c++ - Visual C++ 2015 中的 SIGINT 处理程序重置

Question

考虑下面的测试程序

#include <csignal>
#include <iostream>

volatile std::sig_atomic_t signal_raised = 0;

void set_signal_raised(int signal) {
    signal_raised = 1;
}

void check(std::sig_atomic_t expected) {
    if (signal_raised != expected) {
        std::cerr << signal_raised << " != " << expected << std::endl;
        abort();
    }
}

int main() {
    using namespace std;
    check(0);
    std::signal(SIGINT, set_signal_raised);
    check(0);
    std::raise(SIGINT);
    check(1);
    signal_raised = 0;
    check(0);
    std::raise(SIGINT);
    check(1);
    cerr << "OK.\n";
}

对于 GCC 和 Clang，它输出“OK”。但是，对于 Visual Studio 2015，它不输出任何内容。

信号处理程序在处理完第一个信号后被重置。这可以通过添加

来验证

auto prev = std::signal(SIGINT, set_signal_raised);
if (prev != set_signal_raised) {
    std::cerr << "Unexpected handler." << std::endl;
    abort();
}

检查功能。这是允许和预期的吗？

Answer 1

信号配置的重置是 Unix System V 使用的行为。但是 BSD(目前是 glibc)不会重置信号配置。 POSIX 标准允许任何一种行为。 C标准没有指定是否允许“重置”。

来自 signal(2) :

POSIX.1 solved the portability mess by specifying sigaction(2), which provides explicit control of the semantics when a signal handler is invoked; use that interface instead of signal().

In the original UNIX systems, when a handler that was established using signal() was invoked by the delivery of a signal, the disposition of the signal would be reset to SIG_DFL, and the system did not block delivery of further instances of the signal. This is equivalent to calling sigaction(2) with the following flags:

sa.sa_flags = SA_RESETHAND | SA_NODEFER;

System V also provides these semantics for signal(). This was bad because the signal might be delivered again before the handler had a chance to reestablish itself. Furthermore, rapid deliveries of the same signal could result in recursive invocations of the handler.

因此，Visual Studio 似乎遵循了 System V 的行为。

Is this allowed and expected?

这是允许的，但绝对不希望这样做。为此，POSIX 引入了 sigaction() .如果你有 sigaction() 然后使用它。否则，您只需每次在信号处理程序中重新安装处理程序:

void set_signal_raised(int signal) {
    std::signal(SIGINT, set_signal_raised);
    signal_raised = 1;
}