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Help understanding signals


 
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# 1  
Old 10-21-2010
Help understanding signals

I am having trouble with folowing
Code:
sigset_t s;    // now s represents set of signals
sigemptyset(&s) ; // initialize this set and exclude all the signals from it.is it empty?
sigaddset(&s,SIGILL);//this set containts only SIGILL signal
sigprocmask(SIG_BLOCK,&s,NULL);//lost on this one

Can anyone see my comment in the code and tell me if I am right. Last sigprocmask i read in documentation but I didnt really understand it.
Any help would be great
# 2  
Old 10-21-2010
Try reading 'man sigprocmask'. SIG_BLOCK is the operation being performed, and SIGILL is the signal in question. So the code is
  • Creating an empty set of signals
  • Adding SIGILL to that set
  • Blocking every signal in the set, i.e. adding SIGILL to the set of blocked signals
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# 3  
Old 10-21-2010
why?

In your third step you said "Blocking every signal in the set, i.e. adding SIGILL to the set of blocked signals".
Didn't we add SIGILL in previous step?If the set is empty then we only blocking SIGILL?
When we have empty set of signals does that mean there isn't any signals in it?
# 4  
Old 10-21-2010
Quote:
Originally Posted by joker40
In your third step you said "Blocking every signal in the set, i.e. adding SIGILL to the set of blocked signals".
Didn't we add SIGILL in previous step?
You added it to the s variable, is all. The system hasn't been told to use it, that's what sigprocmask is for.
Quote:
If the set is empty then we only blocking SIGILL?
No. From man sigprocmask:
Code:
       SIG_BLOCK
              The set of blocked signals is the union of the current  set  and
              the set argument.

...meaning. you're adding SIGILL to whatever signals are currently being blocked.
Quote:
When we have empty set of signals does that mean there isn't any signals in it?
Yes. an empty set has nothing in it.
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