Protobuf float value transmission over TCP/IP and data corruption

Question

I have two applications that communicate over TCP/IP written in C++ that use protobuf library to exchange data defined into a proto message.

floatExample.proto

syntax = "proto3";

message FloatMessage {
  optional float value = 1;
}

serverFloatExample.cpp

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <cstdint>
#include <iomanip>
#include "floatExample.pb.h"

void error(const char *msg)
{
    perror(msg);
    exit(1);
}

int main(int argc, char *argv[])
{
     int sockfd, newsockfd, portno;
     socklen_t clilen;
     char buffer[1024];
     struct sockaddr_in serv_addr, cli_addr;
     int n;

     if (argc < 2) {
         fprintf(stderr,"ERROR, no port provided\n");
         exit(1);
     }

     // create a socket
     // socket(int domain, int type, int protocol)
     sockfd =  socket(AF_INET, SOCK_STREAM, 0);
     if (sockfd < 0)
        error("ERROR opening socket");

     // clear address structure
     bzero((char *) &serv_addr, sizeof(serv_addr));

     portno = atoi(argv[1]);

     serv_addr.sin_family = AF_INET;
     serv_addr.sin_addr.s_addr = INADDR_ANY;
     serv_addr.sin_port = htons(portno);

     if (bind(sockfd, (struct sockaddr *) &serv_addr, sizeof(serv_addr)) < 0) 
     {
        error("ERROR on binding");
        return -1;
    }

     listen(sockfd,5);

     // The accept() call actually accepts an incoming connection
     clilen = sizeof(cli_addr);


     newsockfd = accept(sockfd,
                 (struct sockaddr *) &cli_addr, &clilen);
     if (newsockfd < 0) {
          error("ERROR on accept");
          return -1;
     }

    printf("server: got connection from %s port %d\n",
    inet_ntoa(cli_addr.sin_addr), ntohs(cli_addr.sin_port));

    while(true) 
    {
        sleep(2);
        bzero(buffer,1024);

        n = read(newsockfd,buffer,1024);
        if (n < 0) error("ERROR reading from socket");
        {
            printf("Here is the message size: <%d>\n", n);
        }   

        FloatMessage receivedMsg = FloatMessage();
        receivedMsg.ParseFromString(buffer);

        printf("VALUE: <%.2f>\n", receivedMsg.value());
        receivedMsg.PrintDebugString();
    }

    close(newsockfd);
    close(sockfd);

    return 0;
}

clientFloatExample.cpp

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netdb.h>
#include "floatExample.pb.h" 

void error(const char *msg)
{
    perror(msg);
    exit(0);
}

int main(int argc, char *argv[])
{
    int sockfd, portno, n;
    struct sockaddr_in serv_addr;
    struct hostent *server;

    char buffer[256];
    if (argc < 3) {
       fprintf(stderr,"usage %s hostname port\n", argv[0]);
       exit(0);
    }
    portno = atoi(argv[2]);
    sockfd = socket(AF_INET, SOCK_STREAM, 0);
    if (sockfd < 0) 
        error("ERROR opening socket");
    server = gethostbyname(argv[1]);
    if (server == NULL) {
        fprintf(stderr,"ERROR, no such host\n");
        exit(0);
    }
    bzero((char *) &serv_addr, sizeof(serv_addr));
    serv_addr.sin_family = AF_INET;
    bcopy((char *)server->h_addr, 
         (char *)&serv_addr.sin_addr.s_addr,
         server->h_length);
    serv_addr.sin_port = htons(portno);
    if (connect(sockfd, (struct sockaddr *) &serv_addr, sizeof(serv_addr)) < 0) 
        error("ERROR connecting");

    FloatMessage float_message;
    float_message.set_value(1.0);  
    float_message.PrintDebugString();

    std::string serialized_message = float_message.SerializeAsString();

    n = write(sockfd, serialized_message.c_str(), serialized_message.size());
    if (n < 0) 
    {
        error("ERROR writing to socket");
    } 

    close(sockfd);

    return 0;
}

The bug is the following: sometimes some of the float values received by server application are not the same as the values transmitted by client application.

For example (using protobuf serialization):

• If Client application serialize floatValue = 1.0 --> Server Application deserialize 0 (ERROR!)
• If Client application serialize floatValue = 1.01 --> Server Application deserialize 1.01 (OK)
• If Client application serialize floatValue = 2.5 --> Server Application deserialize 0 (ERROR!)
• If Client application serialize floatValue = 2.51 --> Server Application deserialize 2.51 (OK)

Can anyone please suggest me where is the problem or help with this?

Answer 1

This line contains the problem:

receivedMsg.ParseFromString(buffer);

ParseFromString is documented as taking a std::string&, but you give it a char*. This invokes the std::string::string(char*) constructor that only reads up to the first NUL byte.

The correct solution is to just use the ParseFromArray method that takes a pointer and a size:

receivedMsg.ParseFromArray(buf, n);