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| <h4 class="subsection">24.6.4 Using <code>kill</code> for Communication</h4> |
| |
| <p><a name="index-interprocess-communication_002c-with-signals-2955"></a>Here is a longer example showing how signals can be used for |
| interprocess communication. This is what the <code>SIGUSR1</code> and |
| <code>SIGUSR2</code> signals are provided for. Since these signals are fatal |
| by default, the process that is supposed to receive them must trap them |
| through <code>signal</code> or <code>sigaction</code>. |
| |
| <p>In this example, a parent process forks a child process and then waits |
| for the child to complete its initialization. The child process tells |
| the parent when it is ready by sending it a <code>SIGUSR1</code> signal, using |
| the <code>kill</code> function. |
| |
| <pre class="smallexample"> #include <signal.h> |
| #include <stdio.h> |
| #include <sys/types.h> |
| #include <unistd.h> |
| |
| /* <span class="roman">When a </span><code>SIGUSR1</code><span class="roman"> signal arrives, set this variable.</span> */ |
| volatile sig_atomic_t usr_interrupt = 0; |
| |
| void |
| synch_signal (int sig) |
| { |
| usr_interrupt = 1; |
| } |
| |
| /* <span class="roman">The child process executes this function.</span> */ |
| void |
| child_function (void) |
| { |
| /* <span class="roman">Perform initialization.</span> */ |
| printf ("I'm here!!! My pid is %d.\n", (int) getpid ()); |
| |
| /* <span class="roman">Let parent know you're done.</span> */ |
| kill (getppid (), SIGUSR1); |
| |
| /* <span class="roman">Continue with execution.</span> */ |
| puts ("Bye, now...."); |
| exit (0); |
| } |
| |
| int |
| main (void) |
| { |
| struct sigaction usr_action; |
| sigset_t block_mask; |
| pid_t child_id; |
| |
| /* <span class="roman">Establish the signal handler.</span> */ |
| sigfillset (&block_mask); |
| usr_action.sa_handler = synch_signal; |
| usr_action.sa_mask = block_mask; |
| usr_action.sa_flags = 0; |
| sigaction (SIGUSR1, &usr_action, NULL); |
| |
| /* <span class="roman">Create the child process.</span> */ |
| child_id = fork (); |
| if (child_id == 0) |
| child_function (); /* <span class="roman">Does not return.</span> */ |
| |
| /* <span class="roman">Busy wait for the child to send a signal.</span> */ |
| while (!usr_interrupt) |
| ; |
| |
| /* <span class="roman">Now continue execution.</span> */ |
| puts ("That's all, folks!"); |
| |
| return 0; |
| } |
| </pre> |
| <p>This example uses a busy wait, which is bad, because it wastes CPU |
| cycles that other programs could otherwise use. It is better to ask the |
| system to wait until the signal arrives. See the example in |
| <a href="Waiting-for-a-Signal.html#Waiting-for-a-Signal">Waiting for a Signal</a>. |
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