add fork example with error

related_info/lab5/fork-related/fork-ex1-with-some-error.c

@@ -0,0 +1,14 @@
+#include <signal.h>
+#include <unistd.h>
+
+int main(void) {
+        pid_t child = fork();
+        if (child) {  // in parent
+                sleep(5);
+                kill(child, SIGKILL);
+        } else {  // in child
+                for (;;);  // loop until killed
+        }
+
+        return 0;
+}

related_info/lab5/fork-related/fork-ex1-with-some-error.md

@@ -0,0 +1,11 @@
+This program compiles with no errors or warnings, not even with -Wall -Wextra -Werror. I recommend you don’t run it though, and here’s why. From the POSIX specification for fork(2):
+
+Upon successful completion, fork() shall return 0 to the child process and shall return the process ID of the child process to the parent process. Both processes shall continue to execute from the fork() function. Otherwise, -1 shall be returned to the parent process, no child process shall be created, and errno shall be set to indicate the error.
+And from the specification for kill(2):
+
+If pid is -1, sig shall be sent to all processes (excluding an unspecified set of system processes) for which the process has permission to send that signal.
+Putting the two together, that program could really ruin our day. If the fork() call fails for some reason2, we store -1 in child. Later, we call kill(-1, SIGKILL), which tries to kill all our processes, and most likely hose our login. Not even screen or tmux will save us!3
+
+It’s a pretty scary failure mode, and neither the library nor the language do anything at all to prevent us from having a terrible day.
+
+