Parsing Json message with sscanf()

Question

My application receives a JSon message via UDP from coordinates of an object as follows:

{
    "FRAME1": {
        "X": "-0.885498",
        "Y": "-0.205078",
        "Z": "0.628174"
    },
    "FRAME2": {
        "X": "1.70898",
        "Y": "-5.67627",
        "Z": "0.305176"
    },
    "ID": "DEVICE9",
    "TS": "7.9900"
}

I need to read the coordinates from FRAME1 and FRAME2 (X,Y,Z values of each frame). The Json message is stored in char[] msg. I use these the following lines of code two get the coordinates of FRAME1:

char Xc[32], Yc[32], Zc[32];
sscanf (msg,
        "{\"X\":\"%s\",\"Y\":\"%s\",\"Z\":\"%s\"}",
        Xc, Yc, Zc);

I display the stored value with printf as:

printf("X coordinate is: %s\n" , Xc);

The output, however is strange:

X coordinate is: |-

What is wrong in the format provided for sscanf()?

Do you check the return value from sscanf? Maybe you should! — Ian Abbott
– Ian Abbott, Commented Feb 19, 2016 at 10:59
Shouldn't the sscanf format string contain some whitespace between the colons and the double quotes? — Ian Abbott
– Ian Abbott, Commented Feb 19, 2016 at 11:02

user3121023 · Accepted Answer · 2016-02-19 12:22:55Z

3

sscanf with the %n specifier and strcmp could be used to parse the string. %n captures the number of characters used in the scan.

#include <stdio.h>
#include <string.h>
#include <stdlib.h>

int main()
{
    int index = 0;
    int offset = 0;
    char sub[300] = "";
    char Xc[32] = "";
    char Yc[32] = "";
    char Zc[32] = "";
    char msg[] = {
    "{\n\
    \"FRAME1\": {\n\
        \"X\": \"-0.885498\",\n\
        \"Y\": \"-0.205078\",\n\
        \"Z\": \"0.628174\"\n\
    },\n\
    \"FRAME2\": {\n\
        \"X\": \"1.70898\",\n\
        \"Y\": \"-5.67627\",\n\
        \"Z\": \"0.305176\"\n\
        },\n\
        \"ID\": \"DEVICE9\",\n\
        \"TS\": \"7.9900\"\n\
    }\n"
    };

    offset = 0;
    while ( ( sscanf ( msg + offset, "%299s%n", sub, &index)) == 1) {
        offset += index;
        if ( strcmp (sub, "\"X\":" ) == 0) {
            if ( ( sscanf ( msg + offset, "%31s%n", Xc, &index)) == 1) {
                offset += index;
                printf ( "Xc is %s\n", Xc);
            }
        }
        if ( strcmp (sub, "\"Y\":" ) == 0) {
            if ( ( sscanf ( msg + offset, "%31s%n", Yc, &index)) == 1) {
                offset += index;
                printf ( "Yc is %s\n", Yc);
            }
        }
        if ( strcmp (sub, "\"Z\":" ) == 0) {
            if ( ( sscanf ( msg + offset, "%31s%n", Zc, &index)) == 1) {
                offset += index;
                printf ( "Zc is %s\n", Zc);
            }
        }
    }

    return 0;
}

The above sscanf's for the coordinates will include the quotes. This could be used to remove the quotes

if ( ( sscanf ( msg + offset, " \"%31[^\"]%*s%n", Xc, &index)) == 1) {

edited Feb 19, 2016 at 12:22

answered Feb 19, 2016 at 12:06

user3121023

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1 Comment

QuestionMark Over a year ago

Thank you for the snippet that compiles! In sscanf ( msg + offset, "%31s%n", Xc, &index) what is the general case if Xc, Yc and Zc have different unknown lengths?

RastaJedi · Accepted Answer · 2016-02-19 11:08:41Z

1

Maybe you can try to use strtok() instead and use the colons as delims or something? Not sure if it makes your life any easier, but I know that strtok() is generally recommended over the *scanf() family of functions.

answered Feb 19, 2016 at 11:08

RastaJedi

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