The collection of signals that are currently blocked is called the signal mask. Each process has its own signal mask. When you create a new process (see Creating a Process), it inherits its parent’s mask. You can block or unblock signals with total flexibility by modifying the signal mask.
The prototype for the
sigprocmask function is in signal.h.
Note that you must not use
sigprocmask in multi-threaded processes,
because each thread has its own signal mask and there is no single process
signal mask. According to POSIX, the behavior of
sigprocmask in a
multi-threaded process is “unspecified”.
Preliminary: | MT-Unsafe race:sigprocmask/bsd(SIG_UNBLOCK) | AS-Unsafe lock/hurd | AC-Unsafe lock/hurd | See POSIX Safety Concepts.
sigprocmask function is used to examine or change the calling
process’s signal mask. The how argument determines how the signal
mask is changed, and must be one of the following values:
Block the signals in
set—add them to the existing mask. In
other words, the new mask is the union of the existing mask and
Unblock the signals in set—remove them from the existing mask.
Use set for 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 the oldset argument.
Similarly, if you want to know what’s in the mask without changing it,
pass a null pointer for set (in this case the how argument
is not significant). The oldset argument is often used to
remember the previous signal mask in order to restore it later. (Since
the signal mask is inherited over
exec calls, you
can’t predict what its contents are when your program starts running.)
sigprocmask causes any pending signals to be
unblocked, at least one of those signals is delivered to the process
sigprocmask returns. The order in which pending signals
are delivered is not specified, but you can control the order explicitly
by making multiple
sigprocmask calls to unblock various signals
one at a time.
sigprocmask function returns
0 if successful, and
to indicate an error. The following
errno error conditions are
defined for this function:
The how argument is invalid.
You can’t block the
SIGSTOP signals, but
if the signal set includes these,
sigprocmask just ignores
them instead of returning an error status.
Remember, too, that blocking program error signals such as
leads to undesirable results for signals generated by an actual program
error (as opposed to signals sent with
This is because your program may be too broken to be able to continue
executing to a point where the signal is unblocked again.
See Program Error Signals.