Data transmitted in a pipe are garbage - c

Question

I am using these two programs of this answer. This answer uses named-pipes and not pipes, am I correct?
I have written main.c, which is actually the code of my actual project, minimized to this specific question (that's why I have a for loop for example).
#include <unistd.h>
#include <sys/wait.h>
#include <stddef.h>
#include <limits.h>
#include <stdio.h>
int main(void) {
pid_t pid;
int i;
for(i = 0; i < 2; ++i) {
pid = fork();
if (pid == -1) {
// error, failed to fork()
perror("failed to fork()");
return 1;
} else if (pid == 0) {
// child code
if(i < 1) {
// the writer.c
char* arg_list[] = { "w", NULL };
execv( "w", arg_list );
printf("exec FAILED\n");
} else {
// the reader.c
char* arg_list[] = { "r", NULL };
execv( "r", arg_list );
printf("exec FAILED\n");
}
}
}
// parent code
int status;
// wait for all children to terminate
while ((pid = wait(&status)) > 0) {
if (status == 1) {
printf("The child process terminated with an error!\n");
return -1;
}
}
printf("All children are done\n");
return 0;
}
The problem is that sometimes, the reader receives garbage (or most likely nothing) and it hangs up.
Sample output:
Received: Hi
All children are done
samaras#samaras-A15:~/test$ ./m
Received: Hi
All children are done
samaras#samaras-A15:~/test$ ./m
Received: <----------------- This is garbage, that is not reproducible
^C
So, what am I missing?
No need to read below that point.
My guesses are (not checked, so if I am correct, I still need clarification):
The reader runs before writer, that's why it has garbage, but then why it hangs?
or
I need to write a wrapper function read_all() (and one for the write case as well?) that collects all the data that the pipe spits, but then why if I replace "Hi" with "H", I have the same behaviour?
EDIT:
In case my first guess is the case, I put a loop for reading, but it will execute forever in the case that reader starts first.
In the case that I see garbage, after running with strace -f I got this:
...
[pid 3326] read(-1, 0xbfddd80c, 1024) = -1 EBADF (Bad file descriptor)^C
Process 3324 resumed
Process 3325 detached
Process 3326 detached

Answer 1

Your loops (or lack of) have nothing to do with it. When your reader opens (open()) the pipe for reading before writer creates the pipe, then the file descriptor your readers waits on is invalid (-1). So even when writer writes something later on, reader just waits on an invalid fd (-1) and is never going to read anything. Trivially, you could solve it with:
while( (fd = open(myfifo, O_RDONLY)) == -1);
in reader so that it waits until pipe is available. I am actually wondering if there can be a better approach than this. One other way I can think of is a loop over access(), but it's not massively different to this...