C - Replacing the nth byte of a 64 bit integer

Question

I'm trying to write a C function that takes a uint64_t and replace it's nth byte to a given one.

void    setByte(uint64_t *bytes, uint8_t byte, pos)

I know I can easily get the nth byte like so

uint8_t getByte(uint64_t bytes, int pos)
{
     return (bytes >> (8 * pos)) & 0xff;
}

But I have no idea how to Set the nth byte

Daniel Walker · Accepted Answer · 2020-08-07T22:38:50.060

3

Try this:

void setByte(uint64_t *bytes, uint8_t byte, int pos) {
    *bytes &= ~((uint64_t)0xff << (8*pos)); // Clear the current byte
    *bytes |= ((uint64_t)byte << (8*pos)); // Set the new byte
}

edited Aug 07 '20 at 22:38

answered Aug 07 '20 at 22:30

Daniel Walker

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Do I need to do some casting to `uint64_t`? – Daniel Walker Aug 07 '20 at 22:34
Yes, you do indeed. – Nate Eldredge Aug 07 '20 at 22:37
2

`0xff` is (usually) 32 bits and `byte` is 8 bits, so the shift is not going to be 64 bits. – eesiraed Aug 07 '20 at 22:37
Ah, thanks. That stuff often trips me up. – Daniel Walker Aug 07 '20 at 22:38
Thanks a lot for the help ! – angauber Aug 07 '20 at 22:57

Sufian Latif · Answer 2 · 2020-08-08T06:48:42.883

2

Use a mask to set every bit of the target byte to 0 (i.e. the mask should be all 1s but 0s at the target byte and then ANDed to the integer), then use another mask with all 0s but the intended value in the target byte and then OR it with the integer.

edited Aug 08 '20 at 06:48

answered Aug 07 '20 at 22:30

Sufian Latif

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C - Replacing the nth byte of a 64 bit integer

2 Answers2