Console Handles - Win32 Programmer's Reference

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Console Handles

A console process uses handles to access the input and screen buffers of its console. A process can use the GetStdHandle, CreateFile, or CreateConsoleScreenBuffer function to open one of these handles.

The GetStdHandle function provides a mechanism for retrieving the standard input (STDIN), standard output (STDOUT), and standard error (STDERR) handles associated with a process. During console creation, the system creates these handles. Initially, STDIN is a handle of the console's input buffer, and STDOUT and STDERR are handles of the console's active screen buffer. However, the SetStdHandle function can redirect the standard handles by changing the handle associated with STDIN, STDOUT, or STDERR. Because the parent's standard handles are inherited by any child process, subsequent calls to GetStdHandle return the redirected handle. A handle returned by GetStdHandle may, therefore, refer to something other than console I/O. For example, before creating a child process, a parent process can use SetStdHandle to set a pipe handle to be the STDIN handle that is inherited by the child process. When the child process calls GetStdHandle, it gets the pipe handle. This means that the parent process can control the standard handles of the child process. The handles returned by GetStdHandle have GENERIC_READ | GENERIC_WRITE access unless SetStdHandle has been used to set the standard handle to have lesser access.

The value of the handles returned by GetStdHandle will not be 0, 1, and 2, so the standard predefined stream constants in the STDIO.H header file (STDIN, STDOUT, and STDERR) cannot be used in functions that require a console handle.

The CreateFile function enables a process to get a handle of its console's input buffer and active screen buffer, even if STDIN and STDOUT have been redirected. To open a handle of a console's input buffer, specify the CONIN$ value in a call to CreateFile. Specify the CONOUT$ value in a call to CreateFile to open a handle of a console's active screen buffer. CreateFile enables you to specify the read-write access of the handle that it returns.

The CreateConsoleScreenBuffer function creates a new screen buffer and returns a handle, which can be used in any function that accepts a handle of console output. The new screen buffer is not active until its handle is specified in a call to the SetConsoleActiveScreenBuffer function. Note that changing the active screen buffer does not affect the handle returned by GetStdHandle. Similarly, using SetStdHandle to change the STDOUT handle does not affect the active screen buffer.

Console handles returned by CreateFile and CreateConsoleScreenBuffer can be used in any of the console functions that require a handle of a console's input buffer or of a console screen buffer. Handles returned by GetStdHandle can be used by the console functions if they have not been redirected to refer to something other than console I/O. If a standard handle has been redirected to refer to a file or a pipe, however, the handle can only be used by the ReadFile and WriteFile functions.

A process can use the DuplicateHandle function to create a duplicate console handle that has different access or inheritability from the original handle. Note, however, that a process can create a duplicate console handle only for its own use. This differs from other handle types (such as file, pipe, or mutex objects), for which DuplicateHandle can create a duplicate that is valid for a different process.

To close a console handle, a process can use the CloseHandle function.

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