在没有主体的情况下从While循环中的FIFO（命名管道）读取

Question

im trying to make a tic tac toe game with server-client in c. On the server side i have to read from FIFO(named pipe) 2 pids. so i made a loop that run until the read (from fifo) return value different from zero(mean that the client wrote pid to the fifo).

I have to say that for some reason, on my laptop it's not working and on my buddy laptop it's working. The same code!! I have no clue why this is happening.

And when i add a body to the first while loop and put a printf("1"); in it. it's work and the pid1 reads the pid from the FIFO.

The code of the server:

#include <stdio.h>
#include <sys/stat.h> 
#include <fcntl.h>
#include <unistd.h>
#include <signal.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <string.h>

void main()
{
    int fd,shmid;
    key_t shmkey;
    void *shm_add;
    pid_t pid,pid1=0,pid2=0;
    mkfifo("fifo_clientTOserver",400);
    fd=open("fifo_clientTOserver",O_NONBLOCK | O_RDONLY);
    pid=fork();
    if(pid==0)
    {
        while(read(fd,&pid1,sizeof(pid_t))==0); //(1) 
    }
    else
    {
        wait();
        while(read(fd,&pid2,sizeof(pid_t))==0)
        {
            if(pid2!=pid1)
                break;
        }   
        remove("fifo_clientTOserver");
    }
    printf("\nfirst pid= %d\nsecond pid= %d\n",pid1,pid2);
}

The code of the Client:

#include <stdio.h>
#include <sys/stat.h> 
#include <fcntl.h>
#include <unistd.h>
#include <signal.h>
#include <stdbool.h>

void my_handler(int signum);

bool over=false;
int board[3][3]={{0,0,0},{0,0,0},{0,0,0}};
char tav;
static bool first=false;

void main()
{
    int fd;
    pid_t pid1=getpid();
    signal(SIGUSR2, my_handler);
    fd=open("fifo_clientTOserver",O_WRONLY);
    write(fd,&pid1,sizeof(pid_t));
    printf("%d\n",pid1);
    while(!over);

}

void my_handler(int signum)
{
    char geth;
    printf("1");
    //Check if the signal is SIGUSR2.
    if (signum == SIGUSR2)
    {
    if(!first)
    {
        tav='x';
        printf("x");
        first=true;
    }
    else
    {
        tav='c';
        printf("c");
    }
    }
}

It's really weird and i dont know how to deal with it! When i change line (1) to while(read(fd,&pid1,sizeof(pid_t))==0){printf("1");} it's working and pid1 get the value.

Please help me.

Answer 1

man read :

If some process has the pipe open for writing and O_NONBLOCK is set, read() shall return -1 and set errno to [EAGAIN].

So, your while loop breaks without anything read.

In any event, busy waiting is bad. Drop the O_NONBLOCK or use

    fd_set readfds;
    FD_ZERO(&readfds);
    FD_SET(fd, &readfds);
    select(fd+1, &readfds, NULL, NULL, NULL);

before the read() .

Answer 2

When you are constantly checking something within a loop, non-blocking and other parallel activities are starved. This busy waiting can be avoided by sleeping between checks or using I/O multiplexing (select).

while( !AreWeThereYet() ) { GetSomeSleep(); }