C Socket programming on windows errors on listen but no errno

Question

I've done socket programming in other languages but never dealt with doing it in C directly. I'm trying to set up a simple server to listen on port 8080 under Windows. I'm using the gcc that comes with mingw to compile the code.



    #include <stdio.h>
    #include <string.h>
    #include <WinSock2.h>
    #include <stdint.h>
    #include <time.h>

        int main(void) {

        int s;      
        int len;
        char  buffer[1025];  
        struct sockaddr_in servAddr; 
        struct sockaddr_in clntAddr; 

        int clntAddrLen; //length of client socket addre

    //Build local (server) socket add

        memset(&servAddr, 0, sizeof(servAddr));
        servAddr.sin_family = AF_INET;
        servAddr.sin_port = htons(8080);
        servAddr.sin_addr.s_addr = htonl(INADDR_ANY);

       //create socket
        if((s=socket(AF_INET, SOCK_STREAM, 0) %lt; 0 )){
            perror("Error: Socket Failed!");
                printf("%d\n", errno);
            exit(1);
        }

    //bind socket to local address and port
        if((bind(s,(struct sockaddr*)&servAddr, sizeof(servAddr)) %lt; 0))
        {
            perror("Error:bind failed!");
                printf("%d\n", errno);
            exit(1);
        }

        for(;;)
        {
            len = recvfrom(s,buffer, sizeof(buffer),0,(struct sockaddr*)&clntAddr, &clntAddrLen);

            //send string
            sendto(s, buffer, len, 0, (struct sockaddr*)&clntAddr, sizeof(clntAddr));
        }

    }

When I run the code it errors out binding but there's no error. This code was taken and slightly modified from this thread: Windows Socket Programming in C

But there's no error code. The output prints that there's no error. It fails at the bind and I'm not sure why. I've been piecing together a few different sources. I literally just want something that can do a hello world/3 way handshake in C on Windows.

Thanks in advance.

hmjd · Accepted Answer · 2013-04-03 20:30:32Z

1

This is incorrect due to operator precedence:

if((s=socket(AF_INET, SOCK_STREAM, 0) < 0 )){

as < has higher precedence than =. This means that the code will assign 1 or 0 to s depending on result of socket(AF_INET, SOCK_STREAM, 0) < 0. Change to:

if((s=socket(AF_INET, SOCK_STREAM, 0)) < 0 ){

Use WSAGetLastError() to obtain failure reason, not errno. From the MSDN bind() reference page:

If no error occurs, bind returns zero. Otherwise, it returns SOCKET_ERROR, and a specific error code can be retrieved by calling WSAGetLastError.

On Windows WSAStartup() must be invoked before any socket functions are called.

edited Apr 3, 2013 at 20:30

answered Apr 3, 2013 at 20:25

hmjd

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1 Comment

ehudt Over a year ago

Also, WSACleanup() should be run when finishing using sockets.

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