When a process requests the sigaction function, the process
can both examine and specify what action to perform when the
specified signal is delivered. The arguments determine the
behavior of the sigaction function as follows:
o Specifying the sig argument identifies the affected signal.
Use any one of the signal values defined in the <signal.h>
header file, except SIGKILL.
If sig is SIGCHLD and the SA_NOCLDSTOP flag is not set in
sa_flags, then a SIGCHLD signal is generated for the calling
process whenever any of its child processes stop. If sig is
SIGCHLD and the SA_NOCLDSTOP flag is set in sa_flags, then
SIGCHLD signal is not generated in this way.
o Specifying the action argument, if not null, points to a
sigaction structure that defines what action to perform when
the signal is received. If the action argument is null, signal
handling remains unchanged, so you can use the call to inquire
about the current handling of the signal.
o Specifying the o_action argument, if not null, points to
a sigaction structure that contains the action previously
attached to the specified signal.
The sigaction structure consists of the following members:
void (*sa_handler)(int);
sigset_t sa_mask;
int sa_flags;
The sigaction structure members are defined as follows:
sa_ This member can contain the following values:
handler
o SIG_DFL - Specifies the default action taken when
the signal is delivered.
o SIG_IGN - Specifies that the signal has no effect on
the receiving process.
o Function pointer - Requests to catch the signal. The
signal causes the function call.
sa_mask This member can request that individual signals, in
addition to those in the process signal mask, are
blocked from delivery while the signal handler function
specified by the sa_handler member is executing.
sa_flags This member can set the flags to enable further control
over the actions taken when a signal is delivered.
The sa_flags member of the sigaction structure has the following
values:
SA_ONSTACK Setting this bit causes the system to run the
signal catching function on the signal stack
specified by the sigstack function. If this bit
is not set, the function runs on the stack of the
process where the signal is delivered.
SA_RESETHAND Setting this bit resets the signal to SIG_DFL. Be
aware that you cannot automatically reset SIGILL
and SIGTRAP.
SA_NODEFER Setting this bit does not automatically block the
signal as it is intercepted.
SA_NOCLDSTOP If this bit is set and the sig argument is equal
to SIGCHLD and a child process of the calling
process stops, then a SIGCHLD signal is sent to
the calling process only if SA_NOCLDSTOP is not
set for SIGCHLD.
When a signal is intercepted by a signal-catching function
installed by sigaction, a new signal mask is calculated and
installed for the duration of the signal-catching function (or
until a call to either sigprocmask or sigsuspend is made. This
mask is formed by taking the union of the current signal mask and
the value of the sa_mask for the signal being delivered unless
SA_NODEFER or SA_RESETHAND is set, and then including the signal
being delivered. If and when the user's signal handler returns
normally, the original signal mask is restored.
Once an action is installed for a specific signal, it remains
installed until another action is explicitly requested (by
another call to sigaction), until the SA_RESETHAND flag causes
resetting of the handler, or until one of the exec functions is
called.
If the previous action for a specified signal had been
established by signal, the values of the fields returned in
the structure pointed to by the o_action argument of sigaction
are unspecified, and in particular o_action->sa_handler is
not necessarily the same value passed to signal. However, if a
pointer to the same structure or a copy thereof is passed to a
subsequent call to sigaction by means of the action argument
of sigaction), the signal is handled as if the original call to
signal were repeated.
If sigaction fails, no new signal handler is installed.
It is unspecified whether an attempt to set the action for a
signal that cannot be intercepted or ignored to SIG_DFL is
ignored or causes an error to be returned with errno set to
EINVAL.
See the "Error and Signal Handling" chapter of the VSI C RTL
Reference Manual for more information on signal handling.
NOTE
The sigvec and signal functions are provided for
compatibility to old UNIX systems; their function is a
subset of that available with the sigaction function.
See also sigvec, signal, wait, read, and write.