---
name: "funlockfile"
section: "3"
platform: "linux"
package: "man-pages"
version: "6.19-1"
release: "Arch Linux 2026-11-01"
platforms: ["linux", "freebsd"]
url: "https://new.carta.tech/flockfile"
---

# funlockfile(3)

lock FILE for stdio

Part of [flockfile(3)](/flockfile/3.md).

## NAME

flockfile, ftrylockfile, funlockfile - lock FILE for stdio

## LIBRARY

Standard C library (*libc*, *-lc*)

## SYNOPSIS

```
#include <stdio.h>
```

**void flockfile(FILE \****filehandle***);** **int ftrylockfile(FILE \****filehandle***);** **void funlockfile(FILE \****filehandle***);**

Feature Test Macro Requirements for glibc (see [**feature\_test\_macros**(7)](/feature_test_macros/7.md)):

All functions shown above:

```
    /* Since glibc 2.24: */ _POSIX_C_SOURCE >= 199309L
        || /* glibc <= 2.23: */ _POSIX_C_SOURCE
        || /* glibc <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE
```

## DESCRIPTION

The stdio functions are thread-safe. This is achieved by assigning to each *FILE* object a lockcount and (if the lockcount is nonzero) an owning thread. For each library call, these functions wait until the *FILE* object is no longer locked by a different thread, then lock it, do the requested I/O, and unlock the object again.

(Note: this locking has nothing to do with the file locking done by functions like [**flock**(2)](/flock/2.md) and [**lockf**(3)](/lockf/3.md).)

All this is invisible to the C-programmer, but there may be two reasons to wish for more detailed control. On the one hand, maybe a series of I/O actions by one thread belongs together, and should not be interrupted by the I/O of some other thread. On the other hand, maybe the locking overhead should be avoided for greater efficiency.

To this end, a thread can explicitly lock the *FILE* object, then do its series of I/O actions, then unlock. This prevents other threads from coming in between. If the reason for doing this was to achieve greater efficiency, one does the I/O with the nonlocking versions of the stdio functions: with [**getc\_unlocked**(3)](/getc_unlocked/3.md) and [**putc\_unlocked**(3)](/putc_unlocked/3.md) instead of [**getc**(3)](/getc/3.md) and [**putc**(3)](/putc/3.md).

The **flockfile**() function waits for *\*filehandle* to be no longer locked by a different thread, then makes the current thread owner of *\*filehandle*, and increments the lockcount.

The **funlockfile**() function decrements the lock count.

The **ftrylockfile**() function is a nonblocking version of **flockfile**(). It does nothing in case some other thread owns *\*filehandle*, and it obtains ownership and increments the lockcount otherwise.

## RETURN VALUE

The **ftrylockfile**() function returns zero for success (the lock was obtained), and nonzero for failure.

## ERRORS

None.

## ATTRIBUTES

For an explanation of the terms used in this section, see [**attributes**(7)](/attributes/7.md).

| | | |
| --- | --- | --- |
| **Interface** | **Attribute** | **Value** |
| **flockfile**(), **ftrylockfile**(), **funlockfile**() | Thread safety | MT-Safe |

## STANDARDS

POSIX.1-2008.

## HISTORY

POSIX.1-2001.

These functions are available when **\_POSIX\_THREAD\_SAFE\_FUNCTIONS** is defined.

## SEE ALSO

[**unlocked\_stdio**(3)](/unlocked_stdio/3.md)