Parsing the Wire Protocol in C. I also wrote a Ruby version earlier, WireProto.

main.c

#include <stdio.h>
#include <sys/socket.h>
#include <stdlib.h>
#include <netinet/in.h>
#include <string.h>

void processMessage(int);
struct wire_proto_message {
    unsigned int msgLen;
    unsigned int requestId;
    unsigned int responseTo;
    unsigned int opCode;
};
struct opquery_message {
    unsigned int flags;
    char* collection_name;
    unsigned int skip;
    unsigned int returned;
};

int main() {
    const int sizeof_sockaddr_in = sizeof(struct sockaddr_in);
    struct sockaddr_in server, client;
    int srvsockfd, clientsockfd;
    int retval;

    server.sin_family = AF_INET;
    server.sin_addr.s_addr = INADDR_ANY;
    server.sin_port = htons(27000);

    srvsockfd = socket(AF_INET, SOCK_STREAM, 0);
    if (srvsockfd == -1) {
        perror("Could not create a socket");
//        printf("Could not create a socket");
//        exit(-1);
    }
    retval = setsockopt(srvsockfd, SOL_SOCKET, SO_REUSEADDR, &(int){1}, sizeof(int));
    if (retval < 0) {
        perror("Could not set socket option");
    }

    retval = bind(srvsockfd, (struct sockaddr *) &server, sizeof_sockaddr_in);
    if (retval < 0) {
        perror("Could not create a server socket");
//        printf("Could not create a server socket");
//        exit(-2);
    }
    listen(srvsockfd, 0);

    clientsockfd = accept(srvsockfd, (struct sockaddr *) &client, (socklen_t *) &sizeof_sockaddr_in);
    if (clientsockfd < 0) {
        perror("Could not accept connection");
    }

    while(1) {
        processMessage(clientsockfd);
    }

    puts("Received data");

    return 0;
}

void processMessage(int clientsockfd) {
    const unsigned int OP_QUERY = 2004;

    struct wire_proto_message msgMeta1;

    const int bufsize=4096;
    int retval;

    char databuf[bufsize];

    retval = recv(clientsockfd, databuf, bufsize, 0);
    if (retval < 0) {
        perror("Could not receive data");
    }

//    unsigned int * msgLenPtr = (unsigned int *) databuf;
//    unsigned int msgLenPtrDeref = * msgLenPtr;
    msgMeta1.msgLen = *((unsigned int *) (databuf));

    int bufPos = 0;
    bufPos += 4;
    msgMeta1.requestId = *((unsigned int *) (((char*)databuf) + bufPos));
    bufPos += 4;
    msgMeta1.responseTo = *((unsigned int *) (((char*)databuf) + bufPos));
    bufPos += 4;
    msgMeta1.opCode = *((unsigned int *) (((char*)databuf) + bufPos));
    bufPos += 4;

    if (msgMeta1.opCode == OP_QUERY) {
        struct opquery_message opqueryMeta;
        opqueryMeta.flags= *((unsigned int *) (((char*)databuf) + bufPos));
        bufPos += 4;
        int collnameLen = strlen(((char*)databuf) + bufPos);
        opqueryMeta.collection_name = ((char*)databuf) + bufPos;
        bufPos += collnameLen + 1;
        opqueryMeta.skip= *((unsigned int *) (((char*)databuf) + bufPos));
        bufPos += 4;
        opqueryMeta.returned= *((unsigned int *) (((char*)databuf) + bufPos));
        bufPos += 4;

//        msgMeta1.msgLen - bufPos;
        unsigned int bsonDocSize = *((unsigned int *) (((char*)databuf) + bufPos));
        bufPos += 4;

        //Set the debugger breakpoint here to examine the data within the 2 structs
        puts("-");
    }

    puts(".");
}