Question

I am trying to send a buffer via UDP sockets in C but I always get an invalid argument error in sendto. I just don’t find the error. Could anyone maybe help me. Thanks in advance.

Here’s my code:

/**/    void    IPCSend(char *pazClientAddress, int iClientPort, char *pazBuffer )
{
    int iSocket;
    /* */
    if ((iSocket = socket(AF_INET, SOCK_DGRAM, 0)) != -1)
    {
        int iSendToReturn;
        struct sockaddr_in sinServerAddress;
        struct hostent *pstHost;
        /* */
        pstHost = (struct hostent *)gethostbyname((char *)pazClientAddress);
        /* */
        sinServerAddress.sin_family = AF_INET;
        sinServerAddress.sin_addr = *((struct in_addr *)pstHost->h_addr);
        sinServerAddress.sin_port = iPortNumber;
        bzero(&(sinServerAddress.sin_zero),8);
        /* */
        fprintf(stdout,"sending '%s' to client '%s:%d'\n",pazBuffer,pazClientAddress,iClientPort);
        iSendToReturn = sendto(iSocket, pazBuffer, sizeof(pazBuffer), 0, (struct sockaddr *)&sinServerAddress, sizeof(sinServerAddress));
        /* */
        if(iSendToReturn != -1)
            fprintf(stdout,"%d bytes sent\n",iSendToReturn);
        else
            perror("SendTo");
        /* */
        close(iSocket);
    }
    else
        fprintf(stdout,"could not connect to server\n");
}

Answer 1

Firstly, you’re making the perennial error of novice C programmers: using sizeof to get the size of a pointer. The variable pazBuffer is a pointer, not an array, so the sizeof operator will not return the array length, which is what you want. Your IPCSend function needs to take in the length of pazBuffer as a separate parameter:

void IPCSend(char *pazClientAddress, int iClientPort, char *pazBuffer, size_t len)

As for the error you’re getting – it might be related to something else. Invalid argument means that one of the parameters you’re passing to sendto is somehow invalid. Since I see that you are properly initializing the socket descriptor, the problem might be that the send buffer is somehow not valid – possibly a null pointer or bad address. This means that the problem is not apparent from the code you posted, and is likely in some code that is calling IPCSend.