linux - Linux 中的同时套接字读/写 ("full-duplex")(特别是 aio)

Question

我正在移植一个构建在 ACE Proactor 框架之上的应用程序。该应用程序在 VxWorks 和 Windows 上都能完美运行，但无法在内核为 2.6.X.X 的 Linux(CentOS 5.5、WindRiver Linux 1.4 和 3.0)上运行 - 使用 librt。

我已将问题缩小为一个非常基本的问题:应用程序在套接字上开始异步(通过 aio_read)读取操作，随后在同一个套接字上开始异步(通过 aio_write)写入。由于协议(protocol)是从应用程序端初始化的，因此无法完成读取操作。- 当套接字处于阻塞模式时，写永远不会到达并且协议(protocol)“挂起”。- 使用 O_NONBLOCK 套接字时，写入成功但读取无限期返回并出现“EWOULDBLOCK/EAGAIN”错误，永远不会恢复(即使重新启动 AIO 操作)。

我浏览了多个论坛，但无法找到关于这对于 Linux AIO 是否可行(我做错了什么)是否可行的明确答案。如果我放弃 AIO 并寻求不同的实现(通过 epoll/poll/select 等)是否可能？

附件是在非阻塞套接字上快速重现问题的示例代码:

#include <aio.h>
#include <stdio.h>
#include <stdlib.h>
#include <netdb.h>
#include <string.h> 
#include <netinet/in.h>
#include <sys/socket.h>
#include <sys/types.h>
#include <assert.h>
#include <errno.h>

#define BUFSIZE (100)

// Global variables
struct aiocb *cblist[2];
int theSocket;

void InitializeAiocbData(struct aiocb* pAiocb, char* pBuffer)
{
    bzero( (char *)pAiocb, sizeof(struct aiocb) );

    pAiocb->aio_fildes = theSocket;
    pAiocb->aio_nbytes = BUFSIZE;
    pAiocb->aio_offset = 0;
    pAiocb->aio_buf = pBuffer;
}

void IssueReadOperation(struct aiocb* pAiocb, char* pBuffer)
{
    InitializeAiocbData(pAiocb, pBuffer);

    int ret = aio_read( pAiocb );
    assert (ret >= 0);
}

void IssueWriteOperation(struct aiocb* pAiocb, char* pBuffer)
{
    InitializeAiocbData(pAiocb, pBuffer);

    int ret = aio_write( pAiocb );
    assert (ret >= 0);
}

int main()
{
    int ret;
    int nPort = 11111;
    char* szServer = "10.10.9.123";

    // Connect to the remote server
    theSocket = socket(AF_INET, SOCK_STREAM, 0);
    assert (theSocket >= 0);

    struct hostent *pServer;
    struct sockaddr_in serv_addr;
    pServer = gethostbyname(szServer);

    bzero((char *) &serv_addr, sizeof(serv_addr));
    serv_addr.sin_family = AF_INET;
    serv_addr.sin_port = htons(nPort);
    bcopy((char *)pServer->h_addr, (char *)&serv_addr.sin_addr.s_addr, pServer->h_length);

    assert (connect(theSocket, (const sockaddr*)(&serv_addr), sizeof(serv_addr)) >= 0);

    // Set the socket to be non-blocking
    int oldFlags = fcntl(theSocket, F_GETFL) ;
    int newFlags = oldFlags | O_NONBLOCK;

    fcntl(theSocket, F_SETFL, newFlags);
    printf("Socket flags: before=%o, after=%o\n", oldFlags, newFlags);

    // Construct the AIO callbacks array
    struct aiocb my_aiocb1, my_aiocb2;
    char* pBuffer = new char[BUFSIZE+1];

    bzero( (char *)cblist, sizeof(cblist) );
    cblist[0] = &my_aiocb1;
    cblist[1] = &my_aiocb2;

    // Start the read and write operations on the same socket
    IssueReadOperation(&my_aiocb1, pBuffer);
    IssueWriteOperation(&my_aiocb2, pBuffer);

    // Wait for I/O completion on both operations
    int nRound = 1;
    printf("\naio_suspend round #%d:\n", nRound++);
    ret = aio_suspend( cblist, 2, NULL );
    assert (ret == 0);

    // Check the error status for the read and write operations
    ret = aio_error(&my_aiocb1);
    assert (ret == EWOULDBLOCK);

    // Get the return code for the read
    {
        ssize_t retcode = aio_return(&my_aiocb1);
        printf("First read operation results: aio_error=%d, aio_return=%d  -  That's the first EWOULDBLOCK\n", ret, retcode);
    }

    ret = aio_error(&my_aiocb2);
    assert (ret == EINPROGRESS);
    printf("Write operation is still \"in progress\"\n");

    // Re-issue the read operation
    IssueReadOperation(&my_aiocb1, pBuffer);

    // Wait for I/O completion on both operations
    printf("\naio_suspend round #%d:\n", nRound++);
    ret = aio_suspend( cblist, 2, NULL );
    assert (ret == 0);

    // Check the error status for the read and write operations for the second time
    ret = aio_error(&my_aiocb1);
    assert (ret == EINPROGRESS);
    printf("Second read operation request is suddenly marked as \"in progress\"\n");

    ret = aio_error(&my_aiocb2);
    assert (ret == 0);

    // Get the return code for the write
    {
        ssize_t retcode = aio_return(&my_aiocb2);
        printf("Write operation has completed with results: aio_error=%d, aio_return=%d\n", ret, retcode);
    }

    // Now try waiting for the read operation to complete - it'll just busy-wait, receiving "EWOULDBLOCK" indefinitely
    do
    {
        printf("\naio_suspend round #%d:\n", nRound++);
        ret = aio_suspend( cblist, 1, NULL );
        assert (ret == 0);

        // Check the error of the read operation and re-issue if needed
        ret = aio_error(&my_aiocb1);
        if (ret == EWOULDBLOCK)
        {
            IssueReadOperation(&my_aiocb1, pBuffer);
            printf("EWOULDBLOCK again on the read operation!\n");
        }
    }
    while (ret == EWOULDBLOCK);
}

提前致谢，约塔姆。

Answer 1

首先，O_NONBLOCK 和 AIO 不能混用。当相应的read 或write 不会被阻塞时，AIO 将报告异步操作完成——而对于O_NONBLOCK，它们永远不会 block ，因此 aio 请求将始终立即完成(aio_return() 给出 EWOULDBLOCK)。

其次，不要对两个同时未完成的 aio 请求使用相同的缓冲区。在发出 aio 请求和 aio_error() 告诉您它已完成之间的时间范围内，缓冲区应被视为完全不受限制。

第三，对同一文件描述符的 AIO 请求被排队，以便给出合理的结果。这意味着在读取完成之前不会发生写入 - 如果您需要先写入数据，则需要以相反的顺序发出 AIO。以下将正常工作，无需设置 O_NONBLOCK:

struct aiocb my_aiocb1, my_aiocb2;
char pBuffer1[BUFSIZE+1], pBuffer2[BUFSIZE+1] = "Some test message";

const struct aiocb *cblist[2] = { &my_aiocb1, &my_aiocb2 };

// Start the read and write operations on the same socket
IssueWriteOperation(&my_aiocb2, pBuffer2);
IssueReadOperation(&my_aiocb1, pBuffer1);

// Wait for I/O completion on both operations
int nRound = 1;
int aio_status1, aio_status2;
do {
    printf("\naio_suspend round #%d:\n", nRound++);
    ret = aio_suspend( cblist, 2, NULL );
    assert (ret == 0);

    // Check the error status for the read and write operations
    aio_status1 = aio_error(&my_aiocb1);
    if (aio_status1 == EINPROGRESS)
        puts("aio1 still in progress.");
    else
        puts("aio1 completed.");

    aio_status2 = aio_error(&my_aiocb2);

    if (aio_status2 == EINPROGRESS)
        puts("aio2 still in progress.");
    else
        puts("aio2 completed.");
} while (aio_status1 == EINPROGRESS || aio_status2 == EINPROGRESS);

 // Get the return code for the read
ssize_t retcode;
retcode = aio_return(&my_aiocb1);
printf("First operation results: aio_error=%d, aio_return=%d\n", aio_status1, retcode);

retcode = aio_return(&my_aiocb1);
printf("Second operation results: aio_error=%d, aio_return=%d\n", aio_status1, retcode);

或者，如果您不关心读取和写入相对于彼此的顺序，您可以使用 dup() 为套接字创建两个文件描述符，并使用一个用于读取另一个用于写入 - 每个都将其 AIO 操作单独排队。