Fopen fails, even when given full path

When could fopen("/full/path/to/file.ext", "rb"). The file is on system partition, yet sometimes fopen still returns 0. Why would it ever fail? Some async read-preventing locks by itself or external app, like antivirus scanner? How to find what app locked the file? These crashes are really annoying and make me paranoid.

>>7
Ok. It was indeed done by library.