In Windows programming, opening a file means asking the operating system for access to a file and, if successful, receiving a handle the application can use for later file operations. The call also specifies what access the application wants, whether other applications may access the file at the same time, and whether the call should open, create, or truncate it. The title’s phrase “Win File Open” is ambiguous; this article uses the supported interpretation of opening files through the Windows Win32 API.
What happens when a Windows application opens a file?
Microsoft describes the Win32 CreateFile function as able to create a new file or open an existing one. The application passes a path and options describing its request. If the call succeeds, Windows returns a handle for subsequent I/O; if it fails, it returns INVALID_HANDLE_VALUE, and the application can call GetLastError to find more information. See Microsoft’s overview of creating and opening files.
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A successful open does not itself read or write the file. It establishes access under the requested conditions. The handle remains usable until its references are closed.
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Desired access
The application requests the access it needs, such as reading, writing, or both. Some operations may request neither read nor write access—for example, when querying certain metadata. “Open” alone does not tell you what the application can do with the file.
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Sharing mode
The sharing mode controls which kinds of access other opens may request while this handle is open: read, write, or delete. A sharing mode of zero requests exclusive access. When another application tries to open the same file, Windows checks its request against the access and sharing modes already in effect; an incompatible combination can fail.
Creation disposition
The disposition determines what happens depending on whether the path already exists. This is the setting that distinguishes a plain open from creating or clearing a file.
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Which creation disposition should you choose?
| Disposition | If the file exists | If the file is missing |
|---|---|---|
OPEN_EXISTING |
Opens it without truncating. | Fails. |
OPEN_ALWAYS |
Opens it without truncating. | Creates it. |
TRUNCATE_EXISTING |
Opens it and sets its size to zero bytes; write access is required. | Fails. |
CREATE_NEW |
Fails if it already exists. | Creates it. |
CREATE_ALWAYS |
Truncates an existing writable file. | Creates it. |
These behaviors are documented in Microsoft’s CreateFileA API reference. Truncation discards the file’s existing contents, so do not treat TRUNCATE_EXISTING or CREATE_ALWAYS as harmless alternatives to opening a file as-is.
Is “open a file” always the same as calling CreateFile?
No. CreateFile is the Win32 API covered here, not a universal explanation of every Windows file-opening operation. Programming languages and higher-level libraries can provide their own file-opening functions and abstractions. The details in this article apply when the application is using the documented Win32 API; other interfaces may express the operation differently.
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