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Top Forums Programming share file descriptor between childs Post 302245575 by redoubtable on Friday 10th of October 2008 11:26:48 AM
Old 10-10-2008
The code you posted has a problem. But even if you solve that problem, there is another one. fork() dup()licates fd's from the parent but if you don't have some kind of synchronous read between the childs, you'll have unexpected results.

Do you really want to use multiple processes to read the same fd?

Try running this several times to see what I'm talking about:
Code:
#include <stdio.h>
#include <unistd.h>
#include <string.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>

#define CHILDS_N 5

void worker(int * fd)
{
        char buf[20];

        memset (buf, 0x0, sizeof (buf));
        read(*fd, buf, sizeof (buf)-1);
        printf ("%s\n", buf);

}


int
main ()
{
        pid_t childs[CHILDS_N];
        int fd;
        int i;
        fd = open ("Client_Files.txt", O_RDONLY);
        if (fd == -1)
                exit(1);


        for (i=0; i < CHILDS_N; i++)
        {
                childs[i] = fork();
                if (childs[i] == 0)
                {
                        worker(&fd);
                }
                else
                {
//                      printf ("parent: %d\n", getpid());
                }
        }
}

 

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SRV(3)							     Library Functions Manual							    SRV(3)

NAME
srv - server registry SYNOPSIS
bind #s /srv #s/service1 #s/service2 ... DESCRIPTION
The srv device provides a one-level directory holding already-open channels to services. In effect, srv is a bulletin board on which pro- cesses may post open file descriptors to make them available to other processes. To install a channel, create a new file such as /srv/myserv and then write a text string (suitable for strtoul; see atof(2)) giving the file descriptor number of an open file. Any process may then open /srv/myserv to acquire another reference to the open file that was reg- istered. An entry in srv holds a reference to the associated file even if no process has the file open. Removing the file from /srv releases that reference. It is an error to write more than one number into a server file, or to create a file with a name that is already being used. EXAMPLE
To drop one end of a pipe into /srv, that is, to create a named pipe: int fd, p[2]; char buf[32]; pipe(p); fd = create("/srv/namedpipe", 1, 0666); sprint(buf, "%d", p[0]); write(fd, buf, strlen(buf)); close(fd); close(p[0]); write(p[1], "hello", 5); At this point, any process may open and read /srv/namedpipe to receive the hello string. Data written to /srv/namedpipe will be received by executing read(p[1], buf, sizeof buf); in the above process. SOURCE
/sys/src/9/port/devsrv.c SRV(3)
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