Inter-process Communication · Signals

1. TODO 1 in mysh_pipe_starter.c: find the word | in argv
2. TODO 2: pipe(fd)
3. TODO 3: child 1: dup2(fd[1], 1), close both ends, run(left)
4. TODO 4: child 2: dup2(fd[0], 0), close both ends, run(right)
5. TODO 5: the shell closes both ends, then waits for both and reports

the finished shell (real session)

mysh> ls / | wc -l
26
mysh> cat /etc/passwd | grep root
root:x:0:0:root:/root:/bin/bash
mysh> yes | head -3
y
y
y
[908 killed by signal 13]
mysh> nosuch | wc -l
mysh: nosuch: No such file or directory
[910 exited with status 127]
0
mysh> echo hello | tr a-z A-Z
HELLO

Before TODO 1: ls / | wc -l runs ls with the arguments "/", "|", "wc", "-l": ls complains that "|" and "wc" do not exist. The | is the shell's job, not the kernel's.

1. TODO 1 in mytimeout_starter.c: install on_alarm for SIGALRM with sigaction, without SA_RESTART
2. TODO 2: alarm(secs) after starting the program
3. TODO 3: waitpid returned EINTR: first alarm → SIGTERM and alarm(2); second alarm → SIGKILL
4. TODO 4: exit status like coreutils: 124 if timed out, else the child's status (or 128 + signal)
5. Test: ./mytimeout 2 sleep 10, ./mytimeout 5 sleep 1, ./mytimeout 1 ./stubborn; compare with timeout

strace ./mytimeout 1 ./stubborn (real)

rt_sigaction(SIGALRM, {…, sa_flags=SA_RESTORER, …}) = 0
alarm(1)                       = 0
wait4(1857, …) = ? ERESTARTSYS
--- SIGALRM {si_code=SI_KERNEL} ---
rt_sigreturn({mask=[]})  = -1 EINTR
kill(1857, SIGTERM)            = 0
alarm(2)
   # stubborn ignores SIGTERM
wait4(1857, …) = ? ERESTARTSYS
--- SIGALRM … ---
rt_sigreturn({mask=[]})  = -1 EINTR
kill(1857, SIGKILL)            = 0
+++ killed by SIGKILL +++     # 1857
wait4 … WTERMSIG(s) == SIGKILL … = 1857
+++ exited with 124 +++

Think: why must SA_RESTART be off here? (What would waitpid do after the alarm if it were on?)