ReadFile (hFile, bBuffer, 100, NULL, &o1) Here’s what an OVERLAPPED structure looks like: typedef struct _OVERLAPPED The word "overlapped" in this context means that the time spent performing the I/O request overlaps the time your thread spends performing other tasks. When performing asynchronous device I/O, you must pass the address to an initialized OVERLAPPED structure via the pOverlapped parameter. After all, you expect these functions to return before the I/O request has completed, so examining the number of bytes transferred is meaningless at this time. By the way, when calling either function for asynchronous I/O, you can (and usually do) pass NULL for the pdwNumBytes parameter. If this flag is specified, the function performs asynchronous device I/O. When either of these functions is called, the function checks to see if the device, identified by the hFile parameter, was opened with the FILE_FLAG_OVERLAPPED flag.
For convenience, I’ll list the function prototypes again: BOOL ReadFile( To queue an I/O request for a device driver, you use the ReadFile and WriteFile functions that you already learned about in “Performing Synchronous Device I/O” on page 302. This flag notifies the system that you intend to access the device asynchronously. To access a device asynchronously, you must first open the device by calling CreateFile, specifying the FILE_FLAG_OVERLAPPED flag in the dwFlagsAndAttributes parameter. Queuing asynchronous I/O requests is the essence of designing a high-performance, scalable application, and it is what the remainder of this chapter is all about.
#Writefile installbuilder platform how to#
For now, let’s concentrate on how to queue asynchronous I/O requests.
#Writefile installbuilder platform driver#
You’ll learn how the device driver notifies you of I/O completions in “Receiving Completed I/O Request Notifications” on page 310. Instead, this thread continues executing and performs other useful tasks.Īt some point, the device driver finishes processing the queued I/O request and must notify the application that data has been sent, data has been received, or an error has occurred.
While the device driver waits for the device to respond, the application’s thread is not suspended as it waits for the I/O request to complete. This I/O request is passed to a device driver, which assumes the responsibility of actually performing the I/O. However, using asynchronous device I/O enables you to better use resources and thus create more efficient applications.Ĭonsider a thread that issues an asynchronous I/O request to a device. The CPU performs arithmetic operations and even paints the screen much faster than it reads data from or writes data to a file or across a network. Compared to most other operations carried out by a computer, device I/O is one of the slowest and most unpredictable.
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