signal()

set handling for exceptional conditions

Synopsis:

#include <signal.h>
void ( *signal(int sig, 
               void (*func)(int)) )( int );

Description:

The signal() function is used to specify an action to take place when certain conditions are detected while a program executes. See the <signal.h> header file for definitions of these conditions, and also refer to the QNX System Architecture manual.

There are three types of actions that can be associated with a signal: SIG_DFL, SIG_IGN or a pointer to a function. Initially, all signals are set to SIG_DFL or SIG_IGN prior to entry of the main() routine. An action can be specified for each of the conditions, depending upon the value of the func argument, as discussed below.

The SIGKILL and SIGSTOP signals can be caught or ignored only if _PPF_IMMORTAL is set in the system flags word of the process; see qnx_pflags() for more information.

func is a function

When func is a function name, that function is called in a manner equivalent to the following code sequence:

    /* "sig_no" is condition being signalled */
    signal( sig_no, SIG_DFL );
    (*func)( sig_no );

The func function may do the following:

• return
• terminate the program by calling the _exit() or abort() function
• call the longjmp() or siglongjmp() function

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 in a previous call to sigsetjmp().

After returning from the signal-catching function, the receiving process resumes execution at the point at which it was interrupted.

The signal catching function is described as follows:

    void func( int sig_no )
    {

      /* body of function */

    }

It isn't possible to catch the SIGSTOP or SIGKILL signals.

Since signal-catching functions are invoked asynchronously with process execution, the type sig_atomic_t may be used to define variables on which an atomic operation (for example, incrementation or decrementation) may be performed.

func is SIG_DFL

If func is SIG_DFL, the default action for the condition is taken.

If the default action is to stop the process, the execution of that process is temporarily suspended. When a process stops, a SIGCHLD signal is generated for its parent process, unless the parent process has set the SA_NOCLDSTOP flag (see the sigaction() function). While a process is stopped, any additional signals that are sent to the process aren't delivered until the process is continued, except SIGKILL, which always terminates the receiving process.

Setting a signal action to SIG_DFL for a signal that's pending, and whose default action is to ignore the signal (for example, SIGCHLD), causes the pending signal to be discarded, whether or not it's blocked.

func is SIG_IGN

If func is SIG_IGN, the indicated condition is ignored.

The action for the SIGSTOP and SIGKILL signals can't be set to SIG_IGN.

Setting a signal action to SIG_IGN for a signal that's pending causes the pending signal to be discarded, whether or not it's blocked.

(QNX extension) If a process sets the action for the SIGCHLD signal to SIG_IGN:

• the child exits without creating a zombie process
• no signal is generated for the process when a child process ends
• the process doesn't need to wait() on the child

Handling a condition

When a condition is detected, it may be handled by a program, it may be ignored, or it may be handled by the usual default action (often causing an error message to be printed on the stderr stream followed by program termination).

A condition can be generated by a program using the raise() or kill() function.

Returns:

A return value of SIG_ERR indicates that the request couldn't be handled, and errno is set to the value EINVAL.

Otherwise, the previous value of func for the indicated condition is returned.

Examples:

#include <signal.h>

sig_atomic_t signal_count;

void MyHandler( int sig_number )
  {
     ++signal_count;
  }

void main()
  {
    signal( SIGFPE, MyHandler );   /* Set own handler */
    signal( SIGABRT, SIG_DFL );    /* Default action */
    signal( SIGFPE, SIG_IGN );     /* Ignore condition */
  }

Classification:

ANSI

The behavior when the action for the SIGCHLD signal is set to SIG_IGN is a QNX extension.

See also:

errno, kill(), qnx_pflags(), raise(), sigaction(), sigprocmask()