c++ - 使用管道从 STDIN 读取 fork 进程时出现问题

Question

我正在尝试创建一个帮助程序类来执行系统命令并通过管道支持获取响应。对于我只需要获得响应(没有 STDIN 用于命令)的情况，它按预期工作，对于管道支持，我遇到了乱码的 STDIN，我找不到根本原因。

处理这个机制的主要函数是(请忽略小错误检查问题)

最小的工作示例

#include <vector>
#include <string>
#include <iostream>
#include <sstream>
#include <unistd.h>
#include <sys/prctl.h>
#include <sys/types.h>
#include <signal.h>
#include <sys/wait.h>
#include <fcntl.h>
#include <stdarg.h>

struct exec_cmd_t {
    exec_cmd_t(std::vector<std::string> args) : args(args), has_executed(false), cpid(-1) { }
    exec_cmd_t(const exec_cmd_t &) = delete;
    exec_cmd_t(exec_cmd_t &&) = delete;
    exec_cmd_t & operator=(const exec_cmd_t &) = delete;
    exec_cmd_t & operator=(exec_cmd_t &&) = delete;

    std::string operator()();

    std::string pipe_cmd(const std::string & input);
    std::string pipe_cmd();
    ~exec_cmd_t();

    private:
    std::vector<std::string> args;
    bool has_executed;
    int cpid;
    std::stringstream in_stream;
    std::stringstream out_stream;

    friend std::string operator | (exec_cmd_t & first, exec_cmd_t & second);
    friend std::string operator | (exec_cmd_t && first, exec_cmd_t && second);
    friend std::string operator | (std::string, exec_cmd_t & second);
    friend std::string operator | (std::string, exec_cmd_t && second);
};

std::string exec_cmd_t::pipe_cmd(const std::string & input) {
    this->has_executed = true;
    const int read_end = 0;
    const int write_end = 1;

    int read_pipe[2];
    int write_pipe[2];

    if (pipe(read_pipe) < 0 || pipe(write_pipe) < 0) {
        this->has_executed = false;
        return std::string{};
    }

    this->in_stream << input;
    std::string line;
    while(getline(this->in_stream, line)) {
        if (line.size() == 0) {
            continue;
        }
       int wr_sz = write(write_pipe[write_end], line.c_str(), line.size());
       if (wr_sz <= 0) {
           break;
       }
       write(write_pipe[write_end], "\n", 1);
    }
    close(write_pipe[write_end]);

    this->cpid = fork();
    if (this->cpid == 0) {
        dup2(write_pipe[read_end], STDIN_FILENO);
        dup2(read_pipe[write_end], STDOUT_FILENO);
        close(read_pipe[read_end]);
        close(write_pipe[write_end]);
        close(read_pipe[write_end]);
        close(write_pipe[read_end]);
        prctl(PR_SET_PDEATHSIG, SIGTERM);
        char * params[args.size()];
        const char * image_path = args[0].c_str();
        for(int i = 1; i < args.size(); i++) {
            params[i-1] = const_cast<char *>(args[i].c_str());
        }
        params[args.size()] = nullptr;
        execv(image_path, params);
        exit(1);
    }

    close(read_pipe[write_end]);
    close(write_pipe[read_end]);

    char buff[256];
    int rd_sz = -1;
    int flags = fcntl(read_pipe[0], F_GETFL, 0);
    fcntl(read_pipe[read_end], F_SETFL, flags | O_NONBLOCK);
    int status = 0;
    waitpid(this->cpid, &status, 0);
    this->has_executed = false;
    int error_code = 0;
    while((rd_sz = read(read_pipe[read_end], buff, sizeof(buff))) > 0) {
        buff[rd_sz] = '\0';
        this->out_stream << std::string{buff};
    }
    close(read_pipe[read_end]);
    return this->out_stream.str();
}


std::string exec_cmd_t::pipe_cmd() {
    static std::string empty_str{};
    return pipe_cmd(empty_str);
}

std::string exec_cmd_t::operator()() {
    return pipe_cmd();
}

exec_cmd_t::~exec_cmd_t() {
    if (this->has_executed) {
        int status;
        waitpid(this->cpid, &status, WNOHANG);
        if (!WIFEXITED(status)) {
            kill(this->cpid, SIGKILL);
            waitpid(this->cpid, &status, 0);
        }
    }
}

std::string operator | (exec_cmd_t & first, exec_cmd_t & second) {
    return second.pipe_cmd(first());
}

std::string operator | (exec_cmd_t && first, exec_cmd_t && second) {
    return second.pipe_cmd(first());
}

std::string operator | (std::string output, exec_cmd_t & second) {
    return second.pipe_cmd(output);
}

std::string operator | (std::string output, exec_cmd_t && second) {
    return second.pipe_cmd(output);
}

int main() {
    auto str = exec_cmd_t{ {"/bin/echo", "echo", "hello\nworld\nor\nnot"} } | exec_cmd_t{ {"/bin/grep", "grep", "world", "-"} };
    std::cout << str << std::endl;
    return 0;
}

给我

grep: =V: No such file or directory                                                                                                                                                   
(standard input):world 

似乎 grep 执行了两次，一次失败，没有这样的文件或目录，另一次成功。任何建议都会非常有帮助:-)。提前致谢。

Answer 1

你至少有一个未定义行为的原因，它可能导致你的程序做它该做的事。您像这样声明并使用超出范围的 VLA:

char* params[args.size()];
...
params[args.size()] = nullptr;
execv(image_path, params);

这使得 params 中的终止 char* 未初始化，因此它可以指向任何地方。 grep 认为它指向一个文件名，尝试打开它但失败了。

由于 VLA:s 不在 C++ 标准中，请考虑将其更改为:

std::vector<char*> params(args.size());
...
params[args.size() - 1] = nullptr;
execv(image_path, params.data());

另一个值得关注的原因是，您在应该使用 ssize_t 的地方使用了 int，即使您阅读或编写的次数极不可能超过 int 可以处理。

在我做出这些更改之后，它开始工作并打印出预期的 world。我什至添加了第三个命令来检查它是否可以处理它。建议的更改:

---

14,15c14,15
<     exec_cmd_t(std::vector<std::string> args) :
<         args(args), has_executed(false), cpid(-1) {}
---
>     exec_cmd_t(std::vector<std::string> Args) :
>         args(Args), has_executed(false), cpid(-1), in_stream{}, out_stream{} {}
59c59
<         int wr_sz = write(write_pipe[write_end], line.c_str(), line.size());
---
>         ssize_t wr_sz = write(write_pipe[write_end], line.c_str(), line.size());
76c76
<         char* params[args.size()];
---
>         std::vector<char*> params(args.size());
78c78
<         for(int i = 1; i < args.size(); i++) {
---
>         for(decltype(args.size()) i = 1; i < args.size(); i++) {
81,82c81,82
<         params[args.size()] = nullptr;
<         execv(image_path, params);
---
>         params[args.size() - 1] = nullptr;
>         execv(image_path, params.data());
90c90
<     int rd_sz = -1;
---
>     ssize_t rd_sz = -1;
96c96
<     int error_code = 0;
---
>     // int error_code = 0; // unused
106,107c106
<     static std::string empty_str{};
<     return pipe_cmd(empty_str);
---
>     return pipe_cmd({});
143c142,143
<                exec_cmd_t{{"/bin/grep", "grep", "world", "-"}};
---
>                exec_cmd_t{{"/bin/grep", "grep", "-A1", "hello"}} |
>                exec_cmd_t{{"/bin/grep", "grep", "world"}};

---

我还意识到您的程序就像管道命令之间的代理，从一个命令读取所有内容并将其写入下一个命令。

您可以同时启动所有程序，并一次性设置启动程序之间的管道。对于三个命令，您需要三个管道:

                     cmd1  cmd2  cmd3
                     |  w--r  w--r  |
                 stdin              read output into program
or fed by your program

如果您决定运行具有大量输出的命令，这将使性能和内存消耗不再是问题。在内部，您只需要通过读取上一个命令的输出来存储您想要存储的内容。我对这种方法做了一个小测试，它非常有效。