C++ Sending vector<uchar> with socket loosing data - c++

Question

Im trying to send an Image with sockets from the server to the client, but for some reason im losing a lot of data.
this is my server:
#include <opencv2/imgcodecs.hpp>
#include <iostream>
#include <unistd.h>
#include <stdio.h>
#include <sys/socket.h>
#include <stdlib.h>
#include <netinet/in.h>
#include <string.h>
#include <string>
#define PORT 8080
using namespace cv;
using namespace std;
int main(int argc, char const *argv[])
{
int server_fd, new_socket, valread;
struct sockaddr_in address;
int opt = 1;
int addrlen = sizeof(address);
char buffer[1024] = {0};
// Creating socket file descriptor
if ((server_fd = socket(AF_INET, SOCK_STREAM, 0)) == 0)
{
perror("socket failed");
exit(EXIT_FAILURE);
}
// Forcefully attaching socket to the port 8080
if (setsockopt(server_fd, SOL_SOCKET, SO_REUSEADDR | SO_REUSEPORT,
&opt, sizeof(opt)))
{
perror("setsockopt");
exit(EXIT_FAILURE);
}
address.sin_family = AF_INET;
address.sin_addr.s_addr = INADDR_ANY;
address.sin_port = htons( PORT );
// Forcefully attaching socket to the port 8080
if (bind(server_fd, (struct sockaddr *)&address,
sizeof(address))<0)
{
perror("bind failed");
exit(EXIT_FAILURE);
}
if (listen(server_fd, 3) < 0)
{
perror("listen");
exit(EXIT_FAILURE);
}
if ((new_socket = accept(server_fd, (struct sockaddr *)&address,
(socklen_t*)&addrlen))<0)
{
perror("accept");
exit(EXIT_FAILURE);
}
Mat image = cv::imread("Rio.jpg",IMREAD_COLOR); //BGR
std::vector< uchar > buf;
cv::imencode(".jpg",image,buf);
cerr << send(new_socket , buf.data() , buf.size(),0);
cerr << buf.data();
return 0;
}
The output of this file is:
562763����
562763 should be the size of data that is send to the client and ���� should be the data.
This is my Client:
#include <opencv2/imgcodecs.hpp>
#include <stdio.h>
#include <sys/socket.h>
#include <arpa/inet.h>
#include <unistd.h>
#include <string.h>
#include <string>
#include <stdlib.h>
#include <netinet/in.h>
#include <iostream>
#define PORT 8080
using namespace std;
using namespace cv;
int main(int argc, char const *argv[])
{
int sock = 0, valread;
struct sockaddr_in serv_addr;
char buffer[1024] = {0};
if ((sock = socket(AF_INET, SOCK_STREAM, 0)) < 0)
{
printf("\n Socket creation error \n");
return -1;
}
serv_addr.sin_family = AF_INET;
serv_addr.sin_port = htons(PORT);
// Convert IPv4 and IPv6 addresses from text to binary form
if(inet_pton(AF_INET, "127.0.0.1", &serv_addr.sin_addr)<=0)
{
printf("\nInvalid address/ Address not supported \n");
return -1;
}
if (connect(sock, (struct sockaddr *)&serv_addr, sizeof(serv_addr)) < 0)
{
printf("\nConnection Failed \n");
return -1;
}
int l = 0;
std::string data = "";
do{
data += buffer;
l += strlen(buffer);
valread = read( sock , buffer, 1024);
}while(valread != 0);
cerr << l;
char* c = const_cast<char*>(data.c_str());
std::vector<uchar> vec(c,c+l);
Mat image2 = cv::imdecode(vec, 1);
// cv::imwrite("test22.jpg",image2);
return 0;
}
The output i get is:
87567Corrupt JPEG data: 175 extraneous bytes before marker 0xec
87567 should be the size of the data received and because there is data missing the jpeg cant be created
When im sending a message like "This is a test" the full text is received by the client.

Answer 1

You have two major flaws, one which could lead to an infinite loop, and one which leads to the problem you experience:
The infinite loop problem will happen if read fails in the client and it returns -1. -1 != 0, and then read will continue to read -1 forever.
The second and main problem is that you treat the data you send between the programs a strings which it is not. A "string" is a null-terminated (i.e. zero-terminated) sequence of characters, your image is not that. In fact, it might even contain embedded zeros inside the data which will give you invalid data in the middle as well.
To solve both problem I suggest you change the reading loop (and the variables used) to something like this:
uchar buffer[1024];
ssize_t read_result;
std::vector<uchar> data;
// While there's no error (read returns -1) or the connection isn't
// closed (read returns 0), continue to append the received data
// into the vector
while ((read_result = read(sock, buffer, sizeof buffer)) > 0)
{
// No errors, no closed connection
// Append the new data (and only the new data) at the end of the vector
data.insert(end(data), buffer, buffer + read_result);
}
After this loop, and if read_result == 0, then data should contain only the data that was sent. And all of it.

Answer 2

In your client you are using buffer before you have read anything to it. You also are assuming that it is null terminated.
Something like this seems better
std::string data = "";
for (;;)
{
valread = read( sock , buffer, 1024);
if (valread <= 0)
break;
data.append(buffer,valread);
}