Linux C ++套接字选择循环

Question

I'm having a little trouble with sockets, when looping I'm not receiving data except for the first loop, it's timing out each time. If I close and reopen the socket each loop though I seem to be getting the data correctly. Any ideas as to why?

Example of looping without closing:

int socketHandle = socket(AF_INET,SOCK_DGRAM,0);

sockaddr_in serverAddr;

serverAddr.sin_family = AF_INET;
serverAddr.sin_addr.s_addr = inet_addr(/*UDP IP ADDRESS*/);
serverAddr.sin_port = htons(/*UDP PORT*/);

struct timeval tv;
fd_set rfds;

FD_ZERO(&rfds);
FD_SET(socketHandle, &rfds);

tv.tv_usec = 0.0;
int recVal = 0;
int sockLen = sizeof(serverAddr);
bind(socketHandle, (struct sockaddr*)&serverAddr, (socklen_t)sockLen);

bool timePassed = false;
time_t startListenTime = time(NULL);

tv.tv_sec = maxUpdateTime;

while(true)
{
    recVal = select(socketHandle + 1, &rfds, NULL, NULL, &tv);
    switch(recVal)
    {
        case(0):
        {
            //Timeout
            break;
        }
        case(-1):
        {
            //Error
             break;
        }
        default:
        {
            /*Packet Data Type*/ pkt;
            if(recvfrom(socketHandle, &pkt, sizeof(/*Packet Data Type*/), 0, (sockaddr*)&serverAddr, (socklen_t*)&sockLen) < 0)
            {
                //Failed to Recieve Data
                break;
            }
            else
            {
                //Recieved Data!!
            }
            break;
        }
    }
}

Example of looping with closing:

while(true)
{
    int socketHandle = socket(AF_INET,SOCK_DGRAM,0);

    sockaddr_in serverAddr;

    serverAddr.sin_family = AF_INET;
    serverAddr.sin_addr.s_addr = inet_addr(/*UDP IP ADDRESS*/);
    serverAddr.sin_port = htons(/*UDP PORT*/);

    struct timeval tv;
    fd_set rfds;

    FD_ZERO(&rfds);
    FD_SET(socketHandle, &rfds);

    tv.tv_usec = 0.0;
    int recVal = 0;
    int sockLen = sizeof(serverAddr);
    bind(socketHandle, (struct sockaddr*)&serverAddr, (socklen_t)sockLen);


    bool timePassed = false;
    time_t startListenTime = time(NULL);

    tv.tv_sec = maxUpdateTime;

    recVal = select(socketHandle + 1, &rfds, NULL, NULL, &tv);
    switch(recVal)
    {
        case(0):
        {
            //Timeout
            break;
        }
        case(-1):
        {
            //Error
             break;
        }
        default:
        {
            /*Packet Datastructure*/ pkt;
            if(recvfrom(socketHandle, &pkt, sizeof(/*Packet Datastructure*/), 0, (sockaddr*)&serverAddr, (socklen_t*)&sockLen) < 0)
            {
                //Failed to read packet
                break;
            }
            else
            {
                //Read Packet!!
            }
            break;
        }
    }
    close(socketHandle);
}

Answer 1

The select() function uses the specified file descriptor mask to determine which file descriptors to monitor for an event (read, write, etc.). When a file descriptor is available for an I/O activity (read, write) the select() function modifies the descriptors to indicate which of the files are ready for the given I/O action.

See this article on the select function and the macros/functions used with the file descriptors .

Old style Unix type programs often treated the file descriptor as a bit mask and just checked the bits. However the actual implementation of the file descriptor can vary by compiler so it is best to use the standard file descriptor macros/functions to set, clear, and test the various file descriptors.

So when using the select() function you need to use FD_ZERO() and FD_SET() so that you will set the specific file descriptors that you want for this particular call to the select() function. When select() returns, it will indicate which of the file descriptors designated are actually ready to be used for the I/O action (read, write, etc.).

So your code will actually be something like:

while(true)
{
    fd_set rfds;

    FD_ZERO(&rfds);
    FD_SET(socketHandle, &rfds);
    recVal = select(socketHandle + 1, &rfds, NULL, NULL, &tv);
    switch(recVal)
    {
        case(0):
        {
            //Timeout
            break;
        }
        case(-1):
        {
            //Error
             break;
        }
        default:
        {
            /*Packet Data Type*/ pkt;
            if(recvfrom(socketHandle, &pkt, sizeof(/*Packet Data Type*/), 0, (sockaddr*)&serverAddr, (socklen_t*)&sockLen) < 0)
            {
                //Failed to Recieve Data
                break;
            }
            else
            {
                //Recieved Data!!
            }
            break;
        }
    }

However what you really should do is to use the FD_ISSET() function to check which particular file descriptors are ready for use. In your case you have only the one but in a situation where there were multiple descriptors you would want to use the FD_ISSET() function as well.

Answer 2

When select returns anything except -1 it is considered success and the fd sets are modified to say which ones are ready or have an error.

From the POSIX spec :

Upon successful completion, the pselect () or select () function shall modify the objects pointed to by the readfds , writefds , and errorfds arguments to indicate which file descriptors are ready for reading, ready for writing, or have an error condition pending, respectively, and shall return the total number of ready descriptors in all the output sets.

If a timeout occurs, then it returns zero meaning none of the descriptors became ready, and no bits will be set in the fd sets.

So when your call times out there are no bits set in rfds , and so on the next loop when you call select you are asking it to wait for an empty set, which will never return a positive value because if you wait for zero FDs then you will never get a non-zero number of ready FDs!

You need to remember that rfds is both an input parameter and an output parameter, and so ensure it is set correctly before each call to select .