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FLOPEN(3) FreeBSD Library Functions Manual FLOPEN(3)

flopen, flopenat
Reliably open and lock a file

System Utilities Library (libutil, -lutil)

#include <sys/fcntl.h>
#include <libutil.h>

flopen(const char *path, int flags);

flopen(const char *path, int flags, mode_t mode);

flopenat(int fd, const char *path, int flags);

flopenat(int fd, 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.

The flopenat() function is equivalent to the flopen() function except in the case where the path specifies a relative path. In this case the file to be opened is determined relative to the directory associated with the file descriptor fd instead of the current working directory. If flopenat() is passed the special value AT_FDCWD in the fd parameter, the current working directory is used and the behavior is identical to a call to flopen().

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 Dag-Erling Smørgrav <>.
July 28, 2017 FreeBSD 13.1-RELEASE

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