Exercise $3$: Using fork, exec, and signals: create two programs, a parent and a child. The parent accepts one command line argument: the number of children to exec. The child accepts one command line argument: the number of $5-$second intervals to delay before exiting; the delay will be NOT done with sleep $(3),$ but rather with alarm $(3),$ which sends the child SIGALRM when the timer expires. Before exec$$ing each child you will have to alter argv$[1]$ to contain the number $($as a character$)$ of $5-$second intervals to wait.

Because the parent will be forking and then exec$$ing a number of children each child will open $(2)$ an output file named $$out$.,$ where is the child$$s pid $($in character form, not in int form!$).$ Because we would have several processes writing to the standard output, we will only let the parent do that; each child will write $(2)$ to an out. file instead of the standard output. This avoids having multiple processes competing for the standard output.

The parent will have $2$ signal handlers $($see p$.324$ in text for an example of how to set this up$),$ one for SIGCHLD $($the signal sent when a child exits$),$ and one for SIGUSR$1($sent by each child before it exits$).$ The child will have $1$ signal handler for SIGALRM. The parent signal handlers should merely write that they received a signal to the standard output $($see sample output below$).$ The child SIGALRM signal handler, however, will not only write that it received a signal in the output file, it will also send a SIGUSR$1$ signal to the parent. In Assignment $6$ this line should be commented out; for Exercise $3$ you put it back in$!$

Parent pseudocode:

Comment: in order to use execve $(2)$ the parent must declare

extern char $**$environ;

convert argv$[1]$ to number $($print error message and exit if argc $!=2)$

set up for receiving SIGCHLD and SIGUSR$1$ signals $($using signal $(3))$

set up full pathname to child executable $($use getcwd$(3))$

for $1$ to number do

fork new process; fork returns a pid

if $($pid $=0)$ you are in the child, then

change argv$[1]$ to be the $1,2,3,...$ number

comment: argv$[1]$ must be in character format

execve $(2)$ the child $$ use execve $($pathname$,$ argv, environ$)$;

comment: if execve returns use perror$(3)$ and exit

end if

end for

comment: in parent here

for $1$ to number$*2$ do

wait for signals using pause $(3)$

end for

exit

Child pseudocode:

set numseconds to argv$[1]*5$; argv$[1]$ must be converted to an int

open file: out. using getpid $(2)$ to get pid of child

open output file: use open $(2)$ with O$\_$CREAT $|$ O$\_$WRONLY, and mode $0644$

set up SIGALRM signal handler $($see pseudocode below$)$

start alarm timer for numseconds, using alarm $(3)$

wait for the signal to arrive using pause $(3)$

close output file

exit

Child alarm signal handler pseudocode:

get parent pid using getppid $(2)$

send the parent SIGUSR$1$ using kill$(2)$ see testing suggestion below

Suggestion: write the child process first $($see Assignment $6),$ and test it$,$ but DO NOT include sending the SIGUSR$1$ signal to the parent $($as executing the child process directly will send the SIGUSR$1$ signal to your shell $-$ which will cause the shell to die!$).$ Once you have written and debugged the parent amend the child code to send the SIGUSR$1$ signal to the parent.

Sample parent output for command: $ parent $2$

Parent forks child $1($pid $54174)$

Parent forks child $2($pid $54175)$

Parent received child user signal $10$

Parent received child death signal $17$

Parent received child user signal $10$

Parent received child death signal $17$

Sample child output $($in file out$\mathrm{.}54174)$:

Child $1($pid $54174)$ starts alarm timer $5$ seconds long

Child pid $54174$ received timer signal $14$

Child sent SIGUSR$1$ to parent $54173$

Sample child outpExercise $3$: Using fork, exec, and signals: create two programs, a parent and a child. The parent accepts one command line argument: the number of children to exec. The child accepts one command line argument: the number of $5-$second intervals to delay before exiting; the delay will be NOT done with sleep $(3),$ but rather with alarm $(3),$ which sends the child SIGALRM when the timer expires. Before exec$$ing each child you will have to alter argv$[1]$ to contain the number $($as a character$)$ of $5-$second intervals to wait.

Because the parent will be forking and then exec$$ing a number of children each child will open $(2)$ an output file named $$out$.,$ where is the child$$s pid $($in character form, not in int form!$).$ Because we would have several processes writing to the standard output, we will only let the parent do that; each child will write $(2)$ to an out. file instead of the standard output. This avoids having multiple processes competing for the standard output.

The parent will have $2$ signal handlers $($see p$.324$ in text for an eut $($in file out$\mathrm{.}54175)$:

Child $1($pid $54174)$ starts alarm timer $10$ seconds long

Child pid $54174$ received timer signal $14$

Child sent SIGUSR$1$ to parent $54173$