send and receive JSON over sockets in server and client application in C

Question

I want to send the data in JSON over sockets in a server-client application written in C.

I am using json-c / libjson library for handling JSON data in C application.

By working on some of the tutorials I am able to create JSON object and able to parse it successfully.

Now I want to use the JSON data format for the communication of server-client.

Here is part of my server and client code

server.c

int main()
{
   int listenfd = 0, connfd = 0;    //related with the server
   struct sockaddr_in serv_addr;

   //json_object * jobj;
   uint8_t buf[158], i;

   memset(&buf, '0', sizeof(buf));
   listenfd = socket(AF_INET, SOCK_STREAM, 0);

   serv_addr.sin_family = AF_INET;
   serv_addr.sin_addr.s_addr = htonl(INADDR_ANY);
   serv_addr.sin_port = htons(8888); 

   bind(listenfd, (struct sockaddr*)&serv_addr, sizeof(serv_addr));
   printf("binding\n");

   listen(listenfd, 5);
   printf("listening\n");
   connfd = accept(listenfd, (struct sockaddr*)NULL, NULL);

   printf("Reading from client\n");

   while ( (read(connfd, buf, 157)) > 0 )
   {
      for ( i = 0; i < 157; i++ )
         //printf("%s\n", json_object_to_json_string(jobj));
         //json_parse(jobj);
         printf("%d\n", buf[i]);
   }

   return 0;
}

client.c

int main()
{ 
    char* str;
    int fd = 0;
    struct sockaddr_in demoserverAddr;

    fd = socket(AF_INET, SOCK_STREAM, 0);

    if (fd < 0)
    {
        printf("Error : Could not create socket\n");
        return 1;
    }
    else
    {
        demoserverAddr.sin_family = AF_INET;
        demoserverAddr.sin_port = htons(8888);
        demoserverAddr.sin_addr.s_addr = inet_addr("127.0.0.1");
        memset(demoserverAddr.sin_zero, '\0', sizeof(demoserverAddr.sin_zero));
    }

    if (connect(fd, (const struct sockaddr *)&demoserverAddr, sizeof(demoserverAddr)) < 0)
    {
         printf("ERROR connecting to server\n");
         return 1;
    }

    /*Creating a json object*/
    json_object *jobj = json_object_new_object();

    /*Creating a json string*/
    json_object *jstring = json_object_new_string("Joys of Programming");

    /*Creating a json integer*/
    json_object *jint = json_object_new_int(10);

    /*Creating a json boolean*/
    json_object *jboolean = json_object_new_boolean(1);

    /*Creating a json double*/
    json_object *jdouble = json_object_new_double(2.14);

    /*Creating a json array*/
    json_object *jarray = json_object_new_array();

    /*Creating json strings*/
    json_object *jstring1 = json_object_new_string("c");
    json_object *jstring2 = json_object_new_string("c++");
    json_object *jstring3 = json_object_new_string("php");

    /*Adding the above created json strings to the array*/
    json_object_array_add(jarray,jstring1);
    json_object_array_add(jarray,jstring2);
    json_object_array_add(jarray,jstring3);

    /*Form the json object*/
    /*Each of these is like a key value pair*/
    json_object_object_add(jobj,"Site Name", jstring);
    json_object_object_add(jobj,"Technical blog", jboolean);
    json_object_object_add(jobj,"Average posts per day", jdouble);
    json_object_object_add(jobj,"Number of posts", jint);
    json_object_object_add(jobj,"Categories", jarray);

    printf("Size of JSON object- %lu\n", sizeof(jobj));
    printf("Size of JSON_TO_STRING- %lu,\n %s\n", sizeof(json_object_to_json_string(jobj)), json_object_to_json_string(jobj));

    //printf("Size of string- %lu\n", sizeof(json_object_to_json_string(jobj)));
    write(fd, json_object_to_json_string(jobj), 157);

    printf("Written data\n");
    return 0;
}

I want to send the json_object jobj from client to server. How to do this?

Some things that I tried:

1. When I use write(fd, jobj, sizeof(jobj)), the client sends only 8 bytes and other characters are null when I receive data on server.

2. I am only able to send complete json_object jobj when I use write(fd, json_object_to_json_string(jobj), 157) where 157 is the total number of characters in that jobj object.

3. On server side if I use read(connfd, jobj, sizeof(jobj)) then I only able to receive 8 bytes (on client side I am using write(fd, jobj, sizeof(jobj))).

4. If I use the above server.c, I am able to receive complete JSON object (if I already know the number of characters in that object). But it is in raw format (receiving byte by byte).

How to send the complete json_object jobj from client side?

And how to receive the complete json_object jobj on server side?

Answer 1

1) jobj is a pointer, so when you use

write(fd, jobj, sizeof(jobj))

you're writing a pointer to that object, not that object.

3) Same as before.

Maybe you should try sending it with something like

if (write(fd, jobj, sizeof(*jobj)) == -1)
    /* error handler /*

On the receive side, you should do a for loop, like

for (;;)
{
    r = read(fd, jobj, SIZE);

    if (r == -1)
        /*error handler*/
    if (r == 0)
        break;
}

if you know the maximum SIZE of the json, or combine malloc() and realloc() otherwise

EDIT:

I did this, and now it works fine.

client.c

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#include <string.h>
#include <json/json.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <arpa/inet.h>

int main()
{
    /* all previous code until
    printf("Size of string- %lu\n", sizeof(json_object_to_json_string(jobj)))*/

    char temp_buff[MAX_SIZE];

    if (strcpy(temp_buff, json_object_to_json_string(jobj)) == NULL)
    {
        perror("strcpy");
        return EXIT_FAILURE;
    }

    if (write(fd, temp_buff, strlen(temp_buff)) == -1)
    {
        perror("write");
        return EXIT_FAILURE;
    }

    printf("Written data\n");
    return EXIT_SUCCESS;
}

server.c

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#include <string.h>
#include <json/json.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <arpa/inet.h>

int main()
{
    /* all previous code until
    printf("Reading from client\n"); */

    ssize_t r;

    char buff[MAX_SIZE];

    for (;;)
    {
        r = read(connfd, buff, MAX_SIZE);

        if (r == -1)
        {
            perror("read");
            return EXIT_FAILURE;
        }
        if (r == 0)
            break;

        printf("READ: %s\n", buff);
    }

    return EXIT_SUCCESS;
}

MAX_SIZE is a macro that specifies the maximum buffer length, set it as you wish. Please next time paste ALL your code (including the #include ...) and indent it properly.