Path: utzoo!news-server.csri.toronto.edu!rutgers!tut.cis.ohio-state.edu!unreplyable!garbage
From: lin@CS.WMICH.EDU (Lite Lin)
Newsgroups: comp.unix.questions
Subject: sending signal sends -1 to the receiving process (question)
Message-ID: <9103141128.AA12846@cs.wmich.edu>
Date: 14 Mar 91 11:28:26 GMT
Sender: daemon@tut.cis.ohio-state.edu
Lines: 86


Hello,
  I have a question regarding signals.  This question occurs to me when 
I'm trying to write a curses based package.  I'd like certain area on 
the screen to blink, but SUN terminal does not support blinking (am I right?),
so I produce a child process and have the child process interrupt the parent
process, say, every 0.2 seconds, and then certain area on the screen changes
state accordingly (A related problem is, the visual effect produced in this way
is not really pleasing, is there any way to get around the problem?).  The
problem comes in when I notice that the parent process keeps receiving unwanted
characters.  This I did not know before; it looks like when a process sends a 
signal to another process, it puts something in the other process's input 
stream as well.  I tried to write a very much simplified program and then I
realized the problem has nothing to do with the cursor package.  I can get
around the problem because the unwanted character is always (char)(-1), so
I simply ignore that, but still I'm wondering what is the reason for this.
Anyone care to explain?  The programs are appended.
  BTW, if it is relevant, I'm using a SPARC IPC running SunOS 4.1 and
compiled the programs with /usr/5bin/cc and relevant libraries & includes,
where appropriate.
  Thanks in advance,
	Lite



========================================================================
#include <signal.h>

void blink();

main()
{
  int pid;
  char ch;

  if ((pid=fork()) == -1) {  /* forks off a process which would interrupt 
			        once in a while; help do blinking */
    perror("fork");
    exit(1);
  }
  else if (pid == 0) {
    execl("/sol3/cs1/lin/599/sysv-nopad/nap", "nap", "2000000", (char *)0);
    perror("execl");
  }
  else 
    signal(SIGALRM, blink); 
  while (ch=getchar()) {
    printf("%d\n", ch); /* this prints -1 */
    printf("%c", ch);
  }
}



void blink()
{
  signal(SIGALRM, blink);
}
========================================================================


>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
#include <signal.h> /* the "nap" program */
#include <sys/types.h>
#include <stdio.h>

main(argc, argv, envp)
int argc;
char *argv[], *envp[];
{
  int usec = 0;
  char *ptr;
  int ppid;

  ppid = getppid();
  ptr = argv[1];
  while (*ptr) {
    usec = usec*10 + *ptr - '0';
    ptr++;
  }
  for (;;) {
    usleep(usec);
    kill(ppid, SIGALRM);
  }
}
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>