Manually Flushing the Formatted I/O Buffers

NI-VISA 15.0 Help

Edition Date: June 2015

Part Number: 370131S-01

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This section describes flushing issues that are related to formatted I/O buffers. The descriptions apply to all buffered read and buffered write operations. For example, the viPrintf() description applies equally to other buffered write operations (viVPrintf() and viBufWrite()). Similarly, the viScanf() description applies to other buffered read operations (viVScanf() and viBufRead()).

Flushing a write buffer immediately sends any queued data to the device. Flushing a read buffer discards the data in the read buffer. An empty read buffer guarantees that the next call to viScanf(), viBufRead(), or a related operation reads data directly from the device rather than from queued data residing in the read buffer.

The easiest way to flush the buffers is with an explicit call to viFlush(). This operation can actually flush the buffers in two ways. The simpler way uses discard flags. These flags tell the driver to discard the contents of the buffers without performing any I/O to the device. For example,

status = viFlush(instr, VI_READ_BUF_DISCARD);

However, the flush operation can also complete the current I/O before flushing the buffer. For a write buffer, this simply means to send the rest of the buffer to the device. However, for a read buffer, the process is more involved. Because you could be in the middle of a read from the device (that is, the device still has information to send), it is possible to have the driver check the buffer for an EOS or END bit/EOI signal. If such a value exists in the buffer, the contents of the buffer are discarded. However, if the driver can find no such value, it begins reading from the device until it detects the end of the communication and then discards the data. This process keeps the program and device in synchronization with each other. See viFlush() for more information.


Related Topics

Automatically Flushing the Formatted I/O Buffers

Formatted I/O Read and Low-Level I/O Receive Buffers

Formatted I/O Write and Low-Level I/O Transmit Buffers

Recommendations for Using the VISA Buffers

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