How to handle SIGTERM and SIGINT to manage processes. Task: create a non-killable process.

Author: Anton Yarkov

multithreading/01 - Signals_SIGINT_SIGTERM_nonkillable_parent.c

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+#include <signal.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <unistd.h>
+#include <sys/wait.h>
+
+#define EXIT_FAILURE 1
+
+pid_t child_pid;
+
+// Compilation:
+// gcc "01 - Signals_SIGINT_SIGTERM_nonkillable_parent.c" -o nonkillable_parent
+
+// Task:
+// Create a program which creates a child process and this child process can be killed by SIGTERM or SIGINT.
+// However, parent should not be killed by first SIGTERM/SIGINT. It may be killed in further signals.
+
+void cant_stop_me_handler(int sig)
+{
+    if (child_pid)
+    {
+        raise(-9); // or kill(-9, child_pid);
+        printf("SIGCHLD Child: killed %d.\n", child_pid);
+    }
+    else
+    {
+        printf("SIGCHLD Parent: Ha-ha, can't kill me this time :)! (try again)\n");
+    }
+}
+
+void cant_stop_me_handler_int(int sig)
+{
+    if (child_pid)
+    {
+        raise(-9); // or kill(-9, child_pid);
+        printf("SIGINT Child: killed %d.\n", child_pid);
+    }
+    else
+    {
+        printf("SIGINT Parent: Ha-ha, can't kill me this time :)! (try again)\n");
+    }
+}
+
+int main()
+{
+    pid_t main_pid = getpid();
+    
+    printf("Start of process %d forever.\n", main_pid);
+    
+    // Do no stop by SIGTERM.
+    struct sigaction cant_stop_me_signal;
+    cant_stop_me_signal.sa_handler = cant_stop_me_handler;
+    sigemptyset(&cant_stop_me_signal.sa_mask);
+    cant_stop_me_signal.sa_flags = SA_RESETHAND; // Non-permanent handler. Handle only first time...
+    
+    if (sigaction(SIGTERM, &cant_stop_me_signal, 0) == -1)
+    {
+        perror("Error of calling sigaction in parent.");
+        exit(EXIT_FAILURE);
+    }
+
+    // Do no stop by SIGINT.
+    struct sigaction cant_stop_me_signal1;
+    cant_stop_me_signal1.sa_handler = cant_stop_me_handler_int;
+    sigemptyset(&cant_stop_me_signal1.sa_mask);
+    cant_stop_me_signal1.sa_flags = SA_RESETHAND; // Non-permanent handler. Handle only first time...
+    
+    if (sigaction(SIGINT, &cant_stop_me_signal1, 0) == -1)
+    {
+        perror("Error of calling sigaction in parent.");
+        exit(EXIT_FAILURE);
+    }
+
+    // Make child.
+    if((child_pid = fork()))
+    {
+        printf("Start of child process %d forever. Press Ctrl+C OR write 'kill -9 %d' to terminate child.\n", child_pid, child_pid);
+    }
+    else
+    {
+        // Do anything in child.
+    }
+
+    while(1)
+    {
+        // Forever... until someone press Ctrl+C or write 'kill -9 xxxx'
+    }
+    
+    printf("End of process %d\n", getpid());
+        
+    exit(0);
+}