In computing, a segmentation fault or access violation is a fault, or failure condition, raised by hardware with memory protection, notifying an operating system the software has attempted to access a restricted area of memory. On standard x86 computers, this is a form of general protection fault; the OS kernel will, in response perform some corrective action 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, but otherwise the OS default signal handler is used causing abnormal termination of the process, 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, they arise due to errors in use of pointers for virtual memory addressing illegal access. Another type of memory access error is a bus error, which has various causes, but is today much rarer. Many programming languages may employ mechanisms designed to avoid segmentation faults and improve memory safety.
For example, the Rust programming language which appeared in 2010 employs an'Ownership' based model to ensure memory safety, garbage collection has been employed since around 1960, which avoids certain classes of memory errors which could lead to segmentation faults. A segmentation fault occurs when a program attempts to access a memory location that it is not allowed to access, or attempts to access a memory location in a way, not allowed; the term "segmentation" has various uses in computing. With memory protection, only the program's own address space is readable, of this, only the stack 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 to provide virtual memory, 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, an invalid page fault leads to a segmentation fault, segmentation faults and page faults are both faults raised by the virtual memory management system. Segmentation faults can occur independently of page faults: illegal access to a valid page is a segmentation fault, but not an invalid page fault, segmentation faults can occur in the middle of a page, for example in a buffer overflow that stays within a page but illegally overwrites memory. At the hardware level, the fault is raised by the memory management unit on illegal access, as part of its memory protection feature, or an invalid page fault. If the problem is not an invalid logical address but instead an invalid physical address, a bus error 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 operating systems, a signal called. On Microsoft Windows, the offending process receives a STATUS_ACCESS_VIOLATION exception; 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, 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 a software bug of some sort. Determining the root cause – debugging the bug – can be simple in some cases, where the program will cause a segmentation fault, while in other cases the bug can be difficult to reproduce and depend on memory allocation on each run; the following are some typical causes of a segmentation fault: Attempting to access a nonexistent memory address Attempting to access memory the program does not have rights to Attempting to write read-only memory These in turn are caused by programming errors that result in invalid memory access: Dereferencing a null pointer, which points to an address that's not part of the process's address space Dereferencing or assigning to an uninitialized pointer Dereferencing or assigning to a freed pointer A buffer overflow A stack overflow Attempting to execute a program that does not compile correctly.
(Some compilers will output an exe
In computer science, control flow is the order in which individual statements, instructions or function calls of an imperative program are executed or evaluated. The emphasis on explicit control flow distinguishes an imperative programming language from a declarative programming language. Within an imperative programming language, a control flow statement is a statement, the execution of which results in a choice being made as to which of two or more paths to follow. For non-strict functional languages and language constructs exist to achieve the same result, but they are not termed control flow statements. A set of statements is in turn structured as a block, which in addition to grouping defines a lexical scope. Interrupts and signals are low-level mechanisms that can alter the flow of control in a way similar to a subroutine, but occur as a response to some external stimulus or event, rather than execution of an in-line control flow statement. At the level of machine language or assembly language, control flow instructions work by altering the program counter.
For some central processing units, the only control flow instructions available are conditional or unconditional branch instructions termed jumps. The kinds of control flow statements supported by different languages vary, but can be categorized by their effect: Continuation at a different statement Executing a set of statements only if some condition is met Executing a set of statements zero or more times, until some condition is met Executing a set of distant statements, after which the flow of control returns Stopping the program, preventing any further execution A label is an explicit name or number assigned to a fixed position within the source code, which may be referenced by control flow statements appearing elsewhere in the source code. A label marks a position within source code, has no other effect. Line numbers are an alternative to a named label, that are whole numbers placed at the start of each line of text in the source code. Languages which use these impose the constraint that the line numbers must increase in value in each following line, but may not require that they be consecutive.
For example, in BASIC: In other languages such as C and Ada, a label is an identifier appearing at the start of a line and followed by a colon. For example, in C: The language ALGOL 60 allowed both whole numbers and identifiers as labels, but few if any other ALGOL variants allowed whole numbers. Early Fortran compilers only allowed whole numbers as labels. Beginning with Fortran-90, alphanumeric labels have been allowed; the goto statement is the most basic form of unconditional transfer of control. Although the keyword may either be in upper or lower case depending on the language, it is written as: goto label The effect of a goto statement is to cause the next statement to be executed to be the statement appearing at the indicated label. Goto statements have been considered harmful by many computer scientists, notably Dijkstra; the terminology for subroutines varies. In the 1950s, computer memories were small by current standards so subroutines were used to reduce program size. A piece of code was written once and used many times from various other places in a program.
Today, subroutines are more used to help make a program more structured, e.g. by isolating some algorithm or hiding some data access method. If many programmers are working on one program, subroutines are one kind of modularity that can help divide the work. In structured programming, the ordered sequencing of successive commands is considered one of the basic control structures, used as a building block for programs alongside iteration and choice. In May 1966, Böhm and Jacopini published an article in Communications of the ACM which showed that any program with gotos could be transformed into a goto-free form involving only choice and loops with duplicated code and/or the addition of Boolean variables. Authors showed that choice can be replaced by loops; that such minimalism is possible does not mean that it is desirable. What Böhm and Jacopini's article showed was that all programs could be goto-free. Other research showed that control structures with one entry and one exit were much easier to understand than any other form because they could be used anywhere as a statement without disrupting the control flow.
In other words, they were composable. Some academics took a purist approach to the Böhm-Jacopini result and argued that instructions like break and return from the middle of loops are bad practice as they are not needed in the Böhm-Jacopini proof, th
C99 is an informal name for ISO/IEC 9899:1999, a past version of the C programming language standard. It extends the previous version with new features for the language and the standard library, helps implementations make better use of available computer hardware, such as IEEE 754-1985 floating-point arithmetic, compiler technology; the C11 version of the C programming language standard, published in 2011, replaces C99. After ANSI produced the official standard for the C programming language in 1989, which became an international standard in 1990, the C language specification remained static for some time, while C++ continued to evolve during its own standardization effort. Normative Amendment 1 created a new standard for C in 1995, but only to correct some details of the 1989 standard and to add more extensive support for international character sets; the standard underwent further revision in the late 1990s, leading to the publication of ISO/IEC 9899:1999 in 1999, adopted as an ANSI standard in May 2000.
The language defined by that version of the standard is referred to as "C99". The international C standard is maintained by the working group ISO/IEC JTC1/SC22/WG14. C99 is, for the most part, backward compatible with C89. In particular, a declaration that lacks a type specifier no longer has int implicitly assumed; the C standards committee decided that it was of more value for compilers to diagnose inadvertent omission of the type specifier than to silently process legacy code that relied on implicit int. In practice, compilers are to display a warning assume int and continue translating the program. C99 introduced several new features, many of, implemented as extensions in several compilers: inline functions intermingled declarations and code: variable declaration is no longer restricted to file scope or the start of a compound statement, facilitating static single assignment form several new data types, including long long int, optional extended integer types, an explicit boolean data type, a complex type to represent complex numbers variable-length arrays flexible array members support for one-line comments beginning with //, as in BCPL, C++ and Java new library functions, such as snprintf new headers, such as <stdbool.h>, <complex.h>, <tgmath.h>, <inttypes.h> type-generic math functions, in <tgmath.h>, which select a math library function based upon float, double, or long double arguments, etc. improved support for IEEE floating point designated initializers compound literals support for variadic macros restrict qualification allows more aggressive code optimization, removing compile-time array access advantages held by FORTRAN over ANSI C universal character names, which allows user variables to contain other characters than the standard character set keyword static in array indices in parameter declarationsParts of the C99 standard are included in the current version of the C++ standard, including integer types and library functions.
Variable-length arrays are not among these included parts because C++'s Standard Template Library includes similar functionality. A major feature of C99 is its numerics support, in particular its support for access to the features of IEEE 754-1985 floating-point hardware present in the vast majority of modern processors. Platforms without IEEE 754 hardware can implement it in software. On platforms with IEEE 754 floating point: float is defined as IEEE 754 single precision, double is defined as double precision, long double is defined as IEEE 754 extended precision, or some form of quad precision where available; the four arithmetic operations and square root are rounded as defined by IEEE 754. Expression evaluation is defined to be performed in one of three well-defined methods, indicating whether floating-point variables are first promoted to a more precise format in expressions: FLT_EVAL_METHOD == 2 indicates that all internal intermediate computations are performed by default at high precision where available, FLT_EVAL_METHOD == 1 performs all internal intermediate expressions in double precision, while FLT_EVAL_METHOD == 0 specifies each operation is evaluated only at the precision of the widest operand of each operator.
The intermediate result type for operands of a given precision are summarized in the adjacent table. FLT_EVAL_METHOD == 2 tends to limit the risk of rounding errors affecting numerically unstable expressions and is the designed default method for x87 hardware, but yields unintuitive behavior for the unwary user. Before C99, compilers could round intermediate results inconsistently when using x87 floating-point hardware, leading to compiler-specific behaviour.