Glossary

QNX SDP8.0User's GuideUser

administrator: See superuser.
alias: A shell feature that lets you create new commands or specify your favorite options. For example, alias my_ls='ls -F' creates an alias called my_ls that the shell replaces with ls -F.
atomic: Of or relating to atoms. :-)
In operating systems, this refers to the requirement that an operation, or sequence of operations, be considered indivisible. For example, a thread may need to move a file position to a given location and read data. These operations must be performed in an atomic manner; otherwise, another thread could preempt the original thread and move the file position to a different location, thus causing the original thread to read data from the second thread's position.
BIOS/ROM Monitor extension signature: A certain sequence of bytes indicating to the BIOS or ROM Monitor that the device is to be considered an extension to the BIOS or ROM Monitor—control is to be transferred to the device by the BIOS or ROM Monitor, with the expectation that the device will perform additional initializations.
On the x86 architecture, the two bytes 0x55 and 0xAA must be present (in that order) as the first two bytes in the device, with control being transferred to offset 0x0003.
budget: In sporadic scheduling, the amount of time a thread is permitted to execute at its normal priority before being dropped to its low priority.
buildfile: A text file containing instructions for mkifs specifying the contents and other details of an image, or for mkefs specifying the contents and other details of an embedded filesystem image.
canonical mode: Also called edited mode or cooked mode. In this mode the character device library performs line-editing operations on each received character. Only when a line is completely entered—typically when a carriage return (CR) is received—will the line of data be made available to application processes. Contrast raw mode.
card information structure (CIS): A data block that maintains information about flash configuration. The CIS description includes the types of memory devices in the regions, the physical geometry of these devices, and the partitions located on the flash.
channel: A kernel object used with message passing.
In QNX OS, message passing is directed towards a connection (made to a channel); threads can receive messages from channels. A thread that wishes to receive messages creates a channel (using ChannelCreate()), and then receives messages from that channel (using MsgReceive()). Another thread that wishes to send a message to the first thread must make a connection to that channel by attaching to the channel (using ConnectAttach()) and then sending data (using MsgSend()).
command completion: A shell feature that saves typing; type enough of the command's name to identify it uniquely, and then press Esc twice. If possible, the shell fills in the rest of the name.
command interpreter: A process that parses what you type on the command line; also known as a shell.
Common Internet File System (CIFS): also known as Server Message Block (SMB)
A protocol that allows a client workstation to perform transparent file access over a network to a Windows server. Client file access calls are converted to CIFS protocol requests and are sent to the server over the network. The server receives the request, performs the actual filesystem operation, and sends a response back to the client.
compound command: A command that includes a shell's reserved words, grouping constructs, and function definitions (e.g., ls -al | less). Contrast simple command.
configurable limit: A special variable that stores system information. Some (e.g., _PC_NAME_MAX) depend on the filesystem and are associated with a path; others (e.g., _SC_ARG_MAX) are independent of paths.
configuration string: A system variable that's like an environment variable, but is more dynamic. When you set an environment variable, the new value affects only the current instance of the shell and any of its children that you create after setting the variable; when you set a configuration string, its new value is immediately available to the entire system.
connection: A kernel object used with message passing.
Connections are created by client threads to connect to the channels made available by servers. Once connections are established, clients can MsgSend*() messages over them.
console: The display adapter, the screen, and the system keyboard are collectively referred to as the physical console. A virtual console emulates a physical console and lets you run more than one terminal session at a time on a machine.
cooked mode: See canonical mode.
core dump: A file describing the state of a process that terminated abnormally.
critical section: A code passage that must be executed serially (i.e., by only one thread at a time). The simplest from of critical section enforcement is via a mutex.
device driver: A process that allows the OS and application programs to make use of the underlying hardware in a generic way (e.g., a disk drive, a network interface). Unlike OSs that require device drivers to be tightly bound into the OS itself, device drivers for QNX OS are standard processes that can be started and stopped dynamically. As a result, adding device drivers doesn't affect any other part of the OS; drivers can be developed and debugged like any other application. Also, device drivers are in their own protected address space, so a bug in a device driver won't cause the entire OS to shut down.
Domain Name System (DNS): An Internet protocol used to convert ASCII domain names into IP addresses.
edge-sensitive: One of two ways in which a PIC (Programmable Interrupt Controller) can be programmed to respond to interrupts. In edge-sensitive mode, the interrupt is noticed upon a transition to/from the rising/falling edge of a pulse. Contrast level-sensitive.
edited mode: See canonical mode.
erasable programmable read-only memory (EPROM): A memory technology that allows the device to be programmed (typically with higher-than-operating voltages, e.g., 12 V), with the characteristic that any bit (or bits) may be individually programmed from a 1 state to a 0 state.
Changing a bit from a 0 state into a 1 state can be accomplished only by erasing the entire device, setting all of the bits to a 1 state. Erasing is accomplished by shining an ultraviolet light through the erase window of the device for a fixed period of time (typically 10-20 minutes). The device is further characterized by having a limited number of erase cycles (typically 10e5 - 10e6). Contrast flash and RAM.
event: A scheme used to notify a thread that a particular condition has occurred. QNX has several mechanisms for notifying a thread about the occurrence of a condition, including pulses, signals, semaphores, updates to memory, thread creation, unblocking of kernel calls, and unblocking of ISTs.
extent: A contiguous sequence of blocks on a disk.
file descriptor (FD): A handle for a client process to use to access a resource such as a file, socket, pipe, etc. In a QNX system, a file descriptor is also a connection ID that the client obtained by connecting to a server by a call such as open() or socket(). The client can then use the file descriptor in subsequent calls to operate on the same resource.
filename completion: A shell feature that saves typing; type enough of the file's name to identify it uniquely, and then press Esc twice. If possible, the shell fills in the rest of the name.
filter: A program that reads from standard input and writes to standard output, such as grep and sort. You can use a pipe (|) to combine filters.
first in, first out (FIFO): A scheduling policy whereby a thread is able to consume CPU at its priority level without bounds. See also round robin and sporadic.
flash memory: A memory technology similar in characteristics to EPROM memory, with the exception that erasing is performed electrically, and, depending upon the organization of the flash memory device, erasing may be accomplished in blocks (typically 64 KB bytes at a time) instead of the entire device. Contrast EPROM and RAM.
group: A collection of users who share similar file permissions.
hard link: See link.
hardware interrupt: See interrupt.
hidden file: A file whose name starts with a dot (.), such as .profile. Commands such as ls don't operate on hidden files unless you explicitly say to.
high availability (HA): In telecommunications and other industries, HA describes a system's ability to remain up and running without interruption for extended periods of time.
image: In the context of embedded QNX OS systems, an image can mean either a structure that contains files (i.e., an OS image) or a structure that can be used in a read-only, read/write, or read/write/reclaim FFS-3-compatible filesystem (i.e., a flash filesystem image).
initial program loader (IPL): The software component that either takes control at the processor's reset vector (e.g., location 0xFFFFFFF0 on the x86), or is a BIOS extension. This component is responsible for putting the machine into a usable state such that the startup program can then perform further initializations. The IPL is written in assembler and C. See also BIOS extension signature and startup code.
inode: Information node—a storage table that holds information about files, other than the files' names. In order to support links for each file, the filename is separated from the other information that describes a file.
interprocess communication (IPC): The ability for two processes (or threads) to communicate. The QNX OS offers several forms of IPC, most notably native messaging (synchronous, client/server relationship), POSIX message queues and pipes (asynchronous), and signals.
interrupt: An event (usually caused by hardware) that interrupts whatever the processor was doing and asks it do something else. The hardware will generate an interrupt whenever it has reached some state where software intervention is required.
interrupt latency: The amount of time elapsed between the generation of a hardware interrupt and the first instruction executed by the relevant interrupt service thread (IST) returning from its blocking function. Also designated as T_il.
interrupt request (IRQ): A hardware request line asserted by a peripheral to indicate that it requires servicing by software. The IRQ is handled by the PIC, which then interrupts the processor, usually causing the processor to execute an Interrupt Service Thread (IST).
interrupt service routine (ISR): A routine responsible for servicing hardware (e.g., reading and/or writing some device ports), for updating some data structures shared between the ISR and the thread(s) running in the application, and for signalling the thread that some kind of event has occurred.
Note:
QNX OS does not use ISRs anymore; see ISTs.
interrupt service thread (IST): A thread that is responsible for servicing an interrupt.
The thread attaches to the interrupt via InterruptAttachThread() or InterruptAttachEvent(), and then blocks waiting for the QNX OS to unblock it after an interrupt has happened.
kernel: See microkernel.
level-sensitive: One of two ways in which a PIC (Programmable Interrupt Controller) can be programmed to respond to interrupts. If the PIC is operating in level-sensitive mode, the IRQ is considered active whenever the corresponding hardware line is active. Contrast edge-sensitive.
link: A filename; a pointer to the file's contents. Contrast symbolic link.
message: A parcel of bytes passed from one process to another. The OS attaches no special meaning to the content of a message—the data in a message has meaning for the sender of the message and for its receiver, but for no one else.
Message passing not only allows processes to pass data to each other, but also provides a means of synchronizing the execution of several processes. As they send, receive, and reply to messages, processes undergo various changes of state that affect when, and for how long, they may run.
metadata: Data about data; for a filesystem, metadata includes all the overhead and attributes involved in storing the user data itself, such as the name of a file, the physical blocks it uses, modification and access timestamps, and so on.
microkernel: A part of the operating system that provides the minimal services used by a team of optional cooperating processes, which in turn provide the higher-level OS functionality. The microkernel itself lacks filesystems and many other services normally expected of an OS; those services are provided by optional processes.
mountpoint: The location in the pathname space where a resource manager has registered itself. For example, the serial port resource manager registers mountpoints for each serial device (/dev/ser1, /dev/ser2, etc.).
mutex: Mutual exclusion lock, a simple synchronization service used to ensure exclusive access to data shared between threads. It's typically acquired (pthread_mutex_lock()) and released (pthread_mutex_unlock()) around the code that accesses the shared data (usually a critical section).
Network File System (NFS): A TCP/IP application that lets you graft remote filesystems (or portions of them) onto your local pathname space. Directories on the remote systems appear as part of your local filesystem and all the utilities you use for listing and managing files (e.g., ls, cp, mv) operate on the remote files exactly as they do on your local files.
pathname prefix: See mountpoint.
pathname-space mapping: The process whereby the Process Manager maintains an association between resource managers and entries in the pathname space.
persistent: When applied to storage media, the ability for the media to retain information across a power-cycle. For example, a hard disk is a persistent storage medium, whereas a ramdisk is not, because the data is lost when power is lost.
POSIX: An IEEE/ISO standard. The term is an acronym (of sorts) for Portable Operating System Interface—the X alludes to UNIX, on which the interface is based.
preemption: The act of suspending the execution of one thread and starting (or resuming) another. The suspended thread is said to have been preempted by the new thread. Whenever a lower-priority thread is actively consuming the CPU, and a higher-priority thread becomes READY on the same CPU, the lower-priority thread is immediately preempted by the higher-priority thread.
prefix tree: The internal representation used by the Process Manager to store the pathname table.
priority inheritance: The characteristic of a thread that causes its priority to be raised or lowered to that of the thread that sent it a message. Also used with mutexes. Priority inheritance is a method used to prevent priority inversion.
priority inversion: A condition that can occur when a low-priority thread consumes CPU at a higher priority than it should. This can be caused by not supporting priority inheritance, such that when the lower-priority thread sends a message to a higher-priority thread, the higher-priority thread consumes CPU on behalf of the lower-priority thread. This is solved by having the higher-priority thread inherit the priority of the thread on whose behalf it's working.
process: A nonschedulable entity, which defines the address space and a few data areas. A process must have at least one thread running in it—this thread is then called the first thread.
process group: A collection of processes that permits the signalling of related processes. Each process in the system is a member of a process group identified by a process group ID. A newly created process joins the process group of its creator.
process group ID: The unique identifier representing a process group during its lifetime. A process group ID is a positive integer. The system may reuse a process group ID after the process group dies.
process group leader: A process whose ID is the same as its process group ID.
process ID (PID): The unique identifier representing a process. A PID is a positive integer. The system may reuse a process ID after the process dies, provided no existing process group has the same ID. Only the Process Manager can have a process ID of 1.
programmable interrupt controller (PIC): A hardware component that handles IRQs. See also edge-sensitive, level-sensitive, and IST.
pseudo-TTY (pty): A character-based device that has two ends: a controller end and a worker end. Data written to the controller end shows up on the worker end and vice versa. These devices are typically used to interface between two programs, one that expects a character device and another that wishes to use that device (e.g., the shell and the sshd process).
quoting: A method of forcing a shell's special characters to be treated as simple characters instead of being interpreted in a special way by the shell. For example, less "my file name" escapes the special meaning of the spaces in a filename.
random access memory (RAM): A memory technology characterized by the ability to read and write any location in the device without limitation. Contrast flash and EPROM.
raw mode: In raw input mode, the character device library performs no editing on received characters. This reduces the processing done on each character to a minimum and provides the highest performance interface for reading data. Also, raw mode is used with devices that typically generate binary data—you don't want any translations of the raw binary stream between the device and the application. Contrast canonical mode.
remote execution: Running commands on a machine other than your own over a network.
replenishment: In sporadic scheduling, the period of time during which a thread is allowed to consume its execution budget.
reset vector: The address at which the processor begins executing instructions after the processor's reset line has been activated. On the x86, for example, this is the address 0xFFFFFFF0.
resource manager: A user-level server program that accepts messages from other programs and, optionally, communicates with hardware. QNX OS resource managers are responsible for presenting an interface to various types of devices, whether actual (e.g., serial ports, parallel ports, network cards, disk drives) or virtual (e.g., /dev/null, a network filesystem, and pseudo-ttys).
In other operating systems, this functionality is traditionally associated with device drivers. But unlike device drivers, QNX OS resource managers don't require any special arrangements with the kernel. In fact, a resource manager looks just like any other user-level program. See also device driver.
root: The superuser, which can do anything on your system. The superuser has what Windows calls administrator's rights.
round robin: A scheduling policy whereby a thread is given a certain period of time to run. Should the thread consume CPU for the entire period of its timeslice, the thread will be placed at the end of the ready queue for its priority, and the next available thread will be made READY. If a thread is the only thread READY at its priority level, it will be able to consume CPU again immediately. See also FIFO and sporadic.
runtime loading: The process whereby a program decides while it's actually running that it wishes to load a particular function from a library. Contrast static linking.
scheduling latency: The amount of time elapsed between the point when one thread makes another thread READY and when the other thread actually gets some CPU time. Note that this latency is almost always at the control of the system designer.
Also designated as T_sl. Contrast interrupt latency.
session: A collection of process groups established for job-control purposes. Each process group is a member of a session. A process belongs to the session that its process group belongs to. A newly created process joins the session of its creator. A process can alter its session membership via setsid(). A session can contain multiple process groups.
session leader: A process whose death causes all processes within its process group to receive a SIGHUP signal.
shell: A process that parses what you type on the command line; also known as a command interpreter.
shell script: A file that contains shell commands.
simple command: A command line that contains a single command, usually a program that you want to run (e.g., less my_file). Contrast compound command.
socket: A virtual endpoint for communication. For example, in TCP/IP, a socket is a combination of an IP address and a port number that uniquely identifies a single network process. Another example is the Unix domain socket, which exchanges data between processes that execute on the same host operating system.
software interrupt: Similar to a hardware interrupt (see interrupt), except that the source of the interrupt is software.
sporadic: A scheduling policy whereby a thread's priority can oscillate dynamically between a foreground or normal priority and a background or low priority. A thread is given an execution budget of time to be consumed within a certain replenishment period. See also FIFO and round robin.
startup code: The software component that gains control after the IPL code has performed the minimum necessary amount of initialization. After gathering information about the system, the startup code transfers control to the OS.
static linking: The process whereby you combine your modules with the modules from the library to form a single executable that's entirely self-contained. The word static implies that it's not going to change—all the required modules are already combined into one. Contrast runtime loading.
superuser: The root user, which can do anything on your system. The superuser has what Windows calls administrator's rights.
symbolic link: A special file that usually has a pathname as its data. Symbolic links are a flexible means of pathname indirection and are often used to provide multiple paths to a single file. Unlike hard links, symbolic links can cross filesystems and can also create links to directories.
system page area: An area in the kernel that is filled by the startup code and contains information about the system (number of bytes of memory, location of serial ports, etc.) This is also called the SYSPAGE area.
thread: The schedulable entity under the QNX OS. A thread is a flow of execution; it exists within the context of a process.
timer: A kernel object used in conjunction with time-based functions. A timer is created via timer_create() and armed via timer_settime(). A timer can then deliver an event, either periodically or on a one-shot basis.
timeslice: A period of time assigned to a round-robin scheduled thread. This period of time is small (on the order of tens of milliseconds); the actual value shouldn't be relied upon by any program (it's considered bad design).

Page updated: August 09, 2024