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A comment on Kill -0. Parent comment
'A value of 0, however, will cause error checking to be performed (with no signal being sent). This can be used to check the validity of pid.'
On Mac OS X, the actual kernel implementation is: if (uap->signum) psignal(p, uap->signum);
So if signum == 0, no signal is sent, but this occurs after the destination (and permissions) are checked for validity.
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'A value of 0, however, will cause error checking to be performed (with no signal being sent). This can be used to check the validity of pid.'
On Mac OS X, the actual kernel implementation is: if (uap->signum) psignal(p, uap->signum);
So if signum == 0, no signal is sent, but this occurs after the destination (and permissions) are checked for validity.