System Concepts

You may view the REAL/IX Operating System as a layered entity, as illustrated in Figure 2-1. Users always interact directly with the user level, usually through the shell. From the shell, one may call utilities and user programs, which in turn may call library routines. In the user-level of the operating system, one opens and closes files (which are actual data files, or are special files that correspond to system devices), reads and writes data, and so forth.

The kernel level of the operating system is accessed through system calls, which are called directly by utility programs and user processes, or through library routines. Kernel-level processes (or functions) handle the details of interacting with the internal structures of files, processes, and devices. The  hardware control level of the system is accessed through kernel-level processes, usually drivers.

Figure 1 - REAL/IX Operating System

 

The discussions in this white paper frequently talk about user-level activities versus kernel-level activities. One of the features which makes UNIX operating systems so popular is that most of the kernel is written in the C programming language, a high-level language that is also used for user-level programming.

However, there are fundamental differences between user-level and kernel-level code:

 

System Calls

System calls are the highest level of the kernel, and are the means by which a user-level process requests service from the kernel. You might access system calls directly from a C language or FORTRAN program, or indirectly (by calling a C subroutine) from a program written in another language. To the programmer, system calls are indistinguishable from library routine calls.

All system calls take the form of functions, meaning they can return a value. A return value of -1 usually indicates unsuccessful execution. When a call returns -1, an external variable called errno (defined in errno.h ) describes the nature of the problem. All valid errnos are defined on the intro(2) manual page; the specific meanings of errnos returned by individual system calls are listed on the individual manual pages.

 

Library Routines

Library routines are part of the user level. In most cases, the library routines call system calls. For example, the fopen library routine calls the open system call; the sleep library routine calls the alarm, pause, and signal system calls. As a general rule, the library routines execute a bit more slowly than the corresponding system calls, but also are safer for the novice programmer, since they usually include some checks and clean up operations that the programmer would otherwise need to code.

 

Privileges

The REAL/IX Operating System recognizes three basic privileges: regular, superuser, and realtime. Regular privileges are the standard privileges that allow you to execute the commands listed in Section 1 of the manual pages, where you can only affect your own files and processes (or those whose owners have granted you access). Superuser privileges are the traditional administrative privileges, which allow access to all files and processes on the system; a number of administrative functions are only executed by the superuser. Realtime privileges assigned to a user or a process are required to use a number of the special REAL/IX features for realtime programming; restricted assignment of these privileges ensures that a time-sharing program cannot inadvertently affect the realtime responsiveness of the system.

Realtime users and processes have more privileges than regular users, but fewer privileges than the superuser. A superuser can do anything that a realtime user can.

Some examples of actions that are allowed for both realtime and superusers are:

Only superusers can do the following:

Realtime privileges are controlled by a system table that is loaded with the setrtusers(1) command, usually during boot time. Processes executed by a user who is listed in this table are allowed to become realtime processes when the setrt(2) system call is issued. Use the rtusers(1) command to get a list of users with realtime permissions on your system.

 

Data Structures and Header Files

The REAL/IX Operating System uses header files to localize information of interest to several user-level and kernel-level processes. Localizing common information reduces the overhead to individual processes, enhances the portability of code, and ensures that all programs compiled on the system are using the same definitions for a specific entity.

A program that needs to access structures in a particular header file uses a #include statement that references the file. If information changes in the header file, all programs that reference that header file automatically incorporate those changes when they are recompiled. Without the header files, all programs would have to declare these structures, and would all require recoding when the definitions changed. The use of header files is a major help when porting programs from one system to another and when two or more processes must agree on the meaning of a defined variable.

System header files are located under the /usr/include  directory. This directory includes a number of definitions that are of interest to user-level processes, such as the definition of the a.out  header of executable object files and the definitions in value.h  of the sizes of various data elements. The sys  subdirectory defines structures that are more involved with the kernel processes and drivers, such as user.h  and proc.h  that describe the basic data structures used in scheduling processes. Header files in the sys  directory also define kernel macros and I/O controls (IOCTLs) that are used by drivers and their associated user-level processes.

Header files are all standard ASCII files coded using C language conventions; for technical users, they also provide a quick reference to data structures implemented on the system.

 

System Time

A number of system and application programs identify the local clock time. The most obvious example is the electronic mail facilities, which indicate the time the mail was sent. For instance, a standard header received from the mail(1) command is:

From user Tue Mar 27 14:09 EST 1990

Similar date outputs are required for reports associated with user applications and so forth. Traditionally, this information is controlled by the $TZ environmental variable, which records standard designation for the time zone (in this case, EST, meaning Eastern Standard Time), the number of whole hours from Greenwich Mean Time (GMT), and whether Daylight Savings Time is observed. If Daylight Savings Time is observed, the time stamp shown above would print "EDT" rather than "EST" during the times Daylight Savings Time is observed in the United States.

This scheme worked well when UNIX operating systems were used mostly by small development groups in the United States, but have become too parochial with so many users working in other countries. To address this problem, this release of the REAL/IX Operating System uses a more inclusive method of defining time conversion satisfying the requirements of the POSIX 1003.1 standard.

 

Manual Page Format

The REAL/IX Operating System provides on line documentation in the form an on line copy of the REAL/IX Operating System reference manuals. Access to the on line reference manuals is through the use of the man(1) command. man locates and prints the entry from the on line reference manuals for the named command in the specified section. If no section is specified, man searches through the entire set of online documentation for the title. By default, the output (commonly referred to as the manual page) is directed to the user's terminal. Figure 2-2 shows the format of the on line manual page. Note that through the use of REAL/IX Operating System pipes and filters, you may redirect the output of the man command to a file or device.

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Figure 2 - Online Manual Page Format

 

• Note that Figure 2-2 is a representation of what you can expect when using the on line documentation feature of the REAL/IX Operating System. For a more detailed explanation of the manual page format, execute the command string man 1 intro for on line documentation or refer to the hardcopy of the REAL/IX Reference Manual intro(1) command.

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