How do I create a JSON object in C? - c

Question

I currently have a string in C that I have coded like this:
#define MESSAGE_STRING "{\"deviceName\":string, \"Telemetry\":{\"voltage\":12,\"current\":0.12}}"
As I am sending it to a cloud service that processes it as a JSON message and receives this:
But I want to do this in an easier way as this could get a lot more complex when I am sending more complicated JSON messages. I was wondering if there was any easy way to create these JSON objects within C easier?
Cheers

Answer 1

Write your JSON in a text file, and replace every parameter with a %d, %f, %s placeholder.
Then read this file, and fill in the values with sprintf()
But pay attention to the order of your parameters.
json.txt:
{
"device_name": "%s",
"Telemetry": {
"voltage": %d,
"current": %f
}
}
main.c
#include <stdio.h>
int main(void)
{
char json_format[4096];
char message_string[4096];
FILE* fp;
// Read in json format
fp = fopen("json.txt", "r");
int len = fread(json_format, 1, 4096, fp);
json_format[len] = '\0';
fclose(fp);
// Fill in parameters
const char* device_name = "string";
int voltage = 12;
float current = 0.12;
sprintf(message_string, json_format, device_name, voltage, current);
printf("%s", message_string);
}
The code is only for understanding the concept.
It works, but don't use it, because it doesn't check for erorrs and the memory management is bad too.