How to test for a Boolean being TRUE/FALSE in cJSON

Question

I'm parsing a json file with cJSON. Everything is going well. But one of the data elements is a Boolean, and the problem is I don't understand how to use cJSON's functions for reading Boolean types. The data file contains lines like:

"isDaytime": true,

I attempted to use a code block like:

if (cJSON_IsTrue("isDaytime")==0) {
    printf("isDaytime = TRUE\n");
} else {
    printf("isDaytime = FALSE\n");
}

When I try to compile this with gcc, I get the following message:

undefined reference to `cJSON_IsTrue'

I think perhaps cJSON represents it as an int? I don't know. At any rate, does anyone know how I should be calling cJSON_IsTrue() ? Or perhaps I shouldn't be calling cJSON_IsTrue() at all. If not, how should I be checking this type of data element? If it was an int, I'd be done. If it was a char *, I'd be done. I'm just not sure how I'm supposed to test whether it's true or false.

EDIT: According to the documentation for the cJSON library, here:https://github.com/DaveGamble/cJSON

Because the entire library is only one C file and one header file, you can just copy cJSON.h and cJSON.c to your projects source and start using it.

So this was how I was using cJSON. I had just included the two files in my project, and everything was working like a charm. Until I tried using the cJSON_IsTrue() function. I wasn't able to see how to correctly use the function from the documentation, or from the source files. I figured I'd just try to use it, "wrongly", and then correct myself via whatever error messages came up.

I didn't expect to receive the "undefined reference" error message. I looked through the source, and indeed the function was there...

David Collins suggested I needed to link to the cjson library when compiling. I was including the source, so I didn't think that was truly the answer. But okay. So I followed the directions and actually installed the library on my system. And I got the same undefined reference error message.

It turned out that the location where the library installed itself: /usr/local/lib, wasn't being referenced. I copied the installed files to the correct directory for my system and the cJSON_IsTrue() function is now available when I compile with the -lcjson flag.

So how to use it? This is how I used it:

`if ( cJSON_IsTrue(cJSON_GetObjectItem(subitem, "isDayTime")) == 1) {
    /* do something if it's true */
 } else {
    /* do something if it's false */
 }`

This seems to have done the trick. Thanks again @David Collins!

Answer 1

cJSON_IsTrue() doesn't expect a string literal; it expects a cJSON object.

Suppose json_string is a char * representation of your full JSON object. Then you could try the following.

First of all parse the top-level JSON object.

cJSON *json = CJSON_Parse(json_string);
if (json == NULL) {
    // Handle error and abort if appropriate
}

Then extract your child object.

cJSON *daytime_json = cJSON_GetObjectItemCaseSensitive(json, "isDayTime");
if (cJSON_IsTrue(daytime_json) == 0) {
    // Whatever you need to do next ...
}
// Etc.

Finally, don't forget to de-allocate memory / clean up afterwards.

cJSON_Delete(json);

Compiling / Linking

You mentioned that

When I try to compile this with gcc, I get the following message:
undefined reference to cJSON_IsTrue

You need to link the cjson library when compiling. E.g., if using the gcc compiler, you would use something like

gcc -o json-test json-test.c -lcjson