IPC using Named Pipes in C++ and Python program hangs

Question

I'm practicing IPC by using named pipes on Unix and trying to write a string in FIFO file using python & reversing it through C++ program. But the program in Python gets hanged & returns no result.

Python Code For writing into file:

import os
path= "/home/myProgram"
os.mkfifo(path)
fifo=open(path,'w')
string=input("Enter String to be reversed:\t ")
fifo.write(string)
fifo.close()

The program hangs and doesn't ask for any input here. I get following error when I break out:

Traceback (most recent call last):
  File "writer.py", line 4, in <module>
    fifo=open(path,'w')
KeyboardInterrupt

C++ code for reading from file:

#include <fcntl.h>
#include <iostream>
#include <sys/stat.h>
#include <unistd.h>
#include <string>
#include <string.h>

#define MAX_BUF 1024
using namespace std;

char* strrev(char *str){
    int i = strlen(str)-1,j=0;

    char ch;
    while(i>j)
    {
        ch = str[i];
        str[i]= str[j];
        str[j] = ch;
        i--;
        j++;
    }
    return str;

}


int main()
{
    int fd;
    char *myfifo = "/home/myProgram";
    char buf[MAX_BUF];

    /* open, read, and display the message from the FIFO */
    fd = open(myfifo, O_RDONLY);
    read(fd, buf, MAX_BUF);
    cout<<"Received:"<< buf<<endl;
    cout<<"The reversed string is \n"<<strrev(buf)<<endl;
    close(fd);
    return 0;
}

Since, the writer program fails to execute, unable to test the reader code and hence can't mention the result here.

Please help.

Answer 1

The python code blocks in open(). It is waiting for a reader.

One might normally switch to nonblocking and use os.open(). With a FIFO, you will get an error, ENXIO. This basically equates to, no reader present.

So, the "owner" of the FIFO should be the reader. This rule might simply be a matter of style. I am not aware of specific reasons for this constraint.

Here is some python code that demonstrates interleaving multiple readers and writers.

    import os
    r1 = os.open('myfifo', os.OS_RDONLY | os.OS_NONBLOCK)
    r2 = os.open('myfifo', os.OS_RDONLY | os.OS_NONBLOCK)
    w1 = os.open('myfifo', os.OS_WRONLY | os.OS_NONBLOCK)
    w2 = os.open('myfifo', os.OS_WRONLY | os.OS_NONBLOCK)
    os.write(w1, b'hello')
    msg = os.read(r1, 100)
    print(msg.decode())
    os.write(w2, b'hello')
    msg = os.read(r2, 100)