lock-file.md

description	title	ms.date	api_name	api_location	api_type	topic_type	f1_keywords	helpviewer_keywords	ms.assetid
Learn more about: _lock_file	_lock_file	4/2/2020	_lock_file _o__lock_file	msvcrt.dll msvcr80.dll msvcr90.dll msvcr100.dll msvcr100_clr0400.dll msvcr110.dll msvcr110_clr0400.dll msvcr120.dll msvcr120_clr0400.dll ucrtbase.dll api-ms-win-crt-filesystem-l1-1-0.dll	DLLExport	apiref	_lock_file lock_file	file locking [C++] _lock_file function lock_file function	75c7e0e6-efff-4747-b6ed-9bcf2b0894c3

_lock_file

Locks a FILE object to ensure consistency for threads accessing the FILE object concurrently.

Syntax

void _lock_file( FILE* file );

Parameters

file
File handle.

Remarks

The _lock_file function locks the FILE object specified by file. The underlying file isn't locked by _lock_file. Use _unlock_file to release the lock on the file. Calls to _lock_file and _unlock_file must be matched in a thread.

By default, this function's global state is scoped to the application. To change this behavior, see Global state in the CRT.

Requirements

Routine	Required header
_lock_file	<stdio.h>

For more compatibility information, see Compatibility.

Example

// crt_lock_file.c
// This example creates multiple threads that write to standard output
// concurrently, first with _file_lock, then without.

#include <stdio.h>
#include <process.h>// _beginthread
#include <windows.h>// HANDLE

void Task_locked( void* str )
{
    for( int i=0; i<1000; ++i )
    {
        _lock_file( stdout );
        for( char* cp = (char*)str; *cp; ++cp )
        {
            _fputc_nolock( *cp, stdout );
        }
        _unlock_file( stdout );
    }
}

void Task_unlocked( void* str )
{
    for( int i=0; i<1000; ++i )
    {
        for( char* cp = (char*)str; *cp; ++cp )
        {
            fputc( *cp, stdout );
        }
    }
}

int main()
{
    HANDLE h[3];
    h[0] = (HANDLE)_beginthread( &Task_locked, 0, "First\n" );
    h[1] = (HANDLE)_beginthread( &Task_locked, 0, "Second\n" );
    h[2] = (HANDLE)_beginthread( &Task_locked, 0, "Third\n" );

    WaitForMultipleObjects( 3, h, true, INFINITE );

    h[0] = (HANDLE)_beginthread( &Task_unlocked, 0, "First\n" );
    h[1] = (HANDLE)_beginthread( &Task_unlocked, 0, "Second\n" );
    h[2] = (HANDLE)_beginthread( &Task_unlocked, 0, "Third\n" );

    WaitForMultipleObjects( 3, h, true, INFINITE );
}

...
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Second
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Second
Third
Second
Third
Second
...
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See also

File handling
_creat, _wcreat
_open, _wopen
_unlock_file