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In
computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes, and development of both hardware and software. Computing has scientific, ...
, a segmentation fault (often shortened to segfault) or access violation is a fault, or failure condition, raised by hardware with memory protection, notifying an
operating system An operating system (OS) is system software that manages computer hardware, software resources, and provides common daemon (computing), services for computer programs. Time-sharing operating systems scheduler (computing), schedule tasks for ef ...
(OS) the software has attempted to access a restricted area of memory (a memory access violation). On standard x86 computers, this is a form of general protection fault. The
operating system kernel The kernel is a computer program at the core of a computer's operating system and generally has complete control over everything in the system. It is the portion of the operating system code that is always resident in memory and facilitates ...
will, in response, usually perform some corrective action, generally passing the fault on to the offending process by sending the process a signal. Processes can in some cases install a custom signal handler, allowing them to recover on their own,''Expert C programming: deep C secrets'' By Peter Van der Linden, page 188 but otherwise the OS default signal handler is used, generally causing abnormal termination of the process (a program
crash Crash or CRASH may refer to: Common meanings * Collision, an impact between two or more objects * Crash (computing), a condition where a program ceases to respond * Cardiac arrest, a medical condition in which the heart stops beating * Couch ...
), and sometimes a core dump. Segmentation faults are a common class of error in programs written in languages like C that provide low-level memory access and few to no safety checks. They arise primarily due to errors in use of pointers for
virtual memory In computing, virtual memory, or virtual storage is a memory management technique that provides an "idealized abstraction of the storage resources that are actually available on a given machine" which "creates the illusion to users of a very ...
addressing, particularly illegal access. Another type of memory access error is a
bus error In computing, a bus error is a fault raised by hardware, notifying an operating system (OS) that a process is trying to access memory that the CPU cannot physically address: an invalid address for the address bus, hence the name. In modern use on ...
, which also has various causes, but is today much rarer; these occur primarily due to incorrect ''physical'' memory addressing, or due to misaligned memory access – these are memory references that the hardware ''cannot'' address, rather than references that a process is not ''allowed'' to address. Many programming languages may employ mechanisms designed to avoid segmentation faults and improve memory safety. For example,
Rust Rust is an iron oxide, a usually reddish-brown oxide formed by the reaction of iron and oxygen in the catalytic presence of water or air moisture. Rust consists of hydrous iron(III) oxides (Fe2O3·nH2O) and iron(III) oxide-hydroxide (FeO( ...
employs an ownership-based model to ensure memory safety. Other languages, such as
Lisp A lisp is a speech impairment in which a person misarticulates sibilants (, , , , , , , ). These misarticulations often result in unclear speech. Types * A frontal lisp occurs when the tongue is placed anterior to the target. Interdental lispin ...
and
Java Java (; id, Jawa, ; jv, ꦗꦮ; su, ) is one of the Greater Sunda Islands in Indonesia. It is bordered by the Indian Ocean to the south and the Java Sea to the north. With a population of 151.6 million people, Java is the world's mo ...
, employ
garbage collection Waste collection is a part of the process of waste management. It is the transfer of solid waste from the point of use and disposal to the point of treatment or landfill. Waste collection also includes the curbside collection of recyclabl ...
, which avoids certain classes of memory errors that could lead to segmentation faults.


Overview

A segmentation fault occurs when a program attempts to access a
memory Memory is the faculty of the mind by which data or information is encoded, stored, and retrieved when needed. It is the retention of information over time for the purpose of influencing future action. If past events could not be remember ...
location that it is not allowed to access, or attempts to access a memory location in a way that is not allowed (for example, attempting to write to a read-only location, or to overwrite part of the
operating system An operating system (OS) is system software that manages computer hardware, software resources, and provides common daemon (computing), services for computer programs. Time-sharing operating systems scheduler (computing), schedule tasks for ef ...
). The term "segmentation" has various uses in computing; in the context of "segmentation fault", a term used since the 1950s, it refers to the address space of a ''program.'' With memory protection, only the program's own address space is readable, and of this, only the
stack Stack may refer to: Places * Stack Island, an island game reserve in Bass Strait, south-eastern Australia, in Tasmania’s Hunter Island Group * Blue Stack Mountains, in Co. Donegal, Ireland People * Stack (surname) (including a list of people ...
and the read/write portion of the data segment of a program are writable, while read-only data and the code segment are not writable. Thus attempting to read outside of the program's address space, or writing to a read-only segment of the address space, results in a segmentation fault, hence the name. On systems using hardware
memory segmentation Memory segmentation is an operating system memory management technique of division of a computer's primary memory into segments or sections. In a computer system using segmentation, a reference to a memory location includes a value that ident ...
to provide
virtual memory In computing, virtual memory, or virtual storage is a memory management technique that provides an "idealized abstraction of the storage resources that are actually available on a given machine" which "creates the illusion to users of a very ...
, a segmentation fault occurs when the hardware detects an attempt to refer to a non-existent segment, or to refer to a location outside the bounds of a segment, or to refer to a location in a fashion not allowed by the permissions granted for that segment. On systems using only
paging In computer operating systems, memory paging is a memory management scheme by which a computer stores and retrieves data from secondary storage for use in main memory. In this scheme, the operating system retrieves data from secondary storage ...
, an invalid page fault generally leads to a segmentation fault, and segmentation faults and page faults are both faults raised by the
virtual memory In computing, virtual memory, or virtual storage is a memory management technique that provides an "idealized abstraction of the storage resources that are actually available on a given machine" which "creates the illusion to users of a very ...
management system. Segmentation faults can also occur independently of page faults: illegal access to a valid page is a segmentation fault, but not an invalid page fault, and segmentation faults can occur in the middle of a page (hence no page fault), for example in a buffer overflow that stays within a page but illegally overwrites memory. At the hardware level, the fault is initially raised by the memory management unit (MMU) on illegal access (if the referenced memory exists), as part of its memory protection feature, or an invalid page fault (if the referenced memory does not exist). If the problem is not an invalid logical address but instead an invalid physical address, a
bus error In computing, a bus error is a fault raised by hardware, notifying an operating system (OS) that a process is trying to access memory that the CPU cannot physically address: an invalid address for the address bus, hence the name. In modern use on ...
is raised instead, though these are not always distinguished. At the operating system level, this fault is caught and a signal is passed on to the offending process, activating the process's handler for that signal. Different operating systems have different signal names to indicate that a segmentation fault has occurred. On
Unix-like A Unix-like (sometimes referred to as UN*X or *nix) operating system is one that behaves in a manner similar to a Unix system, although not necessarily conforming to or being certified to any version of the Single UNIX Specification. A Unix-li ...
operating systems, a signal called SIGSEGV (abbreviated from ''segmentation violation'') is sent to the offending process. On
Microsoft Windows Windows is a group of several proprietary graphical operating system families developed and marketed by Microsoft. Each family caters to a certain sector of the computing industry. For example, Windows NT for consumers, Windows Server for ...
, the offending process receives a STATUS_ACCESS_VIOLATION exception.


Causes

The conditions under which segmentation violations occur and how they manifest themselves are specific to hardware and the operating system: different hardware raises different faults for given conditions, and different operating systems convert these to different signals that are passed on to processes. The proximate cause is a memory access violation, while the underlying cause is generally a
software bug A software bug is an error, flaw or fault in the design, development, or operation of computer software that causes it to produce an incorrect or unexpected result, or to behave in unintended ways. The process of finding and correcting bugs i ...
of some sort. Determining the
root cause Root cause may refer to: * "Root Cause" (''Person of Interest''), a TV show episode * Root cause analysis In science and engineering, root cause analysis (RCA) is a method of problem solving used for identifying the root causes of faults or pro ...
debugging In computer programming and software development, debugging is the process of finding and resolving '' bugs'' (defects or problems that prevent correct operation) within computer programs, software, or systems. Debugging tactics can involve i ...
the bug – can be simple in some cases, where the program will consistently cause a segmentation fault (e.g., dereferencing a
null pointer In computing, a null pointer or null reference is a value saved for indicating that the pointer or reference does not refer to a valid object. Programs routinely use null pointers to represent conditions such as the end of a list of unknown leng ...
), while in other cases the bug can be difficult to reproduce and depend on memory allocation on each run (e.g., dereferencing a dangling pointer). The following are some typical causes of a segmentation fault: * Attempting to access a nonexistent memory address (outside process's address space) * Attempting to access memory the program does not have rights to (such as kernel structures in process context) * Attempting to write read-only memory (such as code segment) These in turn are often caused by programming errors that result in invalid memory access: * Dereferencing a
null pointer In computing, a null pointer or null reference is a value saved for indicating that the pointer or reference does not refer to a valid object. Programs routinely use null pointers to represent conditions such as the end of a list of unknown leng ...
, which usually points to an address that's not part of the process's address space * Dereferencing or assigning to an uninitialized pointer ( wild pointer, which points to a random memory address) * Dereferencing or assigning to a freed pointer ( dangling pointer, which points to memory that has been freed/deallocated/deleted) * A buffer overflow * A stack overflow * Attempting to execute a program that does not compile correctly. (Some compilers will output an executable file despite the presence of compile-time errors.) In C code, segmentation faults most often occur because of errors in pointer use, particularly in C dynamic memory allocation. Dereferencing a null pointer, which results in undefined behavior, will usually cause a segmentation fault. This is because a null pointer cannot be a valid memory address. On the other hand, wild pointers and dangling pointers point to memory that may or may not exist, and may or may not be readable or writable, and thus can result in transient bugs. For example: char *p1 = NULL; // Null pointer char *p2; // Wild pointer: not initialized at all. char *p3 = malloc(10 * sizeof(char)); // Initialized pointer to allocated memory // (assuming malloc did not fail) free(p3); // p3 is now a dangling pointer, as memory has been freed Dereferencing any of these variables could cause a segmentation fault: dereferencing the null pointer generally will cause a segfault, while reading from the wild pointer may instead result in random data but no segfault, and reading from the dangling pointer may result in valid data for a while, and then random data as it is overwritten.


Handling

The default action for a segmentation fault or bus error is abnormal termination of the process that triggered it. A
core file In computing, a core dump, memory dump, crash dump, storage dump, system dump, or ABEND dump consists of the recorded state of the working memory of a computer program at a specific time, generally when the program has crashed or otherwise term ...
may be generated to aid debugging, and other platform-dependent actions may also be performed. For example,
Linux Linux ( or ) is a family of open-source Unix-like operating systems based on the Linux kernel, an operating system kernel first released on September 17, 1991, by Linus Torvalds. Linux is typically packaged as a Linux distribution, whi ...
systems using the grsecurity patch may log SIGSEGV signals in order to monitor for possible intrusion attempts using buffer overflows. On some systems, like Linux and Windows, it is possible for the program itself to handle a segmentation fault. Depending on the architecture and operating system, the running program can not only handle the event but may extract some information about its state like getting a
stack trace In computing, a stack trace (also called stack backtrace or stack traceback) is a report of the active stack frames at a certain point in time during the execution of a program. When a program is run, memory is often dynamically allocated in two ...
,
processor register A processor register is a quickly accessible location available to a computer's processor. Registers usually consist of a small amount of fast storage, although some registers have specific hardware functions, and may be read-only or write-only. ...
values, the line of the source code when it was triggered, memory address that was invalidly accessed and whether the action was a read or a write. Although a segmentation fault generally means that the program has a bug that needs fixing, it is also possible to intentionally cause such failure for the purposes of testing, debugging and also to emulate platforms where direct access to memory is needed. On the latter case, the system must be able to allow the program to run even after the fault occurs. In this case, when the system allows, it is possible to handle the event and increment the processor program counter to "jump" over the failing instruction to continue the execution.


Examples


Writing to read-only memory

Writing to read-only memory raises a segmentation fault. At the level of code errors, this occurs when the program writes to part of its own code segment or the read-only portion of the data segment, as these are loaded by the OS into read-only memory. Here is an example of ANSI C code that will generally cause a segmentation fault on platforms with memory protection. It attempts to modify a string literal, which is undefined behavior according to the ANSI C standard. Most
compiler In computing, a compiler is a computer program that translates computer code written in one programming language (the ''source'' language) into another language (the ''target'' language). The name "compiler" is primarily used for programs tha ...
s will not catch this at compile time, and instead compile this to executable code that will crash: int main(void) When the program containing this code is compiled, the string "hello world" is placed in the
rodata In computing, a data segment (often denoted .data) is a portion of an object file or the corresponding address space of a program that contains initialized static variables, that is, global variables and static local variables. The size of thi ...
section of the program executable file: the read-only section of the data segment. When loaded, the operating system places it with other strings and constant data in a read-only segment of memory. When executed, a variable, ''s'', is set to point to the string's location, and an attempt is made to write an ''H'' character through the variable into the memory, causing a segmentation fault. Compiling such a program with a compiler that does not check for the assignment of read-only locations at compile time, and running it on a Unix-like operating system produces the following
runtime error In computer science, runtime, run time, or execution time is the final phase of a computer programs life cycle, in which the code is being executed on the computer's central processing unit (CPU) as machine code. In other words, "runtime" is ...
: $ gcc segfault.c -g -o segfault $ ./segfault Segmentation fault Backtrace of the core file from GDB: Program received signal SIGSEGV, Segmentation fault. 0x1c0005c2 in main () at segfault.c:6 6 *s = 'H'; This code can be corrected by using an array instead of a character pointer, as this allocates memory on stack and initializes it to the value of the string literal: char s[] = "hello world"; s[0] = 'H'; // equivalently, *s = 'H'; Even though string literals should not be modified (this has undefined behavior in the C standard), in C they are of static char [] type, so there is no implicit conversion in the original code (which points a char * at that array), while in C++ they are of static const char [] type, and thus there is an implicit conversion, so compilers will generally catch this particular error.


Null pointer dereference

In C and C-like languages,
null pointer In computing, a null pointer or null reference is a value saved for indicating that the pointer or reference does not refer to a valid object. Programs routinely use null pointers to represent conditions such as the end of a list of unknown leng ...
s are used to mean "pointer to no object" and as an error indicator, and dereferencing a null pointer (a read or write through a null pointer) is a very common program error. The C standard does not say that the null pointer is the same as the pointer to memory address 0, though that may be the case in practice. Most operating systems map the null pointer's address such that accessing it causes a segmentation fault. This behavior is not guaranteed by the C standard. Dereferencing a null pointer is undefined behavior in C, and a conforming implementation is allowed to assume that any pointer that is dereferenced is not null. int *ptr = NULL; printf("%d", *ptr); This sample code creates a
null pointer In computing, a null pointer or null reference is a value saved for indicating that the pointer or reference does not refer to a valid object. Programs routinely use null pointers to represent conditions such as the end of a list of unknown leng ...
, and then tries to access its value (read the value). Doing so causes a segmentation fault at runtime on many operating systems. Dereferencing a null pointer and then assigning to it (writing a value to a non-existent target) also usually causes a segmentation fault: int *ptr = NULL; *ptr = 1; The following code includes a null pointer dereference, but when compiled will often not result in a segmentation fault, as the value is unused and thus the dereference will often be optimized away by dead code elimination: int *ptr = NULL; *ptr;


Buffer overflow

The following code accesses the character array s beyond its upper boundary. Depending on the compiler and the processor, this may result in a segmentation fault. char s[] = "hello world"; char c = s[20];


Stack overflow

Another example is recursion without a base case: int main(void) which causes the stack to overflow which results in a segmentation fault.What is the difference between a segmentation fault and a stack overflow?
at Stack Overflow Infinite recursion may not necessarily result in a stack overflow depending on the language, optimizations performed by the compiler and the exact structure of a code. In this case, the behavior of unreachable code (the return statement) is undefined, so the compiler can eliminate it and use a tail call optimization that might result in no stack usage. Other optimizations could include translating the recursion into iteration, which given the structure of the example function would result in the program running forever, while probably not overflowing its stack.


See also

* General protection fault * Storage violation * Guru Meditation


References


External links


Process: focus boundary and segmentation fault


* ttp://c-faq.com/null/null1.html A "null pointer" explained
Answer to: NULL is guaranteed to be 0, but the null pointer is not?


{{Error messages Articles with example C code Computer errors Memory management