From 076e324e52881ec7f9ed5a7822d47b441b29f9a6 Mon Sep 17 00:00:00 2001
From: damage <damage@devloop.de>
Date: Tue, 27 Aug 2024 21:52:38 +0200
Subject: [PATCH] use pipes to write back from child to parent

---
 fork.c | 55 ++++++++++++++++++++++++++++++++++++++++++++++++++++---
 1 file changed, 52 insertions(+), 3 deletions(-)

diff --git a/fork.c b/fork.c
index 9220505..60dc321 100644
--- a/fork.c
+++ b/fork.c
@@ -8,8 +8,19 @@
 #define LOOPS_PER_CHILD 2000
 #define CHILDS 30
 
+#define PIPE_READ 0
+#define PIPE_WRITE 1
+
 int main() {
-    for (int start = 0; start < LOOPS_PER_CHILD * CHILDS; start += LOOPS_PER_CHILD) {
+    int pipes[CHILDS][2];
+    pid_t childToPipes[CHILDS];
+
+    for (int child = 0; child < CHILDS; child++) {
+        if (pipe(pipes[child]) == -1) {
+            fprintf(stderr, "Error creating pipe for child %d: %s\n", child, strerror(errno));
+            return 2;
+        }
+
         pid_t pid = fork();
         if (pid == -1) {
             fprintf(stderr, "Error forking: %s\n", strerror(errno));
@@ -18,19 +29,57 @@ int main() {
             // I AM FORK!
             pid_t childPid = getpid();
 
+            // close read of pipe
+            close(pipes[child][PIPE_READ]);
+
+            int start = child * LOOPS_PER_CHILD;
+            long sum = 0;
             for (int i = start; i < start + LOOPS_PER_CHILD; i++) {
                 fprintf(stdout, "%d: %d\n", childPid, i);
                 fflush(stdout);
+                sum += i;
             }
-            printf("Fork %d finished counting from %d\n", childPid, start);
+
+            printf("Fork %d finished counting from %d. Sum is %d\n", childPid, start, sum);
+
+            if (write(pipes[child][PIPE_WRITE], &sum, sizeof(sum)) == -1) {
+                fprintf(stderr, "Error writing pipe for child %d: %s\n", child, strerror(errno));
+                return 4;
+            }
+
+            close(pipes[child][PIPE_WRITE]);
+
             return 0;
         } else {
+            // close write of pipe
+            close(pipes[child][PIPE_WRITE]);
+            childToPipes[child] = pid;
             printf("Fork %d successfull\n", pid);
         }
     }
 
     // wait for all childs to terminate
-    while (wait(NULL) > -1);
+    pid_t childPid;
+    long overallSum = 0;
+    while ((childPid = wait(NULL)) != -1) {
+        printf("Child %d terminated\n", childPid);
+        long sum = -1;
+        for (int child = 0; child < CHILDS && sum == -1; child++) {
+            if (childToPipes[child] == childPid) {
+                read(pipes[child][PIPE_READ], &sum, sizeof(sum));
+                close(pipes[child][PIPE_READ]);
+            }
+        }
+
+        if (sum != -1) {
+            overallSum += sum;
+        } else {
+            fprintf(stderr, "Error reading from pipe\n");
+            return 3;
+        }
+    }
+
+    printf("Overall Sum: %d\n", overallSum);
 
     return 0;