Named pipes linux - c++

Question

I made 2 threads, one has to read the other has to write.
But i get undefined behaviour, sometimes i can read 1 line, sometimes 1000. It does not make alot of sense to me.
What i do is the following:
1. i create a fifo with mkfifo() in main.cpp
2. i start 2 threads, one that reads, the other writes. reader.cpp, writer.cpp
In those threads, every loop i open the fifo and i close it, because it wont work if i only do that once outside the loop, which i find also strange.
I have been looking for good examples but i found none.
My questions is simple, how can i make the fifo (Reader) wait for incoming data and read it when it is available. It should be able to run at 4Mhz.
I hope that somebody can help me out because this is the 3rd day i am breaking my head on this. If it matters im using Qt 4.8.
EDIT: I found the solution to my problem:
main.cpp
#include <QtCore/QCoreApplication>
#include "reader.h"
#include "writer.h"
#include <sys/types.h> // mkfifo
#include <sys/stat.h> // mkfifo
#include <fcntl.h>
int main(int argc, char *argv[]) {
QCoreApplication a(argc, argv);
int fifo = mkfifo("/tmp/fifo", S_IWUSR | S_IRUSR | S_IRGRP | S_IROTH);
Reader r;
Writer w;
r.start();
w.start();
return a.exec();
}
writer.h
#ifndef WRITER_H
#define WRITER_H
#include <QThread>
#include <stdio.h>
#include <iostream>
#include <errno.h>
#include <string.h>
#include <fcntl.h>
class Writer : public QThread {
Q_OBJECT
public:
explicit Writer(QObject *parent = 0);
private:
void run();
};
#endif // WRITER_H
reader.h
#ifndef READER_H
#define READER_H
#include <QThread>
#include <stdio.h>
#include <iostream>
#include <errno.h>
#include <string.h>
#include <fcntl.h>
class Reader : public QThread {
Q_OBJECT
public:
explicit Reader(QObject *parent = 0);
private:
void run();
};
#endif // READER_H
writer.cpp
#include "writer.h"
char * phrase = "Stuff this in your pipe and smoke it\n";
using namespace std;
Writer::Writer(QObject *parent) : QThread(parent) {}
void Writer::run() {
int num, fifo;
if ((fifo = open("/tmp/fifo", O_WRONLY)) < 0) {
printf("%s\n", strerror(errno));
return;
}
while (true) {
if ((num= write(fifo, phrase, strlen(phrase)+1)) < 0) {
printf("ERROR: %s\n", strerror(errno));
}
}
close(fifo);
}
reader.cpp
#include "reader.h"
using namespace std;
Reader::Reader(QObject *parent) : QThread(parent) {}
void Reader::run() {
int num, fifo;
char temp[38];
if ((fifo = open("/tmp/fifo", O_RDONLY)) < 0) {
printf("%s\n", strerror(errno));
return;
}
while (true) {
if ((num = read(fifo, temp, sizeof(temp))) < 0) {
printf("%s\n", strerror(errno));
}
printf("In FIFO is %d %s \n", num, temp);
}
close(fifo);
}

Answer 1

The basic read() and write() functions make no promise to read or write all available data.
You need something like:
int tot = 0;
while (tot < sizeof(temp))
{
num = read(fifo, temp + tot, sizeof(temp) - tot);
if (num < 0)
break;
tot += num;
}
And the same for write.

Answer 2

I met the same problem when periodically opened and closed a single pipe. Re-creation of pipe (in reader process, when EOF is met) will be a solution.