编译器和模板处理

Question

LShadow77

Asked:2022-06-14 22:01:02 +0000 UTC2022-06-14 22:01:02 +0000 UTC 2022-06-14 22:01:02 +0000 UTC

printf() 在 C++17 中不是线程安全的！

772

万事如意！我在各种资料中读到 printf() 函数是线程安全的，并且可以由不同的线程调用，以便将无冲突的输出输出到控制台（当然，前提是参数传递正确）。事实上，这个函数总是处理多线程输出，我实际使用了它......直到我需要编译支持 C ++ 17 标准的程序！一般来说，这里有一个最小可重复的例子来说明问题的本质：

#include <stdio.h>
#include <conio.h>
#include <atomic>
#include <windows.h>

#define NUM_THREADS 4


std::atomic<bool> g_abExit(false);


DWORD WINAPI thread_func (LPVOID num)
{
    while (!g_abExit.load(std::memory_order_acquire))
    {
        printf("*** THREAD #%02u: This is the bug, it's not a feature!\n",(UINT)num);
        Sleep(1);
    }
    
    return 0;
}


int main()
{
    HANDLE threads[NUM_THREADS];

    printf("Enter q to quite...\n");
    Sleep(500);

    for (int i=0; i<NUM_THREADS; ++i)
        if (!(threads[i] = CreateThread(NULL,0,(LPTHREAD_START_ROUTINE)thread_func,(LPVOID)i,0,NULL))) return -1;

    while (getch()!='q');

    g_abExit.store(true,std::memory_order_release);
    for (int i=0; i<NUM_THREADS; ++i)
        WaitForSingleObject(threads[i],INFINITE);
    return 0;
}

我正在使用 MinGW GCC 编译器版本 9.2.0（构建 2）。我默认为 C++ 标准编译（我不知道默认情况下 MinGW 使用什么标准，我没有找到如何查看）并且我得到了预期的正确结果：

Enter q to quite...
*** THREAD #00: This is the bug, it's not a feature!
*** THREAD #01: This is the bug, it's not a feature!
*** THREAD #02: This is the bug, it's not a feature!
*** THREAD #03: This is the bug, it's not a feature!
*** THREAD #03: This is the bug, it's not a feature!
*** THREAD #02: This is the bug, it's not a feature!
*** THREAD #00: This is the bug, it's not a feature!
*** THREAD #01: This is the bug, it's not a feature!
*** THREAD #02: This is the bug, it's not a feature!
*** THREAD #00: This is the bug, it's not a feature!
*** THREAD #03: This is the bug, it's not a feature!
*** THREAD #01: This is the bug, it's not a feature!
*** THREAD #02: This is the bug, it's not a feature!
*** THREAD #00: This is the bug, it's not a feature!
*** THREAD #01: This is the bug, it's not a feature!
*** THREAD #03: This is the bug, it's not a feature!
*** THREAD #02: This is the bug, it's not a feature!
*** THREAD #03: This is the bug, it's not a feature!
*** THREAD #00: This is the bug, it's not a feature!
*** THREAD #01: This is the bug, it's not a feature!
*** THREAD #01: This is the bug, it's not a feature!
*** THREAD #02: This is the bug, it's not a feature!
*** THREAD #03: This is the bug, it's not a feature!
*** THREAD #00: This is the bug, it's not a feature!
*** THREAD #00: This is the bug, it's not a feature!
*** THREAD #03: This is the bug, it's not a feature!
*** THREAD #02: This is the bug, it's not a feature!
*** THREAD #01: This is the bug, it's not a feature!
*** THREAD #02: This is the bug, it's not a feature!
*** THREAD #03: This is the bug, it's not a feature!
*** THREAD #01: This is the bug, it's not a feature!
*** THREAD #00: This is the bug, it's not a feature!
*** THREAD #00: This is the bug, it's not a feature!
*** THREAD #03: This is the bug, it's not a feature!
*** THREAD #02: This is the bug, it's not a feature!
*** THREAD #01: This is the bug, it's not a feature!
*** THREAD #03: This is the bug, it's not a feature!
*** THREAD #02: This is the bug, it's not a feature!
*** THREAD #00: This is the bug, it's not a feature!
*** THREAD #01: This is the bug, it's not a feature!
*** THREAD #00: This is the bug, it's not a feature!
*** THREAD #01: This is the bug, it's not a feature!
*** THREAD #02: This is the bug, it's not a feature!
*** THREAD #03: This is the bug, it's not a feature!
*** THREAD #01: This is the bug, it's not a feature!
*** THREAD #03: This is the bug, it's not a feature!
*** THREAD #02: This is the bug, it's not a feature!
*** THREAD #00: This is the bug, it's not a feature!
*** THREAD #00: This is the bug, it's not a feature!
*** THREAD #02: This is the bug, it's not a feature!
*** THREAD #03: This is the bug, it's not a feature!
*** THREAD #01: This is the bug, it's not a feature!
*** THREAD #02: This is the bug, it's not a feature!
*** THREAD #03: This is the bug, it's not a feature!
*** THREAD #00: This is the bug, it's not a feature!
*** THREAD #01: This is the bug, it's not a feature!
*** THREAD #03: This is the bug, it's not a feature!

但是，当使用 -std=c++17 选项编译时，所有输出线程安全崩溃，导致一团糟：

Enter q to quite...
*** THREAD #00: This is the bug, it's not a feature!
*** THREAD #01: This is the bug, it's not a feature!
*** THREAD #02: This is the bug, it's not a feature!
*** THREAD #03: This is the bug, it's not a feature!
*** THREAD #03: This is the bug, it's not a feature!
*** THREAD #01: This is the bug, it's not a feature!
*** THREAD #02: This is the bug, it's not a feature!
*** THREAD #00: This is the bug, it's not a feature!
*** THREAD #02: This is the bug, it's not a feature!
*** THREAD #03: This is the bug, it's not a feature!
*** THREAD #01: This is the bug, it's not a feature!
*** THREAD #00: This is the bug, it's not a feature!
*** THREAD #02: This is the bug, it's not a feature!
*** THREAD #03: This is the bug, it's not a f*** THREAD #00: This is the bug, it's not a feature!
eature*** THREAD #!
01: This is the bug, it's not a feature!
*** THREAD #03: This is the bug, it's not a feature!
*** THREAD #00: This is the bug, it's not a feature!
*** THREAD #02: This is the bug, it's not a featu*** THREAD #re!
01: This is the bug, it's not a feature!
*** THREAD #00: This is the bug, it's not a feature!
*** THREAD #03: This is the bug, it's not a feature!
*** THREAD #01: This is the bug, it's not a feature!
*** THREAD #02: This is the bug, it's not a feature!
*** THREAD #02: This is the bug, it's not a feature!
*** THREAD #03: This is the bug, it's not a feature!
*** THREAD #00: This is the bug, it's not a feature!
*** THREAD #01: This is the bug, it's not a feature!
*** THREAD #03: This is the bug, it's not a feature!
*** THREAD #00: This is the bug, it's not a feature!
*** THREAD #01: This is the bug, it's not a feature!
*** THREAD #02: This is the bug, it's not a feature!
*** THREAD #01: This is the bug, it's not a feature!
*** THREAD #02: This is the bug, it's not a featu*** THREAD #03: This is the bre!
ug, it's not a feature!
*** THREAD #00: This is the bug, it's not a feature!
*** THREAD #01: This is the bug, it's not a feature!
*** THREAD #03: This is the bug, it's not a feature!
*** THREAD #02: This is the bug, it's not a feature!
*** THREAD #00: This is the bug, it's not a feature!
*** THREAD #00: This is the bug, it's not a feature!
*** THREAD #01: This is the bug, it's not a feature!
*** THREAD #02: This is the bug, it's not a feature!
*** THREAD #03: This is the bug, it's not a feature!
*** THREAD #02: This is the bug, it's not a feature!
*** THREAD #01: This is the bug, it's not a feature!
*** THREAD #00: This is the bug, it's not a feature!
*** THREAD #03: This is the bug, it's not a feature!
*** THREAD #01: This is the bug, it's not a feature!
*** THREAD #00: This is the bug, it's not a feature!
*** THREAD #02: This is the bug, it's not a feature!
*** THREAD #03: This is the bug, it's not a feature!
*** THREAD #03: This is the bug, it's not a feature!
*** THREAD #01: This is the bug, it's not a feature!
*** THREAD #02: This is the bug, it's not a feature!
*** THREAD #00: This is the bug, it's not a feature!
*** THREAD #03: This is the bug, it's not a feature!
*** THREAD #02: This is the bug, it's not a feature!
*** THREAD #01: This is the bug, it's not a feature!
*** THREAD #00: This is the bug, it's not a feature!
*** THREAD #00: This is the bug, it's not a feature!
*** THREAD #01: This is the bug, it's not a feature!
*** THREAD #02: This is the bug, it's not a feature!
*** THREAD #03: This is the bug, it's not a feature!
*** THREAD #01: This is the bug, it's not a feature!
*** THREAD #*03: This is the bug, it's not a feature!
** THREAD #02: This is the bug, it's not a feature!
*** THREAD #00: This is the bug, it's not a feature!
*** THREAD #0*1:**  THThisR iEAD #00s the bu: This is the bg, ugit, it's not a fea's not a feature!
ture!
*** THREAD #02: This is the bug, it's not a feature!
*** THREAD #03: This is the bug, it's not a feature!
*** THREAD #00: This is the bug, it's not a feature!
*** THREAD #01: This is the bug, it's not a feature!
*** THREAD #03: This is the bug, it's not a feature!

请向我解释为什么会这样？由于哪个版本的 C++ 标准具有 printf() 函数不再是线程安全的，为什么？有没有办法在不编写自己的拐杖选项（例如，使控制台输出线程安全的编译器选项）的情况下恢复线程安全？

PS我读到了flockfile()、funlockfile()函数，试过了，但MinGW写道它们是“未声明的”。虽然使用它们也是一个马马虎虎的选择，但我仍然希望 printf() 本身来处理阻塞......

2 个回答

Voted

gbg · Answer 1 · 2022-06-14T23:51:24Z

Best Answer

gbg

2022-06-14T23:51:24Z2022-06-14T23:51:24Z

标准和书籍中没有任何地方说 printf 的结果不能被洗牌。只能保证不会损坏任何东西并且程序不会崩溃。如果您希望所有内容都严格连续打印 - 挂起互斥锁（在某些情况下会花费大量时间）或使用缓冲制作更复杂的系统。

2

LShadow77 · Answer 2 · 2022-06-15T01:07:24Z

LShadow77

2022-06-15T01:07:24Z2022-06-15T01:07:24Z

非常感谢您的澄清！好吧，事实证明，除了使用互斥锁（或临界区）编写您自己的 printf() 版本外，别无他法……

/*
 * File:    mt_printf.cpp
 * Author:  Light Shadow
*/

#include <stdio.h>
#include <cstdarg>
#include "windows.h"

#include "mt_printf.h"

CRITICAL_SECTION g_abMtPrintfCritSec;

void __attribute__ ((constructor)) _init_mt_printf()
{
    InitializeCriticalSection(&g_abMtPrintfCritSec);
}

void __attribute__ ((destructor)) _final_mt_printf()
{
    DeleteCriticalSection(&g_abMtPrintfCritSec);
}


int mt_printf(const char* format, ...)
{
    va_list args;
    va_start(args,format);

    EnterCriticalSection(&g_abMtPrintfCritSec);
    int res = vprintf(format,args);
    LeaveCriticalSection(&g_abMtPrintfCritSec);

    va_end(args);
    return res;
}

可能不是一个非常快速的方法，但出于调试目的，它会做。

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2 个回答

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