CODEWITHSUNDEEP
cpipewait
north212
north212

Reputation: 93

when using wait(), cannot read pipe?

Here is a small piece of code, the parent process write pipe and child read pipe, everything works fine before I add 'wait()' in parent process. I think it should be no difference, but it just stucks when I input. I'm quite new to system programming. Anyone can help with this?

int main() {
      char* msg = malloc(sizeof(100));
      int fd[2];
      pipe(fd);
      int status;


if (fork()!=0){
    close(fd[0]);
    dup2(fd[1],1);
    scanf("%s",msg);
    puts(msg);
    wait(&status);
}
else {
    char* buf = malloc(sizeof(100));
    close(fd[1]);
    dup2(fd[0],0);
    scanf("%s",buf);
    puts(buf);
}
return 0;}

Upvotes: 0

Views: 1595

Answers (3)

Yann Droneaud
Yann Droneaud

Reputation: 5463

You have to flush stdout before calling wait().

Your program was working before without the wait() since an implicit flush was done when the parent process exit.

Upvotes: 0

Basile Starynkevitch
Basile Starynkevitch

Reputation: 1

You should read a good book on advanced unix programming, learn to use multiplexing calls like poll(2) (or select, or pselect, or ppoll), possibly learn to use SIGCHLD. Be careful, a signal handler can call only a very limited set of functions (the async-safe one) and should often set a volatile sig_atomic_t variable.

Upvotes: 0

David Schwartz
David Schwartz

Reputation: 182769

You forgot to close fd[1] after the dup2 call in the parent. Also, you need to write some character that will cause scanf to realize it has read the end of the string. Closing stdout after calling puts would be one way.

Upvotes: 1

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