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In computing, a crash, or system crash, occurs when a computer program such as a software application or an operating system stops functioning properly and exits. On some operating systems or individual applications, a crash reporting service will report the crash and any details relating to it (or give the user the option to do so), usually to the developer(s) of the application. If the program is a critical part of the operating system, the entire system may crash or hang, often resulting in a
kernel panic A kernel panic (sometimes abbreviated as KP) is a safety measure taken by an operating system's kernel upon detecting an internal fatal error in which either it is unable to safely recover or continuing to run the system would have a higher ...
or
fatal system error A fatal system error (also known as a system crash, stop error, kernel error, or bug check) occurs when an operating system halts because it has reached a condition where it can no longer operate safely (''i.e.'' where critical data could be ...
. Most crashes are the result of a software bug. Typical causes include accessing invalid memory addresses, incorrect
address An address is a collection of information, presented in a mostly fixed format, used to give the location of a building, apartment, or other structure or a plot of land, generally using political boundaries and street names as references, along ...
values in the program counter,
buffer overflow In information security and programming, a buffer overflow, or buffer overrun, is an anomaly whereby a program, while writing data to a buffer, overruns the buffer's boundary and overwrites adjacent memory locations. Buffers are areas of memor ...
, overwriting a portion of the affected program code due to an earlier bug, executing invalid machine instructions (an illegal opcode), or triggering an unhandled exception. The original software bug that started this chain of events is typically considered to be the cause of the crash, which is discovered through the process of debugging. The original bug can be far removed from the
code In communications and information processing, code is a system of rules to convert information—such as a letter, word, sound, image, or gesture—into another form, sometimes shortened or secret, for communication through a communication ...
that actually triggered the crash. In early personal computers, attempting to write data to hardware addresses outside the system's main memory could cause hardware damage. Some crashes are exploitable and let a malicious program or hacker execute arbitrary code allowing the replication of viruses or the acquisition of data which would normally be inaccessible.


Application crashes

An application typically crashes when it performs an operation that is not allowed by the operating system. The operating system then triggers an exception or signal in the application. Unix applications traditionally responded to the signal by dumping core. Most Windows and Unix GUI applications respond by displaying a dialogue box (such as the one shown to the right) with the option to attach a debugger if one is installed. Some applications attempt to recover from the error and continue running instead of exiting. An application can also contain code to crash after detecting a severe error. Typical errors that result in application crashes include: *attempting to read or write memory that is not allocated for reading or writing by that application (e.g., segmentation fault, x86-specific
general protection fault A general protection fault (GPF) in the x86 instruction set architectures (ISAs) is a fault (a type of interrupt) initiated by ISA-defined protection mechanisms in response to an access violation caused by some running code, either in the kernel ...
) *attempting to execute privileged or invalid instructions *attempting to perform I/O operations on hardware devices to which it does not have permission to access *passing invalid arguments to system calls *attempting to access other system resources to which the application does not have permission to access *attempting to execute machine instructions with bad arguments (depending on CPU architecture): divide by zero, operations on
denormal number In computer science, subnormal numbers are the subset of denormalized numbers (sometimes called denormals) that fill the underflow gap around zero in floating-point arithmetic. Any non-zero number with magnitude smaller than the smallest normal n ...
or
NaN Nan or NAN may refer to: Places China * Nan County, Yiyang, Hunan, China * Nan Commandery, historical commandery in Hubei, China Thailand * Nan Province ** Nan, Thailand, the administrative capital of Nan Province * Nan River People Given nam ...
(not a number) values, memory access to unaligned addresses, etc.


Crash to desktop

A "crash to desktop" is said to occur when a
program Program, programme, programmer, or programming may refer to: Business and management * Program management, the process of managing several related projects * Time management * Program, a part of planning Arts and entertainment Audio * Programm ...
(commonly a video game) unexpectedly quits, abruptly taking the user back to the
desktop A desktop traditionally refers to: * The surface of a desk (often to distinguish office appliances that fit on a desk, such as photocopiers and printers, from larger equipment covering its own area on the floor) Desktop may refer to various compu ...
. Usually, the term is applied only to crashes where no error is displayed, hence all the user sees as a result of the crash is the desktop. Many times there is no apparent action that causes a crash to desktop. During normal function, the program may freeze for a shorter period of time, and then close by itself. Also during normal function, the program may become a black screen and repeatedly play the last few seconds of sound (depending on the size of the audio
buffer Buffer may refer to: Science * Buffer gas, an inert or nonflammable gas * Buffer solution, a solution used to prevent changes in pH * Buffering agent, the weak acid or base in a buffer solution * Lysis buffer, in cell biology * Metal ion buffer ...
) that was being played before it crashes to desktop. Other times it may appear to be triggered by a certain action, such as loading an area. Crash to desktop bugs are considered particularly problematic for users. Since they frequently display no error message, it can be very difficult to track down the source of the problem, especially if the times they occur and the actions taking place right before the crash do not appear to have any pattern or common ground. One way to track down the source of the problem for games is to run them in windowed-mode. Windows Vista has a feature that can help track down the cause of a CTD problem when it occurs on any program. Windows XP included a similar feature as well. Some computer programs, such as '' StepMania'' and BBC's '' Bamzooki'', also crash to desktop if in full-screen, but display the error in a separate window when the user has returned to the desktop.


Web server crashes

The software running the web server behind a website may crash, rendering it inaccessible entirely or providing only an error message instead of normal content. For example: if a site is using an SQL database (such as MySQL) for a script (such as PHP) and that SQL database server crashes, then PHP will display a connection error.


Operating system crashes

An operating system crash commonly occurs when a
hardware exception In computing and computer programming, exception handling is the process of responding to the occurrence of ''exceptions'' – anomalous or exceptional conditions requiring special processing – during the execution of a program. In general, a ...
occurs that cannot be handled. Operating system crashes can also occur when internal sanity-checking logic within the operating system detects that the operating system has lost its internal self-consistency. Modern multi-tasking operating systems, such as Linux, and macOS, usually remain unharmed when an application program crashes. Some operating systems, e.g., z/OS, have facilities for Reliability, availability and serviceability (RAS) and the OS can recover from the crash of a critical component, whether due to hardware failure, e.g., uncorrectable ECC error, or to software failure, e.g., a reference to an unassigned page.


Security and privacy implications of crashes

Depending on the application, the crash may contain the user's sensitive and private information. Moreover, many software bugs which cause crashes are also exploitable for
arbitrary code execution In computer security, arbitrary code execution (ACE) is an attacker's ability to run any commands or code of the attacker's choice on a target machine or in a target process. An arbitrary code execution vulnerability is a security flaw in softwa ...
and other types of privilege escalation. For example, a stack buffer overflow can overwrite the return address of a subroutine with an invalid value, which will cause, e.g., a segmentation fault, when the subroutine returns. However, if an exploit overwrites the return address with a valid value, the code in that address will be executed.


Crash reproduction

When crashes are collected in the field using a
crash reporter A crash reporter is usually a system software whose function is to identify reporting crash details and to alert when there are crashes, in production or on development / testing environments. Crash reports often include data such as stack traces ...
, the next step for developers is to be able to reproduce them locally. For this, several techniques exist: STAR uses symbolic execution, MuCrash mutates the test code of the application that has crashed, and EvoCrash performs evolutionary search.


See also

* Blue Screen of Death *
Crash-only software Crash-only software refers to computer programs that handle failures by simply restarting, without attempting any sophisticated recovery. Correctly written components of crash-only software can microreboot to a known-good state without the help ...
* Data loss *
Guru Meditation Guru Meditation started as an error notice displayed by the Amiga computer when it crashes. It is now also used by Varnish, a software component used by many content-heavy websites. This has led to many internet users seeing a "Guru Meditation" ...
* Memory corruption * Reboot * Safe mode * SystemRescueCD * Undefined behavior


Notes


References


External links


Picking Up The Pieces After A Computer Crash
{{DEFAULTSORT:Crash (Computing) Computer jargon Computer errors Software anomalies