非阻塞UDP端口打开时打开文件失败

Question

I have a problem with a program. The program should be triggered by UDP messages, that's why I open a nonblocking UDP socket, because I want to use it later again. After that the program should open a file, copy out a certain amount of bytes and send it to a browser.

The problem occurs when I want to open a file, then I get a "resource temporarily not availabe" fault.

Here is a simple program, which creates the same fault.

main part:

udp_openPort(9999);

for(;;){
    if(udp_receiveData(temp, 32) > 0){
        printf("Received Message: %d\n",atoi(temp));
    break;
    }
} 

filefd = open("test.txt",O_RDONLY);
printf("File Open: %s\n",strerror(errno));

read(filefd,buff,sizeof(buff));

printf("Daten: %s",buff);

close(filefd);

udp_closePort();

udp_receiveData():

int udp_receiveData(void* data, int size){
    socklen_t dummy = sizeof(NetworkAddr);
    NetworkAddr sender;
    return recvfrom(sockfd, data, size, MSG_DONTWAIT, (struct sockaddr*) (&sender), &dummy);
}

When I open the socket as a blocking socket, there is no problem with opening the file, but I need I nonblocking socket for my purpose.

Did I make a mistake in coding the program or did I made a mistake, when I planned the program?

Best regards, PG

EDIT: Here is the udp_openPort() function:

int udp_openPort(int portNr){

//Create handle to socket
sockfd = socket(AF_INET, SOCK_DGRAM, 0);
if (sockfd == -1){
    return 0;
}

// Make sure that we don't receive our own packets.
char loop = 0;
if (setsockopt(sockfd, IPPROTO_IP, IP_MULTICAST_LOOP, &loop, sizeof(loop)) == -1){

}


// Bind to the port where we receive UDP messages.
NetworkAddr addr;
memset(&addr, 0, sizeof(addr));

addr.sin_family      = AF_INET;
addr.sin_port        = htons(portNr);
addr.sin_addr.s_addr = htonl(INADDR_ANY);

if (bind(sockfd, (struct sockaddr*) &addr, sizeof(addr)) == -1){
    return 0;
}
return 1;
}

Maybe someone sees a problem in here.

Answer 1

Are you sure that you error after open is not interfere with udp_openPort ? Probably udp_openPort does something wrong and set the errno variable which you lately misinterpret as file open error.

Answer 2

Looking at the manpage of open , the errno is set if and only if the error occurred, ie when returned descriptor is -1 . Hence, your error handling is incorrect. You should have checked the value of filefd in order to determine if the file has been opened or not.

Because the file in fact was opened correctly, errno hasn't been modified and your error message was set by udp_receiveData ; in this case, as your socket is non-blocking, there was no data on UDP socket ( resource temporarily unavailable ). You don't experience this with a blocking socket, as your program is then sleeping waiting for a message to arrive.