transferring a pipe between parent and child within a function

Question

I am having some difficulties in the following thing:
I am trying to send a pipe to a child node within a function, and then have the child writing into it.
The following code parts will explain it better:

int p[2];
int i;
pipe(p);
close(p[1]);
if(fork1() == 0){
  close(p[0]);
  runcmd(parsecmd(buf),p);
}
wait(0);
}
while(read(p[0],&i,sizeof(int)) != 0){
printf(1," id: %d\n",i );}

and runcmd will have the following code:

...
void runcmd(struct cmd *cmd,int pp[]){
int j = getpid();
write(pp[1],&j,sizeof(int));
close(pp[1]);
...

sadly the expected result should be - the parent will print the id (getpid is a function that returns the current running process id ), but it doesn't, it prints nothing when evokes. what did I do wrong?

I don't think that `wait(0);` is allowed. I dind't find in the man pages that one may pass a NULL pointer to `wait()` — Ingo Leonhardt, Mar 26 '15 at 11:24
the wait(0) is allowed because it's xv6 and not normal linux... — Tai, Mar 26 '15 at 11:27

Ingo Leonhardt · Answer 1 · 2015-03-26T12:19:57.937

1

You close the write side of the pipe before forking so the child process can't write to it. You also need to exit() the child. So your code should be sth. like that:

pipe(p);
if(fork1() == 0){
   close(p[0]);
   runcmd(parsecmd(buf),p);
   exit(0);  
}
close(p[1]);
...

Besides I would recommend to add some error handling (fork() may also return -1)`

Edit: this works on Linux

void runcmd(int pp[])
{
    int j = getpid();
    write(pp[1],&j,sizeof(int));
    close(pp[1]);
    exit(0);
}

int main( int argc, char *argv[] )
{
    int p[2];
    int i;
    int status;

    pipe(p);
    if(fork() == 0){  // for linux: fork() instead of fork1()
       close(p[0]);
       runcmd(p);
    }
    close(p[1]);      // close belongs here
    wait(&status);    // Linux: wait(0) is not allowed
    while(read(p[0],&i,sizeof(int)) > 0)  // != 0 causes endless loopn on EOF
    {
         printf(" id: %d\n",i );   // first parameter '1' doesn't belong there
    }
    return( 0 );
}

edited Mar 26 '15 at 12:19

answered Mar 26 '15 at 11:32

Ingo Leonhardt

9,435
2
24
33

I moved the close(p[1]) and it still doesn't work. The child has exit within it (in runcmd)... So these are not the things that cause the problem. – Tai Mar 26 '15 at 11:38
What happens excactly? does `read()` hang or does it return 0? – Ingo Leonhardt Mar 26 '15 at 11:43
Maybe the problem is in the code you haven't posted. I've created a Linux version of your code which wors fine -- see my edit – Ingo Leonhardt Mar 26 '15 at 12:20
I thought the problem was with the pointer maybe ... With (int pp[]) could that be it? – Tai Mar 26 '15 at 12:35
no, that shoud be perfectly ok; of course it would be enough to pass `p[1]` to `runcmd()` – Ingo Leonhardt Mar 26 '15 at 12:37

score 0 · Answer 2 · answered Mar 27 '15 at 10:25

0

the problem has been solved!
the troublesome part was the fact the while loop was after the "wait" call, and not before, after changing that it worked like magic!
thanks for the help!

answered Mar 27 '15 at 10:25

Tai

59
5

transferring a pipe between parent and child within a function

2 Answers2