cJSON memory leak

Question

I use cJSON in my program to convert my values to JSON and write it to file. Here is the example of my code:

void writeStructToFile(IOPipe this, struct structtype somevalues) {
    cJSON *jout = cJSON_CreateObject();
    cJSON_AddItemToObject(jout, "V1", cJSON_CreateNumber(somevalues.v1));
    cJSON_AddItemToObject(jout, "V2", cJSON_CreateNumber(somevalues.v2));
    fprintf(this->outstream, "%s", cJSON_Print(jout));
    cJSON_Delete(jout);
}

Works great, but after some time I found that Linux(embedded) kills my program because of abnormal memory use or device(on Cortex A8) just hangs. After debug I found, that leak appears exactly in this function even though I delete the pointer at the end. Could anyone see that leak?

So, does the memory leak go away when this function is empty (or when you don't call it)? — meaning-matters, Oct 02 '14 at 10:21
yes, just commented this function, checked my program with "ps" in terminal and see stable memory usage. Uncommented and see this value rising, slowly, but after 10 minutes of working it hangs the system so I cannot connect to it again — PaulPonomarev, Oct 02 '14 at 10:28
Now that what I suspected least. But you're right. Leak is in fprintf function. But why and how to fix it? — PaulPonomarev, Oct 02 '14 at 11:06

meaning-matters · Accepted Answer · 2014-10-06T13:13:59.433

8

Initially I thought that it might be FILE I/O's internal buffers. But these are flushed automatically when they become too big.

The real leak is that cJSON_Print allocates memory: a char array. You must free this after you're done:

char* text = cJSON_Print(jout);
fprintf(this->outstream, "%s", text);
free(text);  // As suggested by PaulPonomarev.

cJSON_Delete(jout);

edited Oct 06 '14 at 13:13

answered Oct 02 '14 at 12:33

meaning-matters

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Ok, I've tried to add fflush after fprintf, and "\n" before(and after) writing `fprintf(this->outstream, "\n%s\n", cJSON_Print(jout)); fflush(this->outstream);`. Didn't help. Then I tried to write to file with descriptor instead of fprintf: `int df = fileno(this->outstream); write(df, cJSON_Print(jout), sizeof(cJSON_Print(jout)));` First of all - it didn't work, second - it created memory leak too – PaulPonomarev Oct 06 '14 at 06:57
Yes! Now memory usage is stable. Only thing is when I created char* I need to use standard function `free(text)` – PaulPonomarev Oct 06 '14 at 09:53
@PaulPonomarev: You also need to check that `cJSON_Print` doesn't return a `NULL` pointer. (all the cJSON functions check for `NULL` pointers already) – FSMaxB Feb 02 '18 at 16:59
2

Please consider changing to the `cJSON_free` API (see maintainers comment https://github.com/DaveGamble/cJSON/issues/5) – Chris Merck Jul 07 '18 at 18:17

score -2 · Answer 2 · answered Jan 22 '15 at 20:48

-2

For a char* allocated cJSON_Print, it is said to use cJSON_FreePrintBuffer.

answered Jan 22 '15 at 20:48

PLemoine

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2

2

Correct API is `cJSON_free` – Chris Merck Jul 07 '18 at 18:16

cJSON memory leak

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