Reentrant code confusion

Question

I am trying to understand exactly what re-entrant code means. I have this code:

void HWt::startElement(uint8_t HW, uint8_t val)
{
    writeBitsToregister(REG_VAL + HW*131072, 0, 3, val); //Write bits 0-2
    writeBitToregister(REG_VAL, 3, 1);  //Set bit 3 to 1
    writeBitToregister(REG_VAL, 3, 0); //Set bit 3 to 0
}

I must ensure that all the bit writes are atomic. I was told that this is not re-entrant. I can understand how to make it thread safe, I can just use a mutex.

How to make it re-entrant safe? Also, I do not understand what re-entrant means here? How can the same thread access this function many times? Can that only happen if this is called within an ISR or recursive? It is neither of these things.

Answer 1

Re-entrant safety means that the function can successfully handle an "out of context" call to itself while it is executing. In non-kernel code in modern operating systems, this usually means that another thread will be calling it at the same time so in this case it is equivalent to being thread-safe.

However, e.g. in MS-DOS or in kernel code, an interrupt could occur at any time while the function is executing, possibly invoking the function on its own; this could have strange side-effects unless the function is written specifically to avoid this.

See also: http://en.wikipedia.org/wiki/Reentrancy_%28computing%29