One of your colleagues is thinking of using signals to allow a parent process to count events that occur in a child process. The idea is to notify the parent each time an event occurs by sending it a signal, and letting the parent$$s signal handler increment a global counter variable, which the parent can then inspect after the child has terminated. However, when he runs the test program in Figure $8\mathrm{.}45$ on his system, he discovers that when the parent calls printf, counter always has a value of $2,$ even though the child has sent five signals to the parent. Perplexed, he comes to you for help. Can you explain the bug?

$/*$ Figure $8\mathrm{.}45*/$

#include "csapp.h$"$

int counter $=0$;

void handler$($int sig$)$

$\{$

counter$++$;

sleep$(1)$;

$/*$ Do some work in the handler $*/$

return;

$\}$

int main$()$

$\{$

int i;

Signal$($SIGUSR$2,$ handler$)$;

if $($Fork$()==0)$

$\{$

$/*$ Child $*/$

for $($i $=0$; i $<5$; i$++)$

$\{$

Kill$($getppid$(),$ SIGUSR$2)$;

printf$("$sent SIGUSR$2$ to parent

$")$;

$\}$

exit$(0)$;

$\}$

Wait$($NULL$)$;

printf$("$counter$=\%$d

$",$ counter$)$;

exit$(0)$;

$\}$