C套接字编程：调用recv（）会改变我的套接字文件描述符？

Question

Hey all, I have this strange problem with recv(). I'm programming client/server where client send() a message (a structure to be exact) and server recv() it. I am also working with multiple sockets and select().

while(1)
{
    readset = info->read_set;
    info->copy_set = info->read_set;

    timeout.tv_sec = 1; 
    timeout.tv_usec = 0; // 0.5 seconds

    ready = select(info->max_fd+1, &readset, NULL, NULL, &timeout);

    if (ready == -1)
    {
        printf("S: ERROR: select(): %s\nEXITING...", strerror(errno));
        exit(1);
    }
    else if (ready == 0)
    {
        continue;
    }
    else
    {
        printf("S: oh finally you have contacted me!\n");
        for(i = 0; i < (info->max_fd+1); i++)
        {

            if(FD_ISSET(i, &readset)) //this is where problem begins
            {
                printf("S: %i is set\n", i);
                printf("S: we talking about socket %i son\n", i);  // i = 4
                num_bytes = recv(i, &msg, MAX_MSG_BYTE, 0);
                printf("S: number of bytes recieved in socket %i is %i\n", i, num_bytes); // prints out i = 0 what??

                if (num_bytes == 0)
                {
                    printf("S: socket has been closed\n");
                    break;
                }
                else if (num_bytes == -1)
                {
                    printf("S: ERROR recv: %d %s \n", i, strerror(errno));
                    continue;
                }
                else                    
                {
                    handle_request(arg, &msg);
                    printf("S: msg says %s\n", msg->_payload);
                }
            } // if (FD_ISSET(i, &readset)
            else
                printf("S:  %i is not set\n", i);
        } // for (i = 0; i < maxfd+1; i++) to check sockets for msg
    } // if (ready == -1)   

    info->read_set = info->copy_set;
    printf("S: copied\n");

} 

the problem I have is that in read_set , 0~3 aren't set and 4 is. That is fine. But when i call recv() , i suddently becomes 0. Why is that? It doesn't make sense to me why recv() would take an socket file descriptor number and modify to another number. Is that normal? Am I missing something?

S:  0 is not set
S:  1 is not set
S:  2 is not set
S:  3 is not set
S: 4 is set
S: we talking about socket 4 son
S: i is strangely or unstrangely 0
S: number of bytes recieved in socket 0 is 40

That's what it prints out.

Answer 1

recv cannot modify its first argument, since it is taken by value.

You don't show where you've declared msg or i , but based on this line

printf("S: msg says %s\n", msg->_payload);

Where you use the -> operator on msg , I assume it's probably like this:

struct somestruct* msg = malloc(sizeof(struct somestruct));
int i;

Then you do this:

num_bytes = recv(i, &msg, MAX_MSG_BYTE, 0);

Note that msg is already a pointer, so &msg is a pointer to the pointer .

What this will then do is receive data and try to store it in the place where the msg pointer itself is, not the place that msg points to . Typically, pointers are only 4 bytes long, so this will overflow the storage if you receive more than four bytes. If i is declared on the stack after msg , then it is likely that it is being overwritten by this overflow, and it happens to get overwritten by all zero bytes from the received packet.

Since msg is already a pointer, change your receive line to eliminate the superfluous indirection:

num_bytes = recv(i, msg, MAX_MSG_BYTE, 0);

Similarly, you may want to consider making the same change to the line

handle_request(arg, &msg)

if the handle_request function is not really expecting a pointer-to-pointer.

Answer 2

我的第一个猜测是sizeof(msg) < MAX_MSG_BYTE ，当recv溢出msg它会破坏i 。