SignalAction(), SignalAction_r()

Examine and/or specify actions for signals


#include <sys/neutrino.h>

int SignalAction( pid_t pid,
                  void ( * sigstub)(),
                  int signo,
                  const struct sigaction * act,
                  struct sigaction * oact );

int SignalAction_r( pid_t pid,
                    void * (sigstub)(),
                    int signo,
                    const struct sigaction * act,
                    struct sigaction * oact );


A process ID, or 0 for the current process.
The address of a signal stub handler. This is a small piece of code in the user's space that interfaces the user's signal handler to the kernel. The library provides a standard one, __signalstub().
The signal whose action you want to set or get; see "POSIX signals," below.
NULL, or a pointer to a sigaction structure that specifies the new action for the signal. For more information, see "Signal actions," below.
NULL, or a pointer to a sigaction structure where the function can store the old action.



Use the -l c option to qcc to link against this library. This library is usually included automatically.


The SignalAction() and SignalAction_r() kernel calls let the calling process examine or specify (or both) the action to be associated with a specific signal in the process pid. If pid is zero, the calling process is used. The argument signo specifies the signal.

  • You should call the POSIX sigaction() function or the ANSI signal() function instead of using these kernel calls directly.
  • In order to change the action, your process must have the PROCMGR_AID_SIGNAL ability enabled. For more information, see procmgr_ability().

These functions are identical except in the way they indicate errors. See the Returns section for details.

POSIX signals

The signals are defined in <signal.h>, and so are these global variables:

char * const sys_siglist[]
An array of signal names.
int sys_nsig
The number of entries in the sys_siglist array.

There are 32 POSIX 1003.1a signals, including:

Illegal instruction (not reset when caught).

One possible cause for this signal is trying to perform an operation that requires I/O privileges. A thread can request these privileges by making sure it has the PROCMGR_AID_IO ability enabled (see procmgr_ability()) and then calling ThreadCtl(), specifying the _NTO_TCTL_IO flag:

ThreadCtl( _NTO_TCTL_IO, 0 );
Trace trap (not reset when caught).
IOT instruction.
Used by abort().
EMT instruction.
Floating point exception.
Kill (can't be caught or ignored)
Bus error.
Segmentation violation.
Bad argument to system call.
Write on pipe with no reader.
Realtime alarm clock.
Software termination signal from kill.
User-defined signal 1.
User-defined signal 2.
Death of child.
Power-fail restart.
Window change.
Urgent condition on I/O channel.
System V name for SIGIO.
Asynchronous I/O.
Sendable stop signal not from tty.
Stop signal from tty.
Make a stopped process continue.
Attempted background tty read.
Attempted background tty write.
Note: You can't ignore or catch SIGCONT, SIGKILL, or SIGSTOP.

There are 16 POSIX 1003.1b realtime signals, including:

First realtime signal.
Last realtime signal.

The entire range of signals goes from _SIGMIN (1) to _SIGMAX (64).

QNX Neutrino signals

QNX Neutrino uses the top eight signals for special purposes. They're always masked, and attempts to unmask them are ignored. They include named signals (e.g., SIGSELECT) and unnamed ones.

Signal actions

If act isn't NULL, then the specified signal is modified. If oact isn't NULL, the previous action is stored in the structure it points to. You can use various combinations of act and oact to query or set (or both) the action for a signal.

The structure sigaction contains the following members:

void (*sa_handler)();
The address of a signal handler or action for nonqueued signals.
void (*sa_sigaction) (int signo, siginfo_t *info, void *other);
The address of a signal handler or action for queued signals.
sigset_t sa_mask
An additional set of signals to be masked (blocked) during execution of the signal-catching function.
int sa_flags
Special flags that affect the behavior of the signal:
  • SA_NOCLDSTOP — don't generate a SIGCHLD on the parent for children who stop via SIGSTOP. This flag is used only when the signal is SIGCHLD.
  • SA_SIGINFO — queue this signal. The default is not to queue a signal delivered to a process. If a signal isn't queued, and the same signal is set multiple times on a process or thread before it runs, only the last signal is delivered. If you set the SA_SIGINFO flag, the signals are queued, and they're all delivered.

The sa_handler and sa_sigaction members of act are implemented as a union, and share common storage. They differ only in their prototype, with sa_handler being used for POSIX 1003.1a signals, and sa_sigaction being used for POSIX 1003.1b queued realtime signals. The values stored using either name can be one of:

The address of a signal-catching function. See below for details.
Use the default action for the signal:
  • SIGCHLD, SIGIO, SIGURG, and SIGWINCH — ignore the signal (SIG_IGN).
  • SIGSTOP, SIGTSTP, SIGTTIN, and SIGTTOU — stop the process.
  • SIGCONT — make the program continue.
  • All other signals — kill the process.
Ignore the signal. Setting SIG_IGN for a signal that's pending discards all pending signals, whether it's blocked or not. New signals are discarded. If your process ignores SIGCHLD, its children won't enter the zombie state and the process can't use wait() or waitpid() to wait on their deaths.

The function member of sa_handler or sa_sigaction is always invoked with the following arguments:

void handler(int signo, siginfo_t* info, void* other)

If you have an old-style signal handler of the form:

void handler(int signo)

the microkernel passes the extra arguments, but the function simply ignores them.

While in the handler, signo is masked, preventing nested signals of the same type. In addition, any signals set in the sa_mask member of act are also ORed into the mask. When the handler returns through a normal return, the previous mask is restored, and any pending and now unmasked signals are acted on. You return to the point in the program where it was interrupted. If the thread was blocked in the kernel when the interruption occurred, the kernel call returns with an EINTR (see ChannelCreate() and SyncMutexLock() for exceptions to this).

Note: It isn't safe to use floating-point operations in signal handlers.

When you specify a handler, you must provide the address of a signal stub handler for sigstub. This is a small piece of code in the user's space that interfaces the user's signal handler to the kernel. The library provides a standard one, __signalstub().

The siginfo_t structure of the function in sa_handler or sa_sigaction contains at least the following members:

int si_signo
The signal number, which should match the signo argument to the handler.
int si_code
A signal code, provided by the generator of the signal:
  • SI_USER — the kill() function generated the signal.
  • SI_QUEUE — the sigqueue() function generated the signal.
  • SI_TIMER — a timer generated the signal.
  • SI_ASYNCIO — asynchronous I/O generated the signal.
  • SI_MESGQ — POSIX (not QNX) messages queues generated the signal.
union sigval si_value
A value associated with the signal, provided by the generator of the signal.

Signal handlers and actions are defined for the process and affect all threads in the process. For example, if one thread ignores a signal, then all threads ignore the signal.

You can target a signal at a thread, process or process group (see SignalKill()). When targeted at a process, at most one thread receives the signal. This thread must have the signal unblocked (see SignalProcmask()) to be a candidate for receiving it. All synchronously generated signals (e.g. SIGSEGV) are always delivered to the thread that caused them.

In a multithreaded process, if a signal terminates a thread, by default all threads and thus the process are terminated. You can override this standard POSIX behavior when you create the thread; see ThreadCreate().

If you use longjmp() to return from a signal handler, the signal remains masked. You can use siglongjmp() to restore the mask to the state saved by a previous call to sigsetjmp().

Blocking states

These calls don't block.


The only difference between these functions is the way they indicate errors:

If an error occurs, -1 is returned and errno is set. Any other value returned indicates success.
EOK is returned on success. This function does NOT set errno. If an error occurs, any value in the Errors section may be returned.


The system was unable to allocate a signal handler. This indicated critically low memory.
A fault occurred when the kernel tried to access the buffers provided.
The value of signo is less than 1 or greater than _SIGMAX, or you tried to set SIGKILL or SIGSTOP to something other than SIG_DFL.
The calling process doesn't have the required permission; see procmgr_ability().
The process indicated by pid doesn't exist.


QNX Neutrino

Cancellation point No
Interrupt handler No
Signal handler Yes
Thread Yes