rapidJson: crashed in release mode - crash

Question

I used rapidJson to read json data. I can build my application in both Debug and Release mode, but the application crashes in Release mode.
using namespace rapidjson;
...
char *buffer;
long fileSize;
size_t fileReadingResult;
//obtain file size
fseek(pFile, 0, SEEK_END);
fileSize = ftell(pFile);
if (fileSize <= 0) return false;
rewind(pFile);
//allocate memory to contain the whole file
buffer = (char *)malloc(sizeof(char)*fileSize);
if (buffer == NULL) return false;
//copy the file into the buffer
fileReadingResult = fread(buffer, 1, fileSize, pFile);
if (fileReadingResult != fileSize) return false;
buffer[fileSize] = 0;
Document document;
document.Parse(buffer);
When I run it in Release mode, I encounter an Unhanded exception; A heap has been corrupted.
The application breaks at "res = _heap_alloc(size) in malloc.c file
void * __cdecl _malloc_base (size_t size)
{
void *res = NULL;
// validate size
if (size <= _HEAP_MAXREQ) {
for (;;) {
// allocate memory block
res = _heap_alloc(size);
// if successful allocation, return pointer to memory
// if new handling turned off altogether, return NULL
if (res != NULL)
{
break;
}
if (_newmode == 0)
{
errno = ENOMEM;
break;
}
// call installed new handler
if (!_callnewh(size))
break;
// new handler was successful -- try to allocate again
}
It runs fine in Debug mode.

Answer 1

Maybe it could be a memory leak issue with your Malloc since it runs fine one time in Debug, but when you keep the application up longer it crashes.
Do you free your buffer after using it?

Answer 2

The reason is simple. You allocate a buffer of fileSize bytes but after reading the file, you write at the fileSize+1-th position with buffer[fileSize] = 0;
Fix: change allocation with one larger.
buffer = (char *)malloc(fileSize + 1);
Debug builds pad memory allocations with additional bytes so it does not crash.