1  /* Using kill for Communication
       2     Copyright (C) 1991-2023 Free Software Foundation, Inc.
       3  
       4     This program is free software; you can redistribute it and/or
       5     modify it under the terms of the GNU General Public License
       6     as published by the Free Software Foundation; either version 2
       7     of the License, or (at your option) any later version.
       8  
       9     This program is distributed in the hope that it will be useful,
      10     but WITHOUT ANY WARRANTY; without even the implied warranty of
      11     MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
      12     GNU General Public License for more details.
      13  
      14     You should have received a copy of the GNU General Public License
      15     along with this program; if not, see <https://www.gnu.org/licenses/>.
      16  */
      17  
      18  /*@group*/
      19  #include <signal.h>
      20  #include <stdio.h>
      21  #include <sys/types.h>
      22  #include <unistd.h>
      23  /*@end group*/
      24  
      25  /* When a @code{SIGUSR1} signal arrives, set this variable.  */
      26  volatile sig_atomic_t usr_interrupt = 0;
      27  
      28  void
      29  synch_signal (int sig)
      30  {
      31    usr_interrupt = 1;
      32  }
      33  
      34  /* The child process executes this function. */
      35  void
      36  child_function (void)
      37  {
      38    /* Perform initialization. */
      39    printf ("I'm here!!!  My pid is %d.\n", (int) getpid ());
      40  
      41    /* Let parent know you're done. */
      42    kill (getppid (), SIGUSR1);
      43  
      44    /* Continue with execution. */
      45    puts ("Bye, now....");
      46    exit (0);
      47  }
      48  
      49  int
      50  main (void)
      51  {
      52    struct sigaction usr_action;
      53    sigset_t block_mask;
      54    pid_t child_id;
      55  
      56    /* Establish the signal handler. */
      57    sigfillset (&block_mask);
      58    usr_action.sa_handler = synch_signal;
      59    usr_action.sa_mask = block_mask;
      60    usr_action.sa_flags = 0;
      61    sigaction (SIGUSR1, &usr_action, NULL);
      62  
      63    /* Create the child process. */
      64    child_id = fork ();
      65    if (child_id == 0)
      66      child_function ();		/* Does not return.  */
      67  
      68  /*@group*/
      69    /* Busy wait for the child to send a signal. */
      70    while (!usr_interrupt)
      71      ;
      72  /*@end group*/
      73  
      74    /* Now continue execution. */
      75    puts ("That's all, folks!");
      76  
      77    return 0;
      78  }