open_excl man page
open_excl — open a file for exclusive writing
int open_excl(const char *filename);
open_excl opens the file filename for writing and returns the file handle. The file may not exist before the call to open_excl. The file will be created with mode 0600.
If there was an error creating the file, open_excl returns -1 and sets errno accordingly.
Since open_excl relies on the O_EXCL flag to open, it does not work reliably over NFS (the NFS protocol is broken) and must be emulated using a lock file (create a file with a unique file name and link(2) it to the lock file. Then stat the lock file and see if the link count is 2).