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printf() не является потокобезопасной в C++17!

Автор вопроса: LShadow77

Всем здравия! В разных источниках я читал, что функция printf() является потокобезопасной и может вызываться разными потоками для бесконфликтного вывода на консоль (при условии корректной передачи параметров, разумеется). И действительно, с многопоточным выводом эта функция всегда прекрасно справлялась, чем я собственно пользовался... До тех пор, пока не понадобилось откомпилировать программу с поддержкой стандарта C++17! В общем, вот минимально воспроизводимый пример, иллюстрирующий суть проблемы:

#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;
}

Пользуюсь компилятором MinGW GCC версии 9.2.0 (build 2). Компилирую для стандарта C++ по умолчанию (уж не знаю, какой стандарт использует MinGW по умолчанию, не нашёл как посмотреть) и получаю ожидаемо корректный результат:

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!

Однако, при компиляции с опцией -std=c++17, вся потокобезопасность вывода рушится, получается каша:

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!

Поясните мне, пожалуйста, почему так происходит? Начиная с какой версии стандарта C++ функция printf() перестала быть потокобезопасной и почему? И есть ли способ вернуть ей потокобезопасность без написания собственного варианта-костыля (скажем, опция компилятора, делающая консольный вывод thread safe)?

P.S. Прочитал про функции flockfile(), funlockfile(), попробовал их, однако MinGW пишет, что они есть "undeclared". Хотя их использование - тоже так себе вариант, всё таки хочется, чтобы блокировкой занималась сама printf()...


Источник

Ответы (2 шт):

Автор решения: gbg

Нигде в стандарте и книжках не сказано, что результат printf не может перемешиваться. Гарантируется только то, что ничего не будет испорчено и программа не упадет. Хотите, чтобы все печаталось строго подряд - вешайте мьютекс (дорого по времени в некоторых случаях) или делайте более сложную систему с буферизацией.

→ Ссылка
Автор решения: LShadow77

Спасибо большое за разъяснения! Что ж, получается, что кроме как написать свой вариант printf() с мьютексом (или критической секцией), ничего не остаётся...

/*
 * 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;
}

Наверное, не слишком быстрый способ, но для отладочных целей сойдёт.

→ Ссылка

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