtAddr = (uint8_t(machine->mainMemory[ptrPhysicalAddr + 1]) << 8)
                   | uint8_t(machine->mainMemory[ptrPhysicalAddr]);

answered Oct 29 '15 at 20:40

Ben Voigt

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Is that an inspired guess about the type of `machine->mainMemory`? – Roddy Oct 29 '15 at 20:49
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@Roddy: Not a guess so much as deduction based on the evidence. – Ben Voigt Oct 29 '15 at 21:37

Concatenating hex values in C++

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