Call to dup2 fails when redirecting stdout

Question

I've been struggling with this one for a while. I am trying to write a C++ class around running a subprocess. I am using fork(), pipe(), dup2(), and execv() to launch a subprocess and redirect its stdout and stderr to the parent. As far as I can tell, everything works fine until dup2() is called and it fails with EINVAL (on macOS, I don't think that is an allowed error type in Linux). If I remove all the logic around pipes, the class works as expected.

The class declaration:

class PosixSubprocess : public Subprocess {
    std::string _cmd = {};
    std::string _stdout = "";
    std::string _stderr = "";
    std::vector<std::string> _args = {};
    long _pid = -1;
    int _status = -1;

public:
    void cmd(std::string val) override;
    void addArg(std::string val) override;
    void run() override;

    int status() const override;
    std::string out() const override;
    std::string err() const override;
};

The definition of run():

void PosixSubprocess::run() {
    std::array<int, 2> stdoutPipe = {};
    std::array<int, 2> stderrPipe = {};

    if (pipe(stdoutPipe.data()) < 0)
    {
        throw std::runtime_error("Subprocess: failed to create pipes. Errno: " + std::to_string(errno));
    }

    if (pipe(stderrPipe.data()) < 0)
    {
        throw std::runtime_error("Subprocess: failed to create pipes. Errno: " + std::to_string(errno));
    }

    _pid = fork();

    if (_pid == 0) {

        if (dup2(stderrPipe[1], STDERR_FILENO)) {
            std::cout << "Subprocess: failed to redirect stderr. Errno " << errno << '\n';
            exit(errno);
        }
        if (dup2(stdoutPipe[1], STDOUT_FILENO)) {
            std::cout << "Subprocess: failed to redirect stdout. Errno " << errno << '\n';
            exit(errno);
        }
        close(stdoutPipe[0]);
        close(stdoutPipe[1]);
        close(stderrPipe[0]);
        close(stderrPipe[1]);

        auto argv = std::make_unique<char*[]>(_args.size() + 1);
        argv[_args.size()] = nullptr;

        for (size_t i = 0; i < _args.size(); ++i) {
            argv[i] = &(_args[i].front());
        }

        if(execvp(_cmd.c_str(), argv.get())) {
            std::cerr << "Subprocess: failed to launch. Errno " << errno << '\n';
            exit(errno);
        }
    } else if (_pid > 0) {
        close(stdoutPipe[1]);
        close(stderrPipe[1]);

        std::array<char, 1024> buf;
        auto appendPipe = [&buf](int fd, std::string& str) {
            ssize_t nBytes = 0;
            do {
                nBytes = read(fd, buf.data(), buf.size());
                str.append(buf.data(), nBytes);
                if (nBytes) std::cout << nBytes << '\n';
            } while (nBytes > 0);
        };

        while(!waitpid(_pid, &_status, WNOHANG)) {
            appendPipe(stdoutPipe[0], _stdout);
            appendPipe(stdoutPipe[0], _stderr);
        }
    } else {
        close(stdoutPipe[0]);
        close(stdoutPipe[1]);
        close(stderrPipe[0]);
        close(stderrPipe[1]);

        throw std::runtime_error("Subprocess: fork failed.");
    }
}

Sorry about the wall of code here, there isn't much I can say for sure isn't related to the issue.

Answer 1

From dup2's manual page:

RETURN VALUE

Upon successful completion a non-negative integer, namely the file descriptor, shall be returned; otherwise, -1 shall be returned and errno set to indicate the error.

Since, obviously, dup2() returns the file descriptor the original was dup-ed to, and it is non-zero, the shown code thinks this is an error.

An error condition is indicated by a negative return value.