Question

I want to have a message send & receive through 2 uni-direction FIFO

Flow of data

FIFO1 
stdin--->parent(client) writefd--->FIFO1-->child(server) readfd
FIFO2
child(server) writefd2---->FIFO2--->parent(client) readfd2--->stdout

I need to have boundary structed message mesg_len+mesg_type+mesg_data

The function is that if user input “Knock Knock” on stdin, which directs to client, client send this message to server through FIFO1, server compares the string, if matches with “Knock Knock”, then server reply message “Who’s there?” to client through FIFO2, and client write this message to the stdout.

The interactive part is like:

client:Knock Knock
server:who's there
client:eric
server:eric,welcome
client:exit
all signal terminate

Below is my code:

I need help on kill() signals when client types “exit”.It seems the client process still live after call kill(). So i need to type ctrl+c to end

Please help me. Thanks a lot!

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

#define MAX_BUF 100
#define MAXMESGDATA (MAX_BUF - 2* sizeof(long))
#define MESGHDRSIZE (sizeof(struct mymesg)-MAXMESGDATA)
#define FIFO1 "/tmp/fifo.1"
#define FIFO2 "/tmp/fifo.2"
#define FILE_MODE (S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH)

struct mymesg{
    long mesg_len; //byte in mesg_data
    long mesg_type; //message type
    char mesg_data[MAXMESGDATA];
};

ssize_t mesg_send (int fd, struct mymesg *mptr){
    return (write (fd,mptr,MESGHDRSIZE + mptr->mesg_len));
}

ssize_t mesg_recv(int fd,struct mymesg *mptr){
    size_t len;
    ssize_t n;
    if ((n=read(fd,mptr,MESGHDRSIZE))==0) {//read hear first, get len of data
        return 0; //end of file
    } else if (n!=MESGHDRSIZE){
        printf("message header: expected %d, got %d\n", MESGHDRSIZE,n);
        exit(1);
    }
    if ((len=mptr->mesg_len)>0)
    {
        if ((n=read(fd,mptr->mesg_data,len))!=len)
        {
            printf("message data: expected %d, got %d\n", len,n);
            exit(1);
        }
    }
    return len;
}

void client(int readfd,int writefd){
    size_t len;
    ssize_t n;
    struct mymesg mesg;
    for (;;)
    {
        printf("\nClient:");
        fgets(mesg.mesg_data,MAXMESGDATA,stdin);//read mesg
        len=strlen(mesg.mesg_data);

        if (mesg.mesg_data[len-1]=='\n') //ignore newline
            len--;
        mesg.mesg_len=len;
        mesg.mesg_type=1;

        mesg_send(writefd,&mesg);//write to IPC channel
        //read from IPC,write to std output
        if((n=mesg_recv(readfd,&mesg))>0)
            write(STDOUT_FILENO,mesg.mesg_data,n);
    }
}

void server(int readfd,int writefd){
    ssize_t n;
    struct mymesg mesg;
    for(;;)
    {
        mesg.mesg_type=1;
        //read from IPC channel
        if ((n=mesg_recv(readfd,&mesg))==0){
            printf("Message missing");
            exit(1);
        }

        mesg.mesg_data[n]='\0';
        mesg.mesg_len=strlen(mesg.mesg_data);
        char* str=NULL;

        if (strcasecmp ("Knock Knock", mesg.mesg_data)==0){
             str="Server:Who's there?";
             strcpy(mesg.mesg_data,str);
             mesg.mesg_len=strlen(str)-1;
            }
        else if(strcasecmp ("Eric", mesg.mesg_data)==0){
            str="Server:Eric,Welcome!";
            strcpy(mesg.mesg_data,str);
            mesg.mesg_len=strlen(str)-1;
            }
        else if(strcasecmp ("Exit", mesg.mesg_data)==0){
            kill(getpid(),SIGTERM);
            kill(getppid(),SIGTERM);
            exit(0);
            }
        mesg_send(writefd,&mesg);
    }
}

int main(int argc, char ** argv){
    /*MAXMESGDATA== 92 bytes; sizeof(struct mymesg)== 100 bytes
     2* sizeof(long)== 8 bytes; MESGHDRSIZE ==8 bytes*/

    int readfd,writefd;
    pid_t childpid;
    //create 2 FIFOs
    if ((mkfifo(FIFO1,FILE_MODE)<0) && (errno!=EEXIST)){
        printf("can't create %s",FIFO1);
        exit(1);
    }
    if ((mkfifo(FIFO2,FILE_MODE)<0) && (errno!=EEXIST)){
        printf("can't create %s",FIFO1);
        unlink(FIFO1);
        exit(1);
    }
    if ((childpid=fork()==0)){//child
        readfd=open(FIFO1,O_RDONLY,0);
        writefd=open(FIFO2,O_WRONLY,0);
        server(readfd,writefd);
        exit(0);
    }
    //parent
    writefd=open(FIFO1,O_WRONLY,0);
    readfd=open(FIFO2,O_RDONLY,0);
    client(readfd,writefd);

    waitpid(childpid,NULL,0);
    close(readfd);
    close(writefd);
    unlink(FIFO1);
    unlink(FIFO2);
    return EXIT_SUCCESS;
}

Answer 1

You open FIFO2 as WRONLY and FIFO1 as RDONLY in both the processes. Opening pipe for read will block until the other side opens for write, and you open for read on both sides – hence deadlocked.