epoll 事件不会引发 SIGIO

Question

根据我的研究，您可以将 epoll 文件描述符添加到 poll、select 或其他 epoll 中，如果事件可用，它将返回 POLLIN。根据 epoll(7)：

   Q3  Is the epoll file descriptor itself poll/epoll/selectable?

   A3  Yes.  If an epoll file descriptor has events waiting, then it  will
       indicate as being readable.

这在我的测试中有效。但是，我现在正在尝试将 O_ASYNC 应用于我的 epoll fd，以便在事件准备好时引发 SIGIO：

//Epoll setup code - trySc runs perror on the string and exits if the function returns -1
int epollFd = epoll_create1(EPOLL_CLOEXEC);
trySc(epollFd, "epoll_create1");
trySc(fcntl(epollFd, F_SETOWN, getpid()), "fcntl F_SETOWN");
trySc(fcntl(epollFd, F_SETSIG, SIGIO), "fcntl F_SETSIG");
int oldFlags = fcntl(epollFd, F_GETFL, 0);
trySc(oldFlags, "fcntl F_GETFL");
trySc(fcntl(epollFd, F_SETFL, oldFlags | O_ASYNC), "fcntl F_SETFL");

//Set up SIGIO and get a socket fd, you don't need to see this
//All my handler does is exit - it's a definite method of knowing SIGIO was raised

struct epoll_event event = {
        .events = EPOLLIN | EPOLLET,
        .data.fd = socketFd
};
trySc(epoll_ctl(epollFd, EPOLL_CTL_ADD, socketFd, &event), "epoll_ctl");
//Then connect
while(1){
        struct pollfd pfd = {
                .fd = epollFd,
                .events = POLLIN
        };
        //This blocks until I write something to the socket. Then it enters an infinite loop.
        printf("Returning to main(), poll = %d\n", poll(&pfd, 1, -1));
}

当我这样做时，它不会为 epoll 中的新事件引发 SIGIO。 poll 表示 epollFd 中有准备好的事件，但它应该首先引发 SIGIO（它只是检查事件是否在 epollFd 中并退出）。我知道我可以将 O_ASYNC 应用于套接字（我也尝试过），但我希望在我的事件中包含数据。 Epoll 让我这样做。

这是我的完整代码：

#define _GNU_SOURCE
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <sys/epoll.h>
#include <fcntl.h>
#include <netdb.h>
#include <poll.h>
#include <signal.h>
#include <errno.h>

void handleSIGIO(int, siginfo_t *, void *);

const struct sigaction saSIGIO = {
    .sa_sigaction = handleSIGIO,
    .sa_flags = SA_SIGINFO
};

const struct addrinfo hints = {
    .ai_flags = AI_PASSIVE,
    .ai_family = AF_INET,
    .ai_socktype = SOCK_STREAM
};

void trySc(int err, const char *msg){
    if(err != -1) return;
    perror(msg);
    exit(errno);
}

int main(){
    signal(SIGPIPE, SIG_IGN);
    trySc(sigaction(SIGIO, &saSIGIO, NULL), "sigaction");
    int epollFd = epoll_create1(EPOLL_CLOEXEC);
    trySc(epollFd, "epoll_create1");
    trySc(fcntl(epollFd, F_SETOWN, getpid()), "fcntl F_SETOWN");
    trySc(fcntl(epollFd, F_SETSIG, SIGIO), "fcntl F_SETSIG");
    int oldFlags = fcntl(epollFd, F_GETFL, 0);
    trySc(oldFlags, "fcntl F_GETFL");
    trySc(fcntl(epollFd, F_SETFL, oldFlags | O_ASYNC), "fcntl F_SETFL");
    int socketFd = socket(AF_INET, SOCK_STREAM, 0);
    trySc(socketFd, "socket");
    struct addrinfo *servinfo;
    trySc(getaddrinfo("127.0.0.1", "5000", &hints, &servinfo),
        "getaddrinfo");
    struct epoll_event event = {
        .events = EPOLLIN | EPOLLET,
        .data.fd = socketFd
    };
    trySc(epoll_ctl(epollFd, EPOLL_CTL_ADD, socketFd, &event), "epoll_ctl");
    trySc(connect(socketFd, servinfo->ai_addr, servinfo->ai_addrlen),
        "connect");
    printf("Connected\n");
    while(1){
        struct pollfd pfd = {
            .fd = epollFd,
            .events = POLLIN
        };
        printf("Returning to main(), poll = %d\n", poll(&pfd, 1, -1));
    }
}

void handleSIGIO(int sn, siginfo_t *info, void *ctx){
    printf("SIGIO called\n");
    struct epoll_event event;
    if(epoll_wait(info->si_fd, &event, 1, 0) != 1){
        printf("Warning: no event available\n");
        return;
    }
    printf("Event raised for fd %d\n", event.data.fd);
    exit(0);
}

编辑：根据this website，我正在尝试做的事情应该有效：

Note that an epoll set descriptor can be used much like a regular file 
descriptor. That is, it can be made to generate SIGIO (or another signal)
when input (i.e. events) is available on it; likewise it can be used with 
poll() and can even be stored inside another epoll set.

Answer 1

经过更多研究，我发现这是不可能的。根据open(2)

O_ASYNC

启用信号驱动 I/O：在输入或输出时生成一个信号（默认为 SIGIO，但可以通过 fcntl(2) 更改） 在这个文件描述符上成为可能。 此功能是 仅适用于终端、伪终端、套接字和 （从 Linux 2.6 开始）管道和 FIFO。 请参阅 fcntl(2) 了解更多信息 细节。另请参阅下面的错误。

我真的希望我可以将 epoll 与 SIGIO 一起使用。我会找到其他我可以做的。

我找到了解决这个问题的方法：将O_ASYNC 应用于每个套接字，然后在您的 SIGIO 处理程序中，在您的 epoll fd 上调用 epoll_wait。然后像往常一样处理来自 epoll 的事件。它不适用于不支持 SIGIO 的 FD，即使它们支持 epoll，但它适用于套接字和管道，这可能是此类异步 IO 最常见的用途。