C - Named pipe between top level process and bottom level (leaf) process - c

Question

I'm trying to write to a named pipe from a bottom level leaf process and read from the pipe on the top level process.
To do this, I'm first creating the FIFO in the top level process, then using a for loop to fork more processes. In the for loop I'm checking for leaf processes and if it's a leaf, I'm writing to the FIFO and breaking from the loop. Then, after the loop, I'm trying to read from the FIFO in the top level process. This isn't working, my program just gets stuck and stalls after the leaf process is created.
How can I send a message from a leaf back up to the top parent process through a FIFO?
CODE:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <fcntl.h>
#include <sys/shm.h>
#include <sys/stat.h>
#include <sys/mman.h>
#include <unistd.h>
#include <errno.h>
#define MAX_BUF 1024
int main(int argc, char **argv){
int numprocs = atoi(argv[1]);
int lev = numprocs;
fprintf(stdout,"ALIVE: Level %d process with pid=%d, child of ppid=%d.\n", lev, getpid(), getppid());
//create shared memory
const int SIZE = numprocs * sizeof(int);
const char *name = "dleggio1OS";
int shm_fd;
int *ptr;
shm_fd = shm_open(name, O_CREAT | O_RDWR, 0666);
ftruncate(shm_fd, SIZE);
ptr = mmap(0, SIZE, PROT_WRITE, MAP_SHARED, shm_fd, 0);
*ptr = getpid();
//create fifo
int fd;
char *myfifo = "/tmp/dleggio1fifo";
mkfifo(myfifo, 0666);
//spawn procs
int i;
for(i = 1; i < numprocs; i++){
lev--;
int pfds[2];
char buf[30];
if(pipe(pfds) == -1){
perror("pipe");
exit(1);
}
pid_t pid;
if((pid = fork()) < 0){
perror("fork");
exit(1);
}
if(pid == 0){ //child
const int SIZE = numprocs * sizeof(int);
const char *name = "dleggio1OS";
int shm_fd;
int *ptr;
shm_fd = shm_open(name, O_CREAT | O_RDWR, 0666);
ftruncate(shm_fd, SIZE);
ptr = mmap(0, SIZE, PROT_WRITE, MAP_SHARED, shm_fd, 0);
ptr[i] = getpid();
close(pfds[1]);
if(read(pfds[0], buf, 3) <= 0){
perror("child");
exit(1);
}
int check = atoi(buf);
fprintf(stdout,"ALIVE: Level %d process with pid=%d, child of ppid=%d.\n", check, getpid(), getppid());
if(check == 1){ //leaf
fd = open(myfifo, O_WRONLY);
write(fd,"leaf",sizeof("leaf"));
close(fd);
break;
}
}
else{ //parent
close(pfds[0]);
char hold[3];
sprintf(hold,"%d",lev);
if(write(pfds[1], hold, 3) <= 0){
perror("parent");
exit(1);
}
wait(NULL);
return 0;
}
}
//read fifo
char buff[MAX_BUF];
fd = open(myfifo,O_RDONLY);
read(fd,buff,MAX_BUF);
close(fd);
shm_unlink(name);
unlink(myfifo);
return 0;
}
OUTPUT:
ALIVE: Level 3 process with pid=554, child of ppid=451.
ALIVE: Level 2 process with pid=555, child of ppid=554.
ALIVE: Level 1 process with pid=556, child of ppid=555.
_ // <---- stalls here

Answer 1

All processes are hung on a "wait()" call (i.e. waiting for one of its children to exit), except for the last child forked... which is hung on "open(myfifo, O_WRONLY)"....
The last child will continue to hang until a process opens the fifo for reading...