Psignal(2)                Oct. 1, 1991                 Psignal(2)



NAME
     Psignal - change the way a signal is handled

SYNOPSIS
     #include <signal.h>

     LONG Psignal(WORD sig, LONG handler);

DESCRIPTION
     Psignal changes the handling of the signal indicated by  sig
     (which must be between 1 and 31; symbolic constants for sym-
     bols are defined in  the  file  signal.h).  The  new  signal
     handler applies to this process only.

     If handler is SIG_DFL, then the default action for the  sig-
     nal will occur when the signal is delivered to the process.

     If handler is SIG_IGN, then the signal will  be  ignored  by
     the process, and delivery of the signal will have no notice-
     able effect (in particular, the signal  will  not  interrupt
     the  Pause  system  call, q.v.). If the signal is pending at
     the time of the Psignal call, it is discarded.

     If handler is some other value, it  is  assumed  to  be  the
     address of a user function that will be called when the sig-
     nal is delivered to the process. The user function is called
     with  a  single  LONG  argument  on  the stack, which is the
     number of the signal being delivered (this is done  so  that
     processes may use the same handler for a number of different
     signals). While the signal is being handled, it  is  blocked
     from  delivery;  thus, signal handling is "reliable" (unlike
     Version 7 and early System V Unix implementations, in  which
     delivery of a second signal while it was being handled could
     kill the process). Also note that, unlike some  versions  of
     Unix,  the  signal  handling  is  *not* reset to the default
     action before the handler is called; it remains set  to  the
     given signal handler.

     The signal handler must either return (via  a  normal  680x0
     rts instruction) or call the Psigreturn system call to indi-
     cate when signal handling is complete; in  both  cases,  the
     signal  will  be  unblocked.  Psigreturn  also performs some
     internal clean-up of the kernel stack that is  necessary  if
     the  signal  handler is not planning to return (for example,
     if the C longjmp() function is to be used to continue execu-
     tion at another point in the program).

     Signal handlers may make any GEMDOS, BIOS, or  XBIOS  system
     calls freely.  GEM AES and VDI calls should not be made in a
     signal handler.





Version 0.9   Last change: MiNT Programmer's Manual             1






Psignal(2)                Oct. 1, 1991                 Psignal(2)



     Note that calling Psignal to change behavior of a signal has
     the  side  effect of unmasking that signal, so that delivery
     is possible. This is done so that processes may, while  han-
     dling  a  signal,  reset  the  behavior  and send themselves
     another instance of the signal,  for  example  in  order  to
     suspend  themselves  while  handling a job control signal.
     Signal handling is preserved across Pfork and Pvfork  calls.
     Signals  that  are ignored by the parent are also ignored by
     the child after a Pexec call; signals that were being caught
     for  handling  in  a  function are reset in the child to the
     default behavior.

RETURNS
     The old value of the signal handler on success; this will be
     either SIG_DFL, SIG_IGN, or a function address.

     ERANGE if sig is not a legal signal.

     EACCDN if the signal may not be caught by the user

SEE ALSO
     Pkill(2), Psigblock(2), Psigreturn(2)

BUGS
     Signal handling can be nested only a small (around 3) number
     of  times, i.e. if 4 signals are delivered to a process, and
     the process has established handlers for all 4, and none  of
     the handlers has returned or called Psigreturn,  then  there
     is  a  very  good  chance of a stack  overflow  killing  the
     process off. In practice, this is unlikely to happen.
























Version 0.9   Last change: MiNT Programmer's Manual             2