C++ : fork/exec or pthread?

Question

I'm writing a program, and once a button is pushed, I have to execute a server process (that will stop only if I decide to kill him).
To execute this process, I decided to use fork/execv mechanism :

void Command::RunServer() {

    pid = fork();

    if (pid==0) {
        chdir("./bin");
        char str[10];
        sprintf(str,"%d",port);
        char *argv[] = {"./Server", str};
        execv("./Server",argv);
    }
    else {
        config->pid = pid;
        return;
    }
}

And in the method "button pushed", I do:

command->RunServer();

It seemed to work nicely a few days ago... and now i get error :

main: xcb_io.c:221: poll_for_event: Assertion `(((long) (event_sequence) - (long) (dpy->request)) <= 0)' failed.

Should I try to switch to pthread? Did I do something bad?

Thanks,
eo

Oh yes sorry it changes something. Process description in `ps` is now clean, it was very ugly with lot of args before. — eouti, Jul 19 '12 at 09:05

score 5 · Accepted Answer · edited May 23 '17 at 10:34

5

When you do fork() all file descriptors of your process are duplicated in the new one. And when you do exec*() all file descriptors are also kept, unless they are marked with the flag FD_CLOEXEC.

My guess is that some fd used by some library (Xlib, probably) is inherited by the new process, and that the duplication is causing chaos in your program.

In these cases is useful the BSD function closefrom() (closefrom(3)) if you want to keep the standard I/O opened. Unfortunately, in linux there is no such function, so you have to do a close-all loop or similar cruft:

int open_max = sysconf (_SC_OPEN_MAX);
for (int i = 3; i < open_max; i++)
    close(i);

You can read more about this problem here.

edited May 23 '17 at 10:34

Community

1
1

answered Jul 19 '12 at 08:37

rodrigo

94,151
12
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Nice :) seems to work.. Why does it start at 3? The two first are something specific? – eouti Jul 19 '12 at 08:48
stdin, stdout, stderr. The first _three_ are special. – MSalters Jul 19 '12 at 08:53

score 3 · Answer 2 · answered Jul 19 '12 at 09:21

3

In the call to execv, argv has to be terminated by a null pointer. The preceding line should be:

char* argv[] = { "./Server", str, NULL };

answered Jul 19 '12 at 09:21

James Kanze

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Yes thanks i noticed this, when it was not ending by NULL pointer, I had process name very ugly with unencoded chars. It works well now. – eouti Jul 19 '12 at 09:44

C++ : fork/exec or pthread?

2 Answers2