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Chapter 2 117
Programming Fundamentals
C Programming Using VTL
viClose (vi);
viClose (defaultRM);
}
Including the VISA Declarations File
For C and C++ programs, you must include the visa.h header file at
the beginning of every file that contains VTL function calls:
#include “visa.h”
This header file contains the VISA function prototypes and the
definitions for all VISA constants and error codes. The visa.h header
file includes the visatype.h header file.
The visatype.h header file defines most of the VISA types. The VISA
types are used throughout VTL to specify data types used in the
functions. For example, the viOpenDefaultRM function requires a
pointer to a parameter of type ViSession. If you find ViSession in the
visatype.h header file, you will find that ViSession is eventually
typed as an unsigned long.
Opening a Session
A session is a channel of communication. Sessions must first be opened
on the default resource manager, and then for each device you will be
using. The following is a summary of sessions that can be opened:
• A resource manager session is used to initialize the VISA system.
It is a parent session that knows about all the opened sessions. A
resource manager session must be opened before any other session
can be opened.
• A device session is used to communicate with a device on an
interface. A device session must be opened for each device you will be
using. When you use a device session you can communicate without
worrying about the type of interface to which it si connected. This
insulation makes applications more robust and portable across
interfaces. Typically a device is an instrument, but could be a
computer, a plotter, or a printer.