How to read bytes at an address in C?

Question

I am working with some page table entries and have a virtual address. Each page has data on it and the virtual address is mapped to that page.

If I have a 32 bit virtual address, how can I "grab" the first byte at a specific virtual address?

int *virtualAddress = someaddress;
int byteAtAddress = *(virtualAddress);
int secondByte = *(virtualAddress + 4);

Obviously I am getting 4 bytes instead of getting one byte. What trick can I use here to only get one byte?

Answer 1

The C standard permits you to cast an address of memory that you own to an unsigned char* :

unsigned char* p = (unsigned_char*)someaddress;

You can then extract the memory one byte at a time using pointer arithmetic on p . Be careful not to go beyond the memory that you own - bear in mind that and int could be as small as 16 bits.

Answer 2

I don't understand why you ask, but still is apparently a valid question. I say " I don't understand ", because you say " Obviously I am getting 4 bytes "

unsigned char *virtualAddress = comeaddress;
unsigned char byteAtAddress = virtualAddress[0];

Note, that pointer arithmetic takes consideration of the pointer type so *(virtualAddress + 4) is ¹ actually adding 16 bytes to the base address.

In fact it is equivalent to

*((unsigned char *) virtualAddress + 4 * sizeof(*virtualAddress))

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¹ _{Assuming you are on a regular system where sizeof(int) == 4 whichs is not necessarily true.}