Interpret the JSON response of a Webservice that was called by Arduino

Question

How do I interpret information from a Webservice who was called by the Arduino?

Webservice returns a JSON and I need to interpret it. An example of the answer can be seen below:

[{"valor":"10"}] 

The code below, obtained from that website, shows the call and the interpretation, but is read one byte at a time and, therefore, I have doubts of how could do this for the return in JSON.

The part that reads one byte at a time can be seen below:

if (client.available()) {
  char c = client.read();
  Serial.print(c);
}

Complete code:

#include <SPI.h>
#include <Ethernet.h>

// Enter a MAC address for your controller below.
// Newer Ethernet shields have a MAC address printed on a sticker on the shield
byte mac[] = { 0xDE, 0xAD, 0xBE, 0xEF, 0xFE, 0xED };
// if you don't want to use DNS (and reduce your sketch size)
// use the numeric IP instead of the name for the server:
//IPAddress server(74,125,232,128);  // numeric IP for Google (no DNS)
char server[] = "www.google.com";    // name address for Google (using DNS)

// Set the static IP address to use if the DHCP fails to assign
IPAddress ip(192, 168, 0, 177);

// Initialize the Ethernet client library
// with the IP address and port of the server
// that you want to connect to (port 80 is default for HTTP):
EthernetClient client;

void setup() {
  // Open serial communications and wait for port to open:
  Serial.begin(9600);
  while (!Serial) {
    ; // wait for serial port to connect. Needed for native USB port only
  }

  // start the Ethernet connection:
  if (Ethernet.begin(mac) == 0) {
    Serial.println("Failed to configure Ethernet using DHCP");
    // try to congifure using IP address instead of DHCP:
    Ethernet.begin(mac, ip);
  }
  // give the Ethernet shield a second to initialize:
  delay(1000);
  Serial.println("connecting...");

  // if you get a connection, report back via serial:
  if (client.connect(server, 80)) {
    Serial.println("connected");
    // Make a HTTP request:
    client.println("GET /search?q=arduino HTTP/1.1");
    client.println("Host: www.google.com");
    client.println("Connection: close");
    client.println();
  } else {
    // if you didn't get a connection to the server:
    Serial.println("connection failed");
  }
}

void loop() {
  // if there are incoming bytes available
  // from the server, read them and print them:
  if (client.available()) {
    char c = client.read();
    Serial.print(c);
  }

  // if the server's disconnected, stop the client:
  if (!client.connected()) {
    Serial.println();
    Serial.println("disconnecting.");
    client.stop();

    // do nothing forevermore:
    while (true);
  }
}

Answer 1

Different from what happens in high-level languages like Python and PHP, in C (mainly in Arduino) when making a request, will be returned character by character of the web service and there are no functions ready to assist.

In case, if you print this on your serial, it will look like it all came at once, but make no mistake. To receive data from a web service you need to read the received data, byte by byte, save in a buffer. First you should filter the headers, since they are usually not useful, when receiving two /r/n you will know that the HTTP header is over, so buffer the body of your response.

When the client finishes the request, perform a parser in the buffer looking for your information. It’s really all in hand, it doesn’t have functions ready for this kind of situation and all the libraries I’ve used for parse JSON in C are heavy and spend almost all memory of the Arduino.

Example of how I take the response value and save it to a buffer

[... monte de código acima...]
while (http.connected()) {
            while (http.available()) {

                char read_char = http.read();
                Serial.print(read_char); /* Imprime resposta do servidor */

                if (read_char != '\n' && newLine != 1) {
                  resposta[i] = read_char;
                  i++;
                  resposta[i] = '\0';
                }

                if (read_char == '\n') {
                  newLine = 1;
                }
            }

            i = 0;
        }
[... monte de código abaixo....]

After that I have in the response variable what the server returned me, then I use the strtok to break the answer into pieces

In your example, you have the instruction char c = client.read(); That’s where you’ll get the answer from the server, the problem is that it only has one character, so you’ll have to go throwing it in an array to be able to read the whole answer. Then, at another point, when interpreting what came from the server, read this character array (string) and search for what you want