CODEWITHSUNDEEP
carrays
Tim51
Tim51

Reputation: 141

Array with arithmetic operator?

I have been reviewing some existing code and am hung up on a line. I put all of the code that should be relevant to the question down below. Note, the variable evt is a part of an enumeration type that contains 36 elements, I however only wrote 3 of the 36 for simplicity.

The line in question is:

events[evt / 32] |= (uint32_t)1 << (evt % 32);

I cant seem to wrap my head around how or why one would place evt / 32 in the array index call out. It seems to me that all but 4 product elements from the 36 value enumeration would be truncated. Please excuse my poor terminology as I am still a learning C. If anyone could help me understand what is going on here I would very much appreciate it.

Thank you for your time. 

static volatile uint32_t events[8];

typdef enum event_e
{
EVT_1,
EVT_2,
EVT_36
}event_t;

void event_set(event_t evt)
{
events[evt / 32] |= (uint32_t)1 << (evt % 32);
}

Upvotes: 1

Views: 79

Answers (1)

Barmar
Barmar

Reputation: 780984

events is a bit map, and this is setting the bit that corresponds to the value of evt to 1. The first 32 bits go into events[0], the next 4 bits go into events[1]. evt % 32 gets the remainder, which is the bit offset, and 1 << (evt % 32) shifts 1 to that offset.

Upvotes: 6

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