C: Fork and signals - c

Question

What I want to do is create a parent process that lasts for 5 seconds. I also want it to send a SIGUSR1 signal every second. On this signal I want the child to do something.
The code that I put together so far is:
void alarmHandler(int sig) {
printf("\nalarm called\n");
exit(0);
}
void childHandler(int sig) {
printf("child called");
signal(SIGUSR1, childHandler);
}
int main() {
pid_t val;
if((val = fork())) { //parinte
signal(SIGALRM, alarmHandler);
printf("parent");
alarm(5);
while(1) {
kill(val, SIGUSR1);
sleep(1);
}
}else {
signal(SIGUSR1, childHandler);
printf("child");
}
return 0;
}
What I get is:
child
parent
alarm called
What I want:
child
parent
child called
child called
child called
child called
child called
alarm called

Answer 1

Your parent has the while loop. The child does the following:
signal(SIGUSR1, childHandler);
printf("child");
And then exits.
If it does receive SIGUSR before the exit, this will also be executed
printf("child called");
signal(SIGUSR1, childHandler);
Therefore you have a race condition as the number of child called is printed.
Just put a while (1) {} after the printf("child");

Answer 2

Base on the original code, add two blocks:
flush the printf: setbuf(stdout, NULL);
keep the child running: while (1) pause();
The code list as follow:
#include "stdio.h"
#include "stdlib.h"
#include <signal.h>
/* For a real-world program, printing from a signal handler is not very safe.
* A signal handler should do as little as it can, preferably only setting a flag here or there.
* And the flag should be declared `volatile`.
* Real-world example:
* I once worked on a system that used an Access database as a back end,
* and under certain circumstances a printf() call in a signal handler would write to the .mdb file instead of stdout,
* hosing the database beyond repair.
*/
void alarmHandler(int sig)
{
printf("\nparent signal alarm handler: times up\n");
exit(0);
}
void childHandler(int sig)
{
printf("\nchild signal handler\n");
// The old style should install the handler again.
signal(SIGUSR1, childHandler);
}
int main()
{
pid_t val;
signal(SIGALRM, alarmHandler);
// If not set this, we cann't the child's output.
// The stdout stream is buffered, so will only display what's in the buffer after it reaches a newline (or when it's told to).
setbuf(stdout, NULL);
if ((val = fork())) { //parinte
printf("\nparent\n");
// #Note that there is only one alarm clock per process.
// the alarm() function will return a value if another alarm has been previously set.
//
// sets a timer that generates the SIGALARM signal when times up.
// If we ignore or don’t catch this signal, the process is terminated.
alarm(5);
while(1) {
sleep(1);
kill(val, SIGUSR1);
}
printf("\nparent exit\n");
} else {
signal(SIGUSR1, childHandler);
printf("\nchild\n");
while (1)
pause(); // The pause() function suspends the process until a signal is caught.
printf("\nchild exit\n");
}
return 0;
}
And the output:
parent
child
child signal handler
child signal handler
child signal handler
child signal handler
parent signal alarm handler: times up