Questions about dup2 and multiple thread

Question

I meet a very string problem about dup2 and multiple thread, the code is like this:

pipe out, err;
int forkpid = fork();

if (forkpid == 0) {
    dup2(out.writeFd, STDOUT_FILENO);
    dup2(err.writeFd, STDERR_FILENO);
    printf("hello\n");
}

do {
    int res = poll(&fds, 2, 200);
    if (res) {
        for (int i = 0; i < 2; i++) {
            bytes = read(fds[i].fd, buff, size);
            if (fds[i].fd == out.readFd)
                printf("out\n");
            if (fds[i].fd == err.readFd)
                printf("err\n");
        }
    }
} while(....);

I have tested the code in a separated project, I can see "hello" is output from std, but once I put it in a big project, "hello" is output from err:(

I am using xcode5.0 and mix c++ and objective c. any idea about this? thanks

Answer 1

I have found the reason. I implemented a printf with NSlogV before, and I forgot that. NSlogV default seems to output info to stderr. so that's reason. – Jet