What is wrong with sending signal after fork in below code - c

Question

i wrote the below code: after fork() function the parent send a signal to child. i think the child receive the signal and terminates.
i don't want the solution because i can solve it by two below ways.
i want to know what is the cause of this problem?
what i tried at first is below code:
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <sys/time.h>
#include <wait.h>
#include <time.h>
#include <unistd.h>
#include <signal.h>
#define maxchild 1
int inchild=-1;
void sighandler(int signo){
char buffer[256];
int a,b=0;
switch(signo){
case SIGUSR1:
printf("SIGUSR1\n");
break;
case SIGUSR2:
printf("SIGUSR2\n");
break;
}
}
int main(){
pid_t ids[maxchild];
struct sigaction control;
control.sa_flags=0;
control.sa_handler=sighandler;
sigemptyset(&control.sa_mask);
sigaction(SIGUSR1,&control,NULL);
sigaction(SIGUSR2,&control,NULL);
for(int i=0;i<maxchild;i++){
ids[i]=fork();
printf("here %d\n",ids[i]);
if(ids[i]==0){
printf("in child\n");
inchild=i+1;
break;
}
}
while(inchild>=0) {
}
// sleep(1);
if(inchild==-1){
for(int i=0;i<maxchild;i++){
printf("child: %d\n parent: %d\n",ids[i],getpid());
kill(ids[i],SIGUSR1);
}
}
if (inchild==-1) wait(NULL);
return 0;
}
when i run the code so many times, i expect to see the "SIGUSR1" in output every time but mostly the output is:
here 17573 (or any other positive number)
child: 17573
parent: 17572
and then program terminates suddenly.
i understand i can solve it by two solution:
1. parent most sleep after fork() function:
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <sys/time.h>
#include <wait.h>
#include <time.h>
#include <unistd.h>
#include <signal.h>
#define maxchild 1
int inchild=-1;
void sighandler(int signo){
char buffer[256];
int a,b=0;
switch(signo){
case SIGUSR1:
printf("SIGUSR1\n");
break;
case SIGUSR2:
printf("SIGUSR2\n");
break;
}
}
int main(){
pid_t ids[maxchild];
struct sigaction control;
control.sa_flags=0;
control.sa_handler=sighandler;
sigemptyset(&control.sa_mask);
sigaction(SIGUSR1,&control,NULL);
sigaction(SIGUSR2,&control,NULL);
for(int i=0;i<maxchild;i++){
ids[i]=fork();
printf("here %d\n",ids[i]);
if(ids[i]==0){
printf("in child\n");
inchild=i+1;
break;
}
}
while(inchild>=0) {
}
sleep(1);
if(inchild==-1){
for(int i=0;i<maxchild;i++){
printf("child: %d\n parent: %d\n",ids[i],getpid());
kill(ids[i],SIGUSR1);
}
}
if (inchild==-1) wait(NULL);
return 0;
}
that it will work fine
i can comment the "kill(...)" line too to solve the problem:
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <sys/time.h>
#include <wait.h>
#include <time.h>
#include <unistd.h>
#include <signal.h>
#define maxchild 1
int inchild=-1;
void sighandler(int signo){
char buffer[256];
int a,b=0;
switch(signo){
case SIGUSR1:
printf("SIGUSR1\n");
break;
case SIGUSR2:
printf("SIGUSR2\n");
break;
}
}
int main(){
pid_t ids[maxchild];
struct sigaction control;
control.sa_flags=0;
control.sa_handler=sighandler;
sigemptyset(&control.sa_mask);
sigaction(SIGUSR1,&control,NULL);
sigaction(SIGUSR2,&control,NULL);
for(int i=0;i<maxchild;i++){
ids[i]=fork();
printf("here %d\n",ids[i]);
if(ids[i]==0){
printf("in child\n");
inchild=i+1;
break;
}
}
while(inchild>=0) {
}
// sleep(1);
if(inchild==-1){
for(int i=0;i<maxchild;i++){
printf("child: %d\n parent: %d\n",ids[i],getpid());
// kill(ids[i],SIGUSR1);
}
}
if (inchild==-1) wait(NULL);
return 0;
}
now i want to know the reason of problem.
i want to run the first code and see:
here (an positive number)
here 0
in child
child: (an positive number)
parent: (an positive number)
SIGUSR1
and the program must continue running until i press the Ctrl+c
(of course the order of line in output doesn't matter)
i didn't want to change the code, i want to know what is the cause of problem.
thank you in advance

Answer 1

If a process has a signal-handler installed this gets called on reception of the signal. This process is not ended then. So all the children are stuck in
while (inchild >= 0) {
}
As no child ends, the parent is blocking in wait().
To fix this replace the above snippet by
if (inchild >= 0) {
pause();
}