I am getting incorrect output from my forks - c++

Question

I'm attempting to create a program that creates 2 child processes, and 4 pipes (I know this isn't ideal, but the spec for this specific assignment requires it). While it correctly sorts two of the 5 command line argument integers, the rest are just spat out as what I believe are uninitialized integers, I.E. 7 is printed out as 33234951.
I'm pretty new to pipes, and it's been a little hard to wrap my head around, so I believe this issue has to do with this and not some arbitrary error in code.
I was able to successfully get this done using only 1 parent and child, but as soon as I tried to implement multiple, things got dicey.
I have a lot of unused includes just from messing around with things in attempt to solve the problem.
#include <bits/stdc++.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <unistd.h>
int main(int argc, char **argv) {
printf("Starting\n");
pid_t pid;
pid_t pid2;
int mypipe0[2];
int mypipe1[2];
int mypipe2[2];
int mypipe3[2];
pipe(mypipe0);
pipe(mypipe1);
pipe(mypipe2);
pipe(mypipe3);
/* Create the child process. */
pid = fork();
std::cout << "Fork " << pid << std::endl;
// Child: Sorts Array
if (pid == 0) {
printf("pid == (pid_t) 0 p2\n");
/* This is the child process.
Close other end first. */
close(mypipe0[1]);
char valuesArray[5];
for (int a = 0; a < 5; a++)
read(mypipe0[0], &valuesArray[a], sizeof(char));
printf("finish reading mypipe0");
std::sort(valuesArray, valuesArray + 5);
printf("sorted");
close(mypipe1[0]);
close(mypipe2[0]);
for (int a = 0; a < 5; a++) {
write(mypipe1[1], &valuesArray[a], sizeof(char));
write(mypipe2[1], &valuesArray[a], sizeof(char));
}
close(mypipe1[1]);
close(mypipe2[1]);
exit(0);
}
else if (pid > 1) {
std::cout << "pid == (pid_t) 1" << std::endl;
/* This is the parent process.
Close other end first. */
close(mypipe0[0]); // Closes reading
int valuesArray[5];
valuesArray[0] = atoi(argv[1]);
valuesArray[1] = atoi(argv[2]);
valuesArray[2] = atoi(argv[3]);
valuesArray[3] = atoi(argv[4]);
valuesArray[4] = atoi(argv[5]);
printf("Argv init");
for (int a = 0; a < 5; ++a)
write(mypipe0[1], &valuesArray[a], sizeof(char));
printf("wrote to pipe 1");
close(mypipe0[1]);
wait(NULL);
close(mypipe1[1]); // Closes writing
// char outputArray[6];
int sortedArray[5];
for (int a = 0; a < 5; ++a)
read(mypipe1[0], &sortedArray[a], sizeof(char));
// Printing Array]
for (int a = 1; a < 5; ++a)
printf(", %d", sortedArray[a]);
printf("]");
// wait(NULL);
// close(mypipe2[1]); // Closes writing
// int median;
// read(mypipe1[0], median, sizeof(charian));
exit(0);
}
else {
pid2 = fork();
// Other child
if (pid2 == 0) {
printf("pid == (pid_t) 0\n");
/* This is the child process.
Close other end first. */
close(mypipe0[1]);
char valuesArray[5];
for (int a = 0; a < 5; a++)
read(mypipe0[0], &valuesArray[a], sizeof(char));
printf("finish reading mypipe0");
std::sort(valuesArray, valuesArray + 5);
printf("sorted");
close(mypipe1[0]);
close(mypipe2[0]);
for (int a = 0; a < 5; a++) {
write(mypipe1[1], &valuesArray[a], sizeof(char));
write(mypipe2[1], &valuesArray[a], sizeof(char));
}
close(mypipe1[1]);
close(mypipe2[1]);
exit(0);
}
}
}
I expect the output to be 2 4 5 6 7 from the given command line arguments of 4 2 5 6 7. Instead, I get [1, 28932, 5, 6, -14276913]

Answer 1

The else branch is reached when there is a failure in the first fork call.
Your current code structure is,
pid = fork();
if (pid == 0)
{
runFirstChild();
}
else if (pid > 1)
{
runParent();
}
else // This branch is not taken unless there is a failure
{
pid2 = fork();
if (pid2 == 0)
{
runSecondChild();
}
}
You should instead structure it like so,
pid = fork();
if (pid == 0)
{
runFirstChild();
}
else if (pid > 1)
{
pid2 = fork(); // Fork the second child in the parent process
if (pid2 == 0)
{
runSecondChild();
}
else if (pid2 > 1)
{
runParent();
}
}