Multiple children reading from/writing to a single parent - c

Question

Here I have a program where a parent process creates several child processes, passes
a distinct integer to each of them. Then each child process writes back the integer read to the parent process, which prints the result to standard output:
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <sys/wait.h>
#define R 0
#define W 1
void run_child(int in, int out){
int r;
int it;
while((r = read(in, &it, sizeof(it))) != 0){
if(r == -1){
perror("read");
exit(1);
}
//word[r] = '\0';
int w = write(out, &it, sizeof(it));
if(w == -1){
perror("write");
exit(1);
}
if(close(out) == -1){
perror("close");
exit(1);
}
}
}
int main(int argc, char **argv) {
// Process fan
int i;
int n;
int num_kids;
int from_parent[2];
int to_parent[2];
if(argc != 2) {
fprintf(stderr, "Usage: fan_write <numkids>\n");
exit(1);
}
num_kids = atoi(argv[1]);
int status;
char word[32];
for(i = 0; i < num_kids; i++) {
if(pipe(from_parent) == -1){
perror("pipe");
exit(1);
}
if(pipe(to_parent) == -1){
perror("pipe");
exit(1);
}
int g = i;
write(from_parent[W], &g, sizeof(int));
n = fork();
if(n < 0) {
perror("fork");
exit(1);
}
if(n == 0){
if(close(from_parent[W]) == -1){
perror("close");
exit(1);
}
if(close(to_parent[R]) == -1){
perror("close");
exit(1);
}
dup2(from_parent[R], STDIN_FILENO);
if(close(from_parent[R]) == -1){
perror("close");
exit(1);
}
run_child(STDIN_FILENO, to_parent[W]);
close(to_parent[W]);
exit(0);
}
if(close(from_parent[R]) == -1){
perror("close");
exit(1);
}
if(close(to_parent[W]) == -1){
perror("close");
exit(1);
}
if(close(from_parent[W]) == -1){
perror("close");
exit(1);
}
for(i=0;i<num_kids;i++){
int read_int;
int r = read(to_parent[R], &read_int, sizeof(int));
printf("read %d bytes\n", r);
if(r == -1){
perror("read");
exit(1);
}
printf("%d\n", read_int);
}
}
for(i = 0; i < num_kids; i++){
wait(&status);
}
return 0;
}
With num_kids = 4 I would expect the program to read 4 bytes each time and print the
distinct integer. However, when run it reads 4 bytes in one iteration, and then it reads
0 bytes on the following iterations, and prints the same integer over and over. I'm not sure how to fix it.
Edit: Solved! Hint: use a matrix of file descriptors for pipes.

Answer 1

you have wrong concept to read numbers from all num_kids child process in a loop, your are reading every time from single child and in each child, by loop:
for(i=0;i<num_kids;i++){
int read_int;
int r = read(to_parent[R], &read_int, sizeof(int));
printf("read %d bytes\n", r);
if(r == -1){
perror("read");
exit(1);
}
printf("%d\n", read_int);
}
Is code runs for each child including parent too because you are unconditionally running this loop in child/parent process. But parent can read from single child that the reason you fist time get 4 byte then 0. because child return a number one time. Remove above for loop from your code and do like I am suggesting below:
you should do like(read comments):
if(n == 0){
//child
}
else{
// read in parent without loop
}
So correct way is:
if(n == 0){//child
if(close(from_parent[W]) == -1){
perror("close");
exit(1);
}
if(close(to_parent[R]) == -1){
perror("close");
exit(1);
}
dup2(from_parent[R], STDIN_FILENO);
if(close(from_parent[R]) == -1){
perror("close");
exit(1);
}
run_child(STDIN_FILENO, to_parent[W]);
close(to_parent[W]);
exit(0);
}
else{ // parent
write(from_parent[W], &g, sizeof(int));
int read_int;
int r = read(to_parent[R], &read_int, sizeof(int));
printf("read %d bytes\n", r);
if(r == -1){
perror("read");
exit(1);
}
printf("%d\n", read_int);
}
And its working like:
:~$ ./a.out 4
read 4 bytes
0
read 4 bytes
1
read 4 bytes
2
read 4 bytes
3

Answer 2

if (n == 0) { //child
if(close(from_parent[W]) == -1) {
perror("close");
exit(1);
}
if(close(to_parent[R]) == -1) {
perror("close");
exit(1);
}
run_child(from_parent[R], to_parent[W]);
close(from_parent[R]); // ignore checking return code here!
close(to_parent[W]); // ignore checking return code here!
exit(0);
}
// And this is what run_child looks like
void run_child(int in, int out){
int r;
int it;
while((r = read(in, &it, sizeof(it))) != 0){
if(r == -1){
perror("read");
exit(1);
}
int w = write(out, &it, sizeof(it));
if(w == -1){
perror("write");
exit(1);
}
}
}

Answer 3

I've mixed up the aswers code and this is the final result..
All succesful
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <sys/wait.h>
#define R 0
#define W 1
void run_child(int in, int out){
int r;
int it;
while((r = read(in, &it, sizeof(it))) != 0){
if(r == -1){
perror("read");
exit(1);
}
int w = write(out, &it, sizeof(it));
//perror("write:");
//close(out) ;
exit(1);
}
}
int main(int argc, char **argv) {
// Process fan
int i;
int n;
int num_kids;
int from_parent[2];
int to_parent[2];
if(argc != 2) {
fprintf(stderr, "Usage: fan_write <numkids>\n");
exit(1);
}
num_kids = atoi(argv[1]);
int status;
char word[32];
for(i = 0; i < num_kids; i++) {
if(pipe(from_parent) == -1){
perror("pipe");
exit(1);
}
if(pipe(to_parent) == -1){
perror("pipe");
exit(1);
}
int g = i;
write(from_parent[W], &g, sizeof(int));
n = fork();
if(n < 0) {
perror("fork");
exit(1);
}
if(n == 0){//child
if(close(from_parent[W]) == -1){
perror("close");
exit(1);
}
if(close(to_parent[R]) == -1){
perror("close");
exit(1);
}
run_child(from_parent[R], to_parent[W]);
if(close(from_parent[R]) == -1){
perror("close");
exit(1);
}
if(close(to_parent[W]) == -1){
perror("close");
exit(1);
}
exit(0);
}
else{ // parent
write(from_parent[W], &g, sizeof(int));
int read_int;
int r = read(to_parent[R], &read_int, sizeof(int));
printf("read %d bytes\n", r);
if(r == -1){
perror("read");
exit(1);
}
printf("%d\n", read_int);
}
}
for(i = 0; i < num_kids; i++){
wait(&status);
}
return 0;
}