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man -S 2 fcntl

flock ??

The lockf() function shall lock sections of a file with advisory-mode locks. Calls to lockf() from other threads which attempt to lock the locked file section shall either return an error value or block until the section becomes unlocked.

File * const fp = new File("Filename");
Now the fp cant point to something else.
i guess this will do.

sorry i read wrongly.. i read it as.. how do u lock a pointer to a file

#include <unistd.h>

int lockf(int fildes, int function, off_t size);