我正在移植构建在ACE Proactor框架之上的应用程序.该应用程序适用于VxWorks和Windows,但在使用librt的内核2.6.X.X的Linux(CentOS 5.5,WindRiver Linux 1.4和3.0)上无法运行.
我把问题缩小到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,然后在同一个套接字上开始异步(通过aio_write)写入.由于协议是从应用程序结束初始化的,因此无法实现读取操作.
– 当套接字处于阻塞模式时,永远不会写入并且协议“挂起”.
– 使用O_NONBLOCK套接字时,写入成功,但读取无限期地返回“EWOULDBLOCK / EAGAIN”错误,永远不会恢复(即使重新启动AIO操作).
我经历了多个论坛,无法找到一个明确的答案,说明这是否应该可行(而且我做错了)或Linux AIO不可能.是否可以放弃AIO并寻求不同的实现(通过epoll / poll / select等)？
附件是一个示例代码,可以在非阻塞套接字上快速重现问题：
#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,
Yotam.
解决方法:
首先,O_NONBLOCK和AIO不混合.当相应的读或写不会被阻塞时,AIO将报告异步操作完成 – 并且使用O_NONBLOCK,它们永远不会阻塞,因此aio请求将始终立即完成(使用aio_return()给出EWOULDBLOCK).
其次,不要为两个同时发生的未完成的aio请求使用相同的缓冲区.在发出aio请求的时间和aio_error()告诉你已完成的时间之间,缓冲区应被视为完全禁止.
第三,AIO对同一文件描述符的请求排队,以便给出明智的结果.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.以下工作正常,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,如果您不关心相互之间的读取和写入,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而另一个用于写入 – 每个都将具有AIO操作单独排队.