fLock (Function)

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The lock was not set
Locking a file
Position in the file
Operating mode in Windows Vista (and later)

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fLock (Function)

In french: fBloque

Entirely or partially locks an external file. The file will be unlocked:

using the fUnlock function,
when the file is closed.

The application that locked the file can read this file and write into this file. The other applications cannot read this file or write into this file.

Example

See additional examples

// Open an external file
FileID is int
ResLock is boolean
FileID = fOpen("C:\MyDirectories\File.txt", foReadWrite)
IF FileID <> -1 THEN
...
// Lock the external file
ResLock = fLock(FileID)
...
END

// Open an external file
MyTextFile is DiskFile
ResLock is boolean
IF fOpen(MyTextFile, "C:\MyDirectories\File.txt", foReadWrite) = True THEN
...
// Lock the external file
ResLock = fLock(MyTextFile)
...
END

Syntax

<Result> = fLock(<File used> [, <First byte to lock> [, <Number of bytes to lock>]])

<Result>: Boolean

True if the file was locked,
False otherwise.

<File used>: Integer or DiskFile variable

Identifier of the file to use, defined by fOpen or fCreate.
Name of the DiskFile variable initialized with fOpen or fCreate.

<First byte to lock>: Optional integer

Position of the first byte to lock (0 corresponds to the first byte of the file).

<Number of bytes to lock>: Optional constant or integer

Number of bytes to lock in the file.
Constant:
fLockEmptyFile fLock has the same behavior whether the file is empty or not (see Remarks).

Remarks

The lock was not set

fLock returns False if the file was not locked. The lock can be set only if:

the external file is opened,
the file identifier is valid,
the file or the bytes to lock are not already locked by another computer or by another application.

Locking a file

The file is entirely locked if <First byte to lock> and <Number of bytes to lock> are not specified.
The file is locked from the first byte to lock (<First byte to lock>) until the end if <Number of bytes to lock> is not specified.
By default, fLock does not lock any bytes if the file is empty. In this case, several applications can successfully "lock" an empty file. However, the file cannot be overwritten as long as it is locked.

To get the same behavior no matter whether the file is empty or not, use the fLockEmptyFile constant.

The lock is effective between two different sites or contexts. Unlike Windows, a process cannot lock a file that was previouly locked.

Position in the file

When opening a file, the current position corresponds to:

the first file byte (by default),
the last file byte if the file is opened in "addition" mode (foAdd constant).

This position can be modified by fSeek.

The current position does not change when fLock is used.

Operating mode in Windows Vista (and later)

If this function does not work properly in Windows Vista (and later), check whether the file or directory used is not in one of the system directories (Windows directory or "Program Files" directory).

In Windows Vista (and later), with the UAC mechanism (User Account Control) enabled, you must have administrator privileges to handle and/or modify the files or directories in system directories (Windows directory or "Program Files" directory).

Programming tip: If you need to manipulate / modify files or directories, without needing administrator privileges, it is advisable:

avoid writing to the Windows directory or to the "Program Files" directory,
use the system directory of the application (returned by SysDir with the srAppDataCommun constant, for example).

Related Examples:

Unit examples (WINDEV): Handling text files

[ + ] Handling "text" files with WINDEV:
- Create a text file
- Write into a text file
- Read in a text file

Unit examples (WEBDEV): Handling text files

[ + ] This example explains how to handle "non HFSQL" files with WEBDEV and it allows you to:
- Create a text file
- Write into a text file
- Read in a text file

Business / UI classification: Business Logic

Component: wd300std.dll