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From: mar@hpclmar.HP.COM (Michelle Ruscetta)
Newsgroups: comp.unix.questions
Subject: question about signal
Message-ID: <720001@hpclmar.HP.COM>
Date: Thu, 24-Sep-87 22:54:19 EDT
Article-I.D.: hpclmar.720001
Posted: Thu Sep 24 22:54:19 1987
Date-Received: Sun, 27-Sep-87 02:07:57 EDT
Organization: HP ITG/ISO Computer Language Lab
Lines: 42


 Question about signal():

  Should the return from a signal handling routine specified by a call
  to signal() go to the instruction which caused the signal, or to the 
  instruction immediately following that which caused the signal?

  For example: 

  void (*p_my_handler)();

  main()
  {  
     extern void my_handler();
     int y = 0;
     int x = 3;
     p_my_handler = my_handler:
     signal(SIGFPE, p_my_handler);    
     x = x/y; 			      /* should my_handler return to here? */
     printf("return from signal\n");  /* or should my_handler return to here? */
  }

  void my_handler() 
  {
     signal(SIGFPE, SIG_IGN);
     /* do something useful */
     signal(SIGFPE, p_my_handler); 
  }


 ---------------------------

  I have run this example on two different machines, with different results; 
  (both were running implementations of System V) one returned to the statement 
  following that which cause the signal (the printf), and the other returned 
  to the instruction which caused the signal -- hence throwing the program 
  into an infinite loop continuously executing x/y and catching the floating 
  point exception.

  I know it is not very good practice to return from signal-handlers
  but this question came up in a discussion and I would be interested to
  hear opinions as to which is the 'correct' way for signal to handle this.