Lab 3 — Signals, Signal Masks, and Signal Handling

Overview

This lab introduces POSIX signals on Linux: basic signal handling, signal masks, and using kill(2) to generate signals between a parent and child process. You’ll practice registering handlers with sigaction, masking with sigprocmask/sigset_t, and synchronizing with sigsuspend.

Learning Objectives

• Install a signal handler using sigaction (POSIX interface).
• Construct and manipulate signal sets (sigfillset, sigdelset).
• Block and unblock signals with sigprocmask.
• Use sigsuspend to atomically swap a mask and wait for a signal.
• Send signals via kill and reason about race conditions and deadlock.

Resources

• signal(7) man page
• GNU C signal masks
• Signals guide; interrupts and signals
• kill and sigaction man pages
• startstop.c; POSIX signal handling; week 3 examples
• gdb demo
• Textbook: Chapter 2.8 (Linux)

Use man 7 signal, man 2 sigaction, man 2 kill, etc.

Step 1 — Setup

$ cd ~
$ cd cs3600
$ ./lab-start.sh
$ cd 3
$ cp /home/fac/dfanucchi/public_html/cs3600/examples/3/*.c .
$ cp /home/fac/dfanucchi/public_html/cs3600/examples/3/Makefile .
$ make

Read the headers and comments in each example; run them as directed to build intuition before coding vrlab3.c.

Step 2 — Assignment (vrlab3.c)

Create the vrlab3.c file, it is not included this week.

Your program forks one child. The child logs messages around a sigsuspend wait; the parent sends SIGTERM then SIGUSR1 to the child, and waits for the child to exit.

Required functions (use in your solution)

• A custom handler function
• sigfillset, sigdelset
• sigprocmask
• sigaction
• sigsuspend
• kill, wait
• open, write, close

#include <stdio.h>
#include <stdlib.h>
#include <sys/wait.h>
#include <sys/file.h>
#include <unistd.h>
#include <errno.h>
#include <signal.h>

High-level flow

1. Declare a sigset_t mask; fill it with all signals via sigfillset.
2. Block every signal in the current process via sigprocmask(SIG_BLOCK, &mask, ...).
3. Install a SIGUSR1 handler using sigaction before fork().
4. fork() a child.
   • Child: open log, write "Go CSUB", prepare a mask that blocks all signals except SIGUSR1 (sigdelset(&unblock_usr1, SIGUSR1)), then call sigsuspend(&unblock_usr1). In the handler, append " (got the signal) " to the log and return. After sigsuspend returns, write "Roadrunners!", close the log, and exit(0).
   • Parent: immediately send SIGTERM then SIGUSR1 to the child via kill, then wait(&status). Print the child’s exit status.

Build & Run

$ make
$ ./vrlab3
child terminated with code 0

$ cat log
Go CSUB (got the signal) Roadrunners!

Trace with strace

$ strace -e trace=signal -f ./vrlab3 2> err
child terminated with code 0
$ cat err
rt_sigprocmask(SIG_BLOCK, ~[RTMIN RT_1], NULL, 8) = 0
rt_sigaction(SIGUSR1, {0x400ba5, ~[RTMIN RT_1], SA_RESTORER|SA_RESTART, 0x7fdbcbbd21e0}, NULL, 8) = 0
Process [CHILD pid] attached
[pid PARENT] kill(CHILD, SIGTERM) = 0
[pid PARENT] kill(CHILD, SIGUSR1) = 0
Process [PARENT pid] suspended
[pid CHILD ] rt_sigsuspend(~[USR1 RTMIN RT_1]) = ? ERESTARTNOHAND (To be restarted_
[pid CHILD ] --- SIGUSR1 --- (User defined signal 1) @ 0 (0) ---
[pid CHILD ] rt_sigreturn(0x3) = -1 EINTR (Interrupted system call)
Process [PARENT pid] resumed
Process [CHILD pid] detached

$ cat log
Go CSUB (got the signal) Roadrunners!