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(libc.info)Signal Sets

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Signal Sets
-----------

   All of the signal blocking functions use a data structure called a
"signal set" to specify what signals are affected.  Thus, every
activity involves two stages: creating the signal set, and then passing
it as an argument to a library function.

   These facilities are declared in the header file `signal.h'.

 - Data Type: sigset_t
     The `sigset_t' data type is used to represent a signal set.
     Internally, it may be implemented as either an integer or structure
     type.

     For portability, use only the functions described in this section
     to initialize, change, and retrieve information from `sigset_t'
     objects--don't try to manipulate them directly.

   There are two ways to initialize a signal set.  You can initially
specify it to be empty with `sigemptyset' and then add specified
signals individually.  Or you can specify it to be full with
`sigfillset' and then delete specified signals individually.

   You must always initialize the signal set with one of these two
functions before using it in any other way.  Don't try to set all the
signals explicitly because the `sigset_t' object might include some
other information (like a version field) that needs to be initialized as
well.  (In addition, it's not wise to put into your program an
assumption that the system has no signals aside from the ones you know
about.)

 - Function: int sigemptyset (sigset_t *SET)
     This function initializes the signal set SET to exclude all of the
     defined signals.  It always returns `0'.

 - Function: int sigfillset (sigset_t *SET)
     This function initializes the signal set SET to include all of the
     defined signals.  Again, the return value is `0'.

 - Function: int sigaddset (sigset_t *SET, int SIGNUM)
     This function adds the signal SIGNUM to the signal set SET.  All
     `sigaddset' does is modify SET; it does not block or unblock any
     signals.

     The return value is `0' on success and `-1' on failure.  The
     following `errno' error condition is defined for this function:

    `EINVAL'
          The SIGNUM argument doesn't specify a valid signal.

 - Function: int sigdelset (sigset_t *SET, int SIGNUM)
     This function removes the signal SIGNUM from the signal set SET.
     All `sigdelset' does is modify SET; it does not block or unblock
     any signals.  The return value and error conditions are the same
     as for `sigaddset'.

   Finally, there is a function to test what signals are in a signal
set:

 - Function: int sigismember (const sigset_t *SET, int SIGNUM)
     The `sigismember' function tests whether the signal SIGNUM is a
     member of the signal set SET.  It returns `1' if the signal is in
     the set, `0' if not, and `-1' if there is an error.

     The following `errno' error condition is defined for this function:

    `EINVAL'
          The SIGNUM argument doesn't specify a valid signal.