flopen man page
flopen — reliably open and lock a file
flopen(const char *path, int flags);
flopen(const char *path, int flags, mode_t mode);
The flopen() function opens or creates a file and acquires an exclusive lock on it. It is essentially equivalent with calling open() with the same parameters followed by flock() with an operation argument of LOCK_EX, except that flopen() will attempt to detect and handle races that may occur between opening / creating the file and locking it. Thus, it is well suited for opening lock files, PID files, spool files, mailboxes and other kinds of files which are used for synchronization between processes.
If flags includes O_NONBLOCK and the file is already locked, flopen() will fail and set errno to EWOULDBLOCK.
As with open(), the additional mode argument is required if flags includes O_CREAT.
If successful, flopen() returns a valid file descriptor. Otherwise, it returns -1, and sets errno as described in flock(2) and open(2).
errno(2), flock(2), open(2)
The flopen function and this manual page were written by⟨des@FreeBSD.org⟩.