---
name: "flopen"
section: "3bsd"
platform: "linux"
package: "libbsd"
version: "0.12.2-2"
release: "Arch Linux 2026-11-01"
platforms: ["linux"]
url: "https://new.carta.tech/flopen/3bsd"
---

# flopen(3bsd)

Reliably open and lock a file

## NAME

**flopen**, **flopenat** – Reliably open and lock a file

## LIBRARY

library “libbsd”

## SYNOPSIS

**#include \<sys/fcntl.h\>**

**#include \<libutil.h\>**

(See [libbsd(7)](/libbsd/7.md) for include usage.)

*int*\

**flopen**(*const char \*path*, *int flags*);

*int*\

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

*int*\

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

*int*\

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

## DESCRIPTION

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**().

## RETURN VALUES

If successful, **flopen**() returns a valid file descriptor. Otherwise, it returns -1, and sets *errno* as described in [flock(2)](/flock/2.md) and [open(2)](/open/2.md).

## SEE ALSO

[errno(2)](/errno/2.md), [flock(2)](/flock/2.md), [open(2)](/open/2.md)

## AUTHORS

The **flopen** function and this manual page were written by Dag-Erling Smørgrav ⟨*des@FreeBSD.org*⟩.