Creating multiple child processes with a single pipe - unix

Question

I need to create three child processes, each of which reads a string from the command line arguments and writes the string to a single pipe. The parent would then read the strings from the pipe and display all three of them on the screen. I tried doing it for two processes to test and it is printing one of the strings twice as opposed to both of them.
#include <stdio.h>
#include <unistd.h>
int main (int argc, char *argv[]) {
char *character1 = argv[1];
char *character2 = argv[2];
char inbuf[100]; //creating an array with a max size of 100
int p[2]; // Pipe descriptor array
pid_t pid1; // defining pid1 of type pid_t
pid_t pid2; // defining pid2 of type pid_t
if (pipe(p) == -1) {
fprintf(stderr, "Pipe Failed"); // pipe fail
}
pid1 = fork(); // fork
if (pid1 < 0) {
fprintf(stderr, "Fork Failed"); // fork fail
}
else if (pid1 == 0){ // if child process 1
close(p[0]); // close the read end
write(p[1], character1, sizeof(&inbuf[0])); // write character 1 to the pipe
}
else { // if parent, create a second child process, child process 2
pid2 = fork();
if (pid2 < 0) {
fprintf(stderr, "Fork Failed"); // fork fail
}
if (pid2 = 0) { // if child process 2
close(p[0]); // close the read end
write(p[1], character2, sizeof(&inbuf[0])); // write character 2 to the pipe
}
else { // if parent process
close(p[1]); // close the write end
read(p[0], inbuf, sizeof(&inbuf[0])); // Read the pipe that both children write to
printf("%s\n", inbuf); // print
read(p[0], inbuf, sizeof(&inbuf[0])); // Read the pipe that both children write to
printf("%s\n", inbuf); // print
}
}
}

Answer 1

Your code doesn't keep looping until there's no more data to read. It does a single read. It also doesn't check the value returned by read(), but it should.
I've abstracted the fork() and write() (and error check) code into a function. This seems to work:
#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
static void child(int fd, const char *string)
{
pid_t pid = fork();
int len = strlen(string);
if (pid < 0)
{
fprintf(stderr, "%.5d: failed to fork (%d: %s)\n",
(int)getpid(), errno, strerror(errno));
exit(1);
}
else if (pid > 0)
return;
else if (write(fd, string, len) != len)
{
fprintf(stderr, "%.5d: failed to write on pipe %d (%d: %s)\n",
(int)getpid(), fd, errno, strerror(errno));
exit(1);
}
else
exit(0);
}
int main (int argc, char *argv[])
{
char inbuf[100]; //creating an array with a max size of 100
int p[2]; // Pipe descriptor array
if (argc != 4)
{
fprintf(stderr, "Usage: %s str1 str2 str3\n", argv[0]);
return 1;
}
if (pipe(p) == -1)
{
fprintf(stderr, "Pipe Failed"); // pipe fail
return 1;
}
for (int i = 0; i < 3; i++)
child(p[1], argv[i+1]);
int nbytes;
close(p[1]); // close the write end
while ((nbytes = read(p[0], inbuf, sizeof(inbuf))) > 0)
printf("%.*s\n", nbytes, inbuf); // print
return 0;
}
I ran the command multiple times, each time using the command line:
./p3 'message 1' 'the second message' 'a third message for the third process'
On one run, the output was:
the second messagemessage 1
a third message for the third process
On another, I got:
the second messagemessage 1a third message for the third process
And on another, I got:
message 1
the second messagea third message for the third process
(This is on a MacBook Pro with Intel Core i7, running Mac OS X 10.8.3, and using GCC 4.7.1.)