recv only gets first 4 bytes of package

Question

I'm writing a client application in C++ to receive binary data via UDP broadcast, however I'm unable to receive anything beyond the first 4 bytes, regardless of the buffer size. I've checked the packets in Wireshark, and I can see that my machine is receiving roughly 1200 bytes of data. When I compare this to what my client is receiving, I can see that I'm getting the packets, however the remaining data is lost. Here's my code:

#define BUFFERSIZE 4096
int main()
{
    struct addrinfo hints, *servinfo, *p;
    int sockfd, i, nbyt;

    int *buf = (int*) malloc(BUFFERSIZE);

    memset(&hints, 0, sizeof hints);
    hints.ai_family = AF_INET;
    hints.ai_socktype = SOCK_DGRAM;
    hints.ai_flags = AI_PASSIVE;

    getaddrinfo(NULL, "9011", &hints, &servinfo);

    sockfd = socket(servinfo->ai_family, servinfo->ai_socktype, servinfo->ai_protocol);
    bind(sockfd, servinfo->ai_addr, servinfo->ai_addrlen);

    FILE *file;
    file = fopen("file.txt", "w+");

    for (i=0;i<=45;i++)
    {
        nbyt = recv(sockfd, buf, BUFFERSIZE, 0);            
        fprintf(file, "%s %d; %s %x\n", "Bytes received:", nbyt, "Message:", *buf);
    }

    fclose(file);
    close(sockfd);
    free(buf);

    printf("%s\n", "Execution ended.");

    return 0;
}

An example of some of the data I'm receiving:

Bytes received: 1131; Message: 5b0
Bytes received: 1131; Message: 5b3
Bytes received: 1131; Message: 5b6
Bytes received: 1092; Message: 4e0

I've tried setting the socket to non-blocking with the MSG_DONTWAIT flag, as this fixed the problem for a similar Python application, however this only returns errors:

Bytes received: -1; Message: 0

I'm relatively new to C++ and sockets, so it's possible there's something I'm just not seeing. Hopefully somebody can help me figure out what's wrong? I can provide additional information if necessary.

Answer 1

You're receiving it just fine. Your debug code just doesn't do what you think it does.

The x specifier to printf prints the unsigned int argument in hex. Integers on your platform are probably 32-bits. So you're only printing the first 4 bytes.

If you want to print the entire buffer in hex, you need to write the loop to do that yourself (or use a library, of course).

For example, and I warn you my C is a little rusty (and this is by far not the best performing way to do this) ... Also, this assumes your buffer is always a multiple of sizeof(int), or it'll lose some data:

nbyt = recv(sockfd, buf, BUFFERSIZE, 0);            
fprintf(file, "%s %d; %s\n", "Bytes received:", nbyt, "Message:");
for (offset = 0; offset < nbyt/sizeof(int); ++offset) {
    fprintf(file, "%x ", buf[offset]);
}
fprintf(file, "\n");

Answer 2

*buf in your code means the first integer from the array of the received data, so you are printing just four first bytes of the received UDP payload.