McIDAS system overview

Now that you've seen McIDAS-X from a user's perspective, you need to understand how McIDAS-X operates at the system level. When McIDAS-X is started, it creates the environment in which it will run. In this section, you will learn about the system-level components of the McIDAS-X environment and how they fit together.

Shared memory

When a McIDAS-X session is started, a block of shared memory is created as an inter-process communication mechanism between McIDAS-X applications, resident programs and the display environment. This shared memory block is composed of:

The shared memory key associated with the memory is stored in the environment variable MCENV_POSUC.

Positive user common

Positive UC contains the current state of the McIDAS-X session. This information is available as long as the session that created positive UC is active. Values in positive UC may change. For example, if frames are looping or you're moving the cursor on the screen, the values in positive UC related to the current frame number or cursor position will change.

Negative user common

Negative UC is not part of the shared memory block created when the McIDAS-X session is started. It is created by mmap when a McIDAS-X command is started, and is initialized with the values in positive UC at the time the command is entered. The file descriptor key associated with negative UC is then placed in the environment variable MCENV_NEGUC.

Negative UC values are not changed by other activities in the McIDAS-X session, such as frame looping or cursor motion. When a command finishes running, its copy of negative UC is deleted if it was not inherited from a parent process.

Visibility and inheritance

As a McIDAS-X programmer, you need to understand the concepts of visibility and inheritance when accessing positive and negative UC.

Visibility describes the portions of the McIDAS-X session that can access specific UC information.
Inheritance refers to the origin of the UC that an application receives at initialization.

The User Common policy for visibility and inheritance when running a McIDAS-X command is described below.

Does positive UC exist?

If positive UC does not exist, the McIDAS-X software creates the shared memory block and assigns the memory key to MCENV_POSUC (positive UC). Otherwise, the software attaches positive UC to the McIDAS-X command.
Once positive UC exists, it is visible to all the commands run in that session.

Should this application inherit negative UC?

The parent process determines if an application inherits negative UC.
If a McIDAS-X command runs synchronously, it inherits negative UC from its parent process. This happens, for example, when a command is run from a McIDAS-X command file. The application inherits the parent by attaching itself to the file descriptor key stored in the parent's MCENV_NEGUC. Values in positive UC are not copied into negative UC when a command inherits its parent's copy of negative UC. The visibility of this copy lasts until the parent process that created it ends.
If a McIDAS-X command runs asynchronously relative to its parent process, the command creates its own negative UC and associated MCENV_NEGUC, and initializes it from values in positive UC. This can occur, for example, when a command is started from the McIDAS-X Text and Command Window. The version of negative UC created is visible only while the command that created it is active.

For a complete list of the contents of User Common, see Chapter 6, Format of the Data Files.

Image frame objects

Image frame objects contain information describing the contents and appearance of a frame displayed on the McIDAS-X Image Window. McIDAS-X provides a variety of library functions and commands to manipulate the contents of frame objects. An individual frame object contains not only pixel values to display, but also tables for image color enhancements, graphics color enhancements and stretching tables.

Resident programs

The second component of the McIDAS-X environment is resident programs. When a user types the mcidas command, the user-specified configuration information, which is in the text file $HOME/.mcidasrc, is determined.

Once the configuration information is extracted, the resident programs below are started.

mcenv

The mcenv program performs the following procedures to start a McIDAS-X session:

Creates the shared memory segment that will contain positive UC, the frame objects and the redirection table
Assigns the shared memory key ID to the environment variable MCENV_POSUC
Creates a temporary directory in $HOME/.mctmp with the same name as the shared memory key
Starts a command, if one is specified; note that when the mcidas script starts mcenv, it specifies that mctext should be started

The temporary directory created by mcenv stores session-dependent files such as frame directories, string tables and frame enhancement tables.

mctext

The mctext program performs the tasks below, as shown in Figure 2-8:

Maintains the Text and Command Window for McIDAS-X
Allocates enough memory to contain the command line and alphanumeric text components needed for the session
Monitors keyboard entry and the placement of the alphanumeric output generated by the commands

Figure 2-8. The mctext program controls command line input from the keyboard and displays text output on the Text and Command Window, while mcimage controls the McIDAS-X Image Window for displaying images and graphics.

The command line arguments for mctext can alter the number of text lines in the window buffer and recall the number of previously run commands. mctext can also start several other commands once it initializes its window environment.

When the mcidas script starts a session, one of the command arguments passed to mctext is mcimage.

mcimage

As shown in Figure 2-8, the mcimage program does the following:

Translates the contents of the frame objects into display characteristics for the workstation windowing system
Combines the brightness values, color enhancement information and stretching tables into pixel values that are displayed on the McIDAS-X Image Window
Controls the display of the McIDAS-X cursor

Applications programs

The third component of the McIDAS-X environment is applications programs. A McIDAS-X applications program is any program that runs from the McIDAS-X command line. McIDAS-X provides Application Programming Interface (API) functions that interact with all the McIDAS-X components described in this chapter. These API functions interact with the Image Window, Text and Command Window, and mouse. The programs access data both locally and through ADDE, and manipulate the contents of the session, as shown in Figure 2-9 below.

Figure 2-9. McIDAS-X applications rely on the API functions to read data files for them.

Figure 2-10 combines the three components of the McIDAS-X environment: the shared memory, resident programs and applications. The arrows represent APIs and imply the flow of data or information.

Figure 2-10. The McIDAS-X environment consists of shared memory, the resident programs mctext and mcimage, and user applications.