CODEWITHSUNDEEP
cmultiprocessingipcnamed-pipesfifo
javac
javac

Reputation: 461

Reading a named pipe while multiple processes are writing to it

What kind of a way is correct for solving this problem?

For example I have a program named write.c that has 4 child processes and the child processes are write their PIDs to a single global named pipe.

Another program named read.cshould read this PIDs.

I have an approach like in below, but that approach has some problems.It can not read all PIDs, sometimes 3 of them and sometimes 2 of them.I think there is a synchronization problem , how can I solve this problem? :

writer.c:

#include <stdio.h> 
#include <string.h> 
#include <stdlib.h> 
#include <fcntl.h> 
#include <sys/stat.h> 
#include <sys/types.h> 
#include <sys/wait.h> 
#include <unistd.h> 

int main(){ 
    int fd; 
    char * myfifo = "/tmp/myfifo"; //FIFO file
    char buffer[50]; 

    mkfifo(myfifo, 0666); //creating the FIFO

    for(int i=0;i<4;i++){ //creating 4 child process
        if(fork() == 0) { 
            fd = open(myfifo, O_WRONLY); //each child process opens the FIFO for writing their own PID.

            sprintf(buffer, "%d", getpid()); //each child process gets pid and assign it to buffer
            printf("write:%s\n", buffer);  // each child process prints to see the buffer clearly

            write(fd, buffer, strlen(buffer)+1); //each child process writes the buffer to the FIFO
            close(fd);

            exit(0); 
        } 
    } 
    for(int i=0;i<4;i++) { //waiting the termination of all 4 child processes.
        wait(NULL); 
    }
    //parent area
}

reader.c

#include <stdio.h> 
#include <unistd.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <sys/stat.h> 
#include <time.h>
#include <string.h>
#include <fcntl.h> 

int main(int argc, char **argv) { 

    int fd1; 

    // FIFO file path 
    char * myfifo = "/tmp/myfifo"; 

    // Creating the named file(FIFO) 
    mkfifo(myfifo, 0666); 

    char str1[80]; //str2[80]; 
    while (1) 
    { 
        // First open in read only and read 
        fd1 = open(myfifo,O_RDONLY); 
        read(fd1, str1, 80); 

        // Print the read string and close 
        printf("read: %s\n", str1); 
        close(fd1); 
    } 
}

Upvotes: 0

Views: 1668

Answers (1)

Yakov Galka
Yakov Galka

Reputation: 72539

This line writes the null byte into the fifo:

write(fd, buffer, strlen(buffer)+1);

as a result if you have two pids in the pipe you'll read the following string:

1234\02345\0

And the printf will print only till the first \0:

1234

To fix it, it's easier to transfer the PID as binary rather than formatting and parsing text:

Writer:

    if(fork() == 0) { 
        fd = open(myfifo, O_WRONLY);
        pid_t pid = getpid();
        write(fd, &pid, sizeof(pid));
        close(fd);
        exit(0); 
    }

The reader:

fd1 = open(myfifo,O_RDONLY); 
pid_t pid;
while (1) // whatever is your termination condition
{ 
    read(fd1, &pid, sizeof(pid)); 
    printf("read: %d\n", pid); 
} 
close(fd1);

Upvotes: 3

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