Implementing sigprocmask(signals) in xv6

Question

I am trying to implement sigprocmask in XV6. In that if the first parameter i.e how=SIG_UNBLOCK then as per my understanding the process mask must be removed. For that I was instructed to do

curproc->sigmask &= ~(*set);

But I am unable to understand this calculation. Can someone please explain? Thanks!

Answer 1

In your question, you don't precise the type of sigmask or set, I will assume that they are 32 bits integer (int) and pointer on 32 bits integer (int *).

The line

curproc->sigmask &= ~(*set);

can be rewritten:

// get current mask
int mask = cupproc->sigmask;

int temp;

// get the signal(s) to set
temp = *set;

// invert the signal(s) to set to convert it signal to mask
temp = ~temp;

// update the mask with the signal to mask:
mask = mask & temp;

// save the new mask
cupproc->sigmask = mask;

So for instance, if you want to set the signal whose code is 3 (SIGINT in linux), you must first raise the third bit of an integer:

sig = 1 << ( 3 - 1) // to set only the third bit in sig.

Then call the function, set will be equal to &sig,

After line temp = ~temp;, temp will have all bits set but third

After line mask = mask & temp; all bits in mask will be kept but third that will be clear.