GNU Info

(libc.info)Process Signal Mask

---

Next: Testing for Delivery Prev: Signal Sets Up: Blocking Signals

---

Enter node , (file) or (file)node

Process Signal Mask
-------------------

   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 (Note: 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'.

 - Function: int sigprocmask (int HOW, const sigset_t *restrict SET,
          sigset_t *restrict OLDSET)
     The `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:

    `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
          and SET.

    `SIG_UNBLOCK'
          Unblock the signals in SET--remove them from the existing
          mask.

    `SIG_SETMASK'
          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 `fork' and `exec' calls, you can't
     predict what its contents are when your program starts running.)

     If invoking `sigprocmask' causes any pending signals to be
     unblocked, at least one of those signals is delivered to the
     process before `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.

     The `sigprocmask' function returns `0' if successful, and `-1' to
     indicate an error.  The following `errno' error conditions are
     defined for this function:

    `EINVAL'
          The HOW argument is invalid.

     You can't block the `SIGKILL' and `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 `SIGFPE'
     leads to undesirable results for signals generated by an actual
     program error (as opposed to signals sent with `raise' or `kill').
     This is because your program may be too broken to be able to
     continue executing to a point where the signal is unblocked again.
     Note: Program Error Signals.