Thesigprocmaskfunction is used to examine or change the calling process's signal mask. Thehowargument determines how the signal mask is changed,and must be one of the following values:
SIG_BLOCK-
Block the signals in
set---add them to the existing mask. In other words,the new mask is the union of the existing mask andset.
SIG_UNBLOCK- Unblock the signals inset---remove them from the existing mask.
SIG_SETMASK- Usesetfor the mask; ignore the previous value of the mask.
The last argument,oldset,is used to return information about the old process signal mask. If you just want to change the mask without looking at it,pass a null pointer as theoldsetargument. Similarly,if you want to know what's in the mask without changing it,pass a null pointer forset(in this case thehowargument is not significant). Theoldsetargument is often used to remember the previous signal mask in order to restore it later. (Since the signal mask is inherited overforkandexeccalls,you can't predict what its contents are when your program starts running.)
If invokingsigprocmaskcauses any pending signals to be unblocked,at least one of those signals is delivered to the process beforesigprocmaskreturns. The order in which pending signals are delivered is not specified,but you can control the order explicitly by making multiplesigprocmaskcalls to unblock various signals one at a time.
Thesigprocmaskfunction returns0if successful,and-1to indicate an error. The followingerrnoerror conditions are defined for this function:
EINVAL- Thehowargument is invalid.
You can't block theSIGKILLandSIGSTOPsignals,but if the signal set includes these,sigprocmaskjust ignores them instead of returning an error status.
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