N children send message to parent - c++

Question

What I did is I created n children and than the parent sent the message "start" to them using n pipes. One pipe for each child.Now what I'm struggling to do is to send the parent back the number of each child.
This is my code until now:
int main()
{
int n=5;
int p[n-1][2];
int i;
for(i=0;i<n;i++){
if(pipe(p[i])>0){
perror("pipe error");
exit(1);
}
}
for(i=0;i<n;i++){
pid_t pid=fork();
if(pid<0){
perror("fork error");
exit(1);
}
if(pid==0){
int j;
for(j=0;j<n;j++){
close(p[j][1]);
}
for(j=0;j<i;j++){
close(p[j][0]);
}
char msg[256];
int h;
read(p[i][0],&h,sizeof(int));
read(p[i][0],msg,h*sizeof(char));
cout<<i<<"_"<<msg<<endl;
close(p[i][0]);//here I would like to send the number i to the parent
for(j=i+1;j<n;j++){
close(p[j][0]);
}
exit(0);
}
}
char ms[256];
strcpy(ms,"start");
int ho=strlen(ms);
for(i=0;i<n;i++){
if(write(p[i][1],&ho,sizeof(int))==-1){
perror("write error");
exit(1);
}
if(write(p[i][1],ms,ho*sizeof(ms))==-1){
perror("write error");
exit(1);
}
close(p[i][1]);
}
for(int j=0;j<n;j++)
close(p[j][0]);//then read the number of each child and print it
while(wait(NULL)>0){};
exit(0);
}
And this is the output:
0_start
2_start
1_start
4_start
3_start
So I successfully sent the message start to each child.But I can't figure out how will the parent receive the numbers sent by the children.

Answer 1

You can do a similar process But here the parent has the read end of the pipe and the children the write end. Extended the example above to include one pipe only . You could have multiple pipes one each for each child.
int main()
{
int n=5;
int p[n-1][2];
int pw[2]; // pipe child writes into
int i;
for(i=0;i<n;i++){
if(pipe(p[i])>0){
perror("pipe error");
exit(1);
}
}
if(pipe(pw)>0){
perror("pipe error");
exit(1);
}
for(i=0;i<n;i++){
pid_t pid=fork();
if(pid<0){
perror("fork error");
exit(1);
}
if(pid==0){
int j;
for(j=0;j<n;j++){
close(p[j][1]);
}
close(pw[0]);// close read end - child
for(j=0;j<n;j++){
if ( i!= j ) close(p[j][0]);
}
char msg[256];
int h;
read(p[i][0],&h,sizeof(int));
read(p[i][0],msg,h*sizeof(char));
cout<<i<<"_"<<msg<<endl;
close(p[i][0]);//here I would like to send the number i to the parent
write(pw[1],&i,sizeof(int)); // send i
close(pw[1]);
exit(0);
}
}
char ms[256];
strcpy(ms,"start");
int ho=strlen(ms);
int value;
for(i=0;i<n;i++){
if(write(p[i][1],&ho,sizeof(int))==-1){
perror("write error");
exit(1);
}
if(write(p[i][1],ms,ho*sizeof(ms))==-1){
perror("write error");
exit(1);
}
close(p[i][1]);
close(pw[1]); //close write end
if(read(pw[0],&value,sizeof(int))==-1){ // read from child process
perror("write error");
exit(1);
}
cout << " in main "<<value<<endl; // display number
}
for(int j=0;j<n;j++)
close(p[j][0]);//then read the number of each child and print it
while( wait(NULL) > 0 ){;}
exit(0);
}