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62-477: A global interpreter lock (GIL) is a mutual-exclusion lock held by a programming language interpreter thread to avoid sharing code that is not thread-safe with other threads. In implementations with a GIL, there is always one GIL for each interpreter process . Applications running on implementations with a GIL can be designed to use separate processes to achieve full parallelism, as each process has its own interpreter and in turn has its own GIL. Otherwise,

124-405: A byte and a word are primitives). A data aggregate (or just aggregate ) is a group of primitives that are logically contiguous in memory and that are viewed collectively as one datum (for instance, an aggregate could be 3 logically contiguous bytes, the values of which represent the 3 coordinates of a point in space). When an aggregate is entirely composed of the same type of primitive,

186-412: A is later accessed again, its new value will be 8. This example may be clearer if memory is examined directly. Assume that a is located at address 0x8130 in memory and ptr at 0x8134; also assume this is a 32-bit machine such that an int is 32-bits wide. The following is what would be in memory after the following code snippet is executed: (The NULL pointer shown here is 0x00000000.) By assigning

248-420: A race condition ) can be avoided by using the requirement of mutual exclusion to ensure that simultaneous updates to the same part of the list cannot occur. The term mutual exclusion is also used in reference to the simultaneous writing of a memory address by one thread while the aforementioned memory address is being manipulated or read by one or more other threads. The problem which mutual exclusion addresses

310-432: A GIL. An example of an interpreted language without a GIL is Tcl , which is used in the benchmarking tool HammerDB . Mutual exclusion In computer science , mutual exclusion is a property of concurrency control , which is instituted for the purpose of preventing race conditions . It is the requirement that one thread of execution never enters a critical section while a concurrent thread of execution

372-724: A consistent interface, some architectures provide memory-mapped I/O , which allows some addresses to refer to units of memory while others refer to device registers of other devices in the computer. There are analogous concepts such as file offsets, array indices, and remote object references that serve some of the same purposes as addresses for other types of objects. Pointers are directly supported without restrictions in languages such as PL/I , C , C++ , Pascal , FreeBASIC , and implicitly in most assembly languages . They are primarily used for constructing references , which in turn are fundamental to constructing nearly all data structures , as well as in passing data between different parts of

434-491: A form of manual memory segmentation , and share many of its advantages and disadvantages. A two-byte offset, containing a 16-bit, unsigned integer, can be used to provide relative addressing for up to 64 KiB (2 bytes) of a data structure. This can easily be extended to 128, 256 or 512 KiB if the address pointed to is forced to be aligned on a half-word, word or double-word boundary (but, requiring an additional "shift left" bitwise operation —by 1, 2 or 3 bits—in order to adjust

496-565: A function. Pointers can also be used to allocate and deallocate dynamic variables and arrays in memory. Since a variable will often become redundant after it has served its purpose, it is a waste of memory to keep it, and therefore it is good practice to deallocate it (using the original pointer reference) when it is no longer needed. Failure to do so may result in a memory leak (where available free memory gradually, or in severe cases rapidly, diminishes because of an accumulation of numerous redundant memory blocks). The basic syntax to define

558-408: A higher-priority thread waits for a lower-priority thread; and high latency, in which response to interrupts is not prompt. Much research is aimed at eliminating the above effects, often with the goal of guaranteeing non-blocking progress . No perfect scheme is known. Blocking system calls used to sleep an entire process. Until such calls became threadsafe , there was no proper mechanism for sleeping

620-400: A level of indirection: A pointer's value determines which memory address (that is, which datum) is to be used in a calculation. Because indirection is a fundamental aspect of algorithms, pointers are often expressed as a fundamental data type in programming languages ; in statically (or strongly ) typed programming languages, the type of a pointer determines the type of the datum to which

682-401: A location in memory, and obtaining the value stored at that location is known as dereferencing the pointer. As an analogy, a page number in a book's index could be considered a pointer to the corresponding page; dereferencing such a pointer would be done by flipping to the page with the given page number and reading the text found on that page. The actual format and content of a pointer variable

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744-417: A location in shared memory, and since the operation is atomic, only one process can set the flag at a time. Any process that is unsuccessful in setting the flag can either go on to do other tasks and try again later, release the processor to another process and try again later, or continue to loop while checking the flag until it is successful in acquiring it. Preemption is still possible, so this method allows

806-443: A non-canonical pointer is dereferenced, the processor raises a general protection fault . On the other hand, some systems have more units of memory than there are addresses. In this case, a more complex scheme such as memory segmentation or paging is employed to use different parts of the memory at different times. The last incarnations of the x86 architecture support up to 36 bits of physical memory addresses, which were mapped to

868-404: A pointer as an arrow that points to a specific spot in a computer's memory, allowing you to interact with the data stored at that location. A memory pointer (or just pointer ) is a primitive, the value of which is intended to be used as a memory address; it is said that a pointer points to a memory address . It is also said that a pointer points to a datum [in memory] when the pointer's value

930-475: A pointer be explicitly initialized to the null pointer value, which is traditionally specified in C with the standardized macro NULL : Dereferencing a null pointer in C produces undefined behavior , which could be catastrophic. However, most implementations simply halt execution of the program in question, usually with a segmentation fault . However, initializing pointers unnecessarily could hinder program analysis, thereby hiding bugs. In any case, once

992-443: A pointer has been declared, the next logical step is for it to point at something: This assigns the value of the address of a to ptr . For example, if a is stored at memory location of 0x8130 then the value of ptr will be 0x8130 after the assignment. To dereference the pointer, an asterisk is used again: This means take the contents of ptr (which is 0x8130), "locate" that address in memory and set its value to 8. If

1054-415: A pointer is: This declares ptr as the identifier of an object of the following type: This is usually stated more succinctly as " ptr is a pointer to int ." Because the C language does not specify an implicit initialization for objects of automatic storage duration, care should often be taken to ensure that the address to which ptr points is valid; this is why it is sometimes suggested that

1116-420: A pointer to the desired data element in the array. In other data structures, such as linked lists , pointers are used as references to explicitly tie one piece of the structure to another. Pointers are used to pass parameters by reference. This is useful if the programmer wants a function's modifications to a parameter to be visible to the function's caller. This is also useful for returning multiple values from

1178-414: A pointer whose value is not a valid memory address could cause a program to crash (or contain invalid data). To alleviate this potential problem, as a matter of type safety , pointers are considered a separate type parameterized by the type of data they point to, even if the underlying representation is an integer. Other measures may also be taken (such as validation and bounds checking ), to verify that

1240-405: A problem in concurrent programming control", which is credited as the first topic in the study of concurrent algorithms. A simple example of why mutual exclusion is important in practice can be visualized using a singly linked list of four items, where the second and third are to be removed. The removal of a node that sits between two other nodes is performed by changing the next pointer of

1302-437: A process is already performing write operation on a data object [critical section] no other process/thread is allowed to access/modify the same object until the first process has finished writing upon the data object [critical section] and released the object for other processes to read and write upon. The requirement of mutual exclusion was first identified and solved by Edsger W. Dijkstra in his seminal 1965 paper "Solution of

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1364-413: A process's critical section. This will prevent any interrupt service routines from running (effectively preventing a process from being preempted ). Although this solution is effective, it leads to many problems. If a critical section is long, then the system clock will drift every time a critical section is executed because the timer interrupt is no longer serviced, so tracking time is impossible during

1426-538: A program. In functional programming languages that rely heavily on lists, data references are managed abstractly by using primitive constructs like cons and the corresponding elements car and cdr , which can be thought of as specialised pointers to the first and second components of a cons-cell. This gives rise to some of the idiomatic "flavour" of functional programming. By structuring data in such cons-lists , these languages facilitate recursive means for building and processing data—for example, by recursively accessing

1488-546: A single thread within a process (see polling ). Pointer (computer programming) I do consider assignment statements and pointer variables to be among computer science's "most valuable treasures." Donald Knuth , Structured Programming, with go to Statements In computer science , a pointer is an object in many programming languages that stores a memory address . This can be that of another value located in computer memory , or in some cases, that of memory-mapped computer hardware . A pointer references

1550-429: A solution built with a test&set register can possibly lead to the starvation of some processes which become caught in the trying section. In fact, Ω ( n ) {\displaystyle \Omega ({\sqrt {n}})} distinct memory states are required to avoid lockout. To avoid unbounded waiting, n distinct memory states are required. Most algorithms for mutual exclusion are designed with

1612-516: A usable sequential index and then to an absolute address without a lookup table . In C, array indexing is formally defined in terms of pointer arithmetic; that is, the language specification requires that array[i] be equivalent to *(array + i) . Thus in C, arrays can be thought of as pointers to consecutive areas of memory (with no gaps), and the syntax for accessing arrays is identical for that which can be used to dereference pointers. For example, an array array can be declared and used in

1674-468: A very large array . The system would then also provide an operation to retrieve the value stored in the memory unit at a given address (usually utilizing the machine's general-purpose registers ). In the usual case, a pointer is large enough to hold more addresses than there are units of memory in the system. This introduces the possibility that a program may attempt to access an address which corresponds to no unit of memory, either because not enough memory

1736-468: Is a problem of resource sharing: how can a software system control multiple processes' access to a shared resource, when each process needs exclusive control of that resource while doing its work? The mutual-exclusion solution to this makes the shared resource available only while the process is in a specific code segment called the critical section . It controls access to the shared resource by controlling each mutual execution of that part of its program where

1798-478: Is already accessing said critical section, which refers to an interval of time during which a thread of execution accesses a shared resource or shared memory . The shared resource is a data object , which two or more concurrent threads are trying to modify (where two concurrent read operations are permitted but, no two concurrent write operations or one read and one write are permitted, since it leads to data inconsistency). Mutual exclusion algorithms ensure that if

1860-412: Is dependent on the underlying computer architecture . Using pointers significantly improves performance for repetitive operations, like traversing iterable data structures (e.g. strings , lookup tables , control tables , linked lists , and tree structures). In particular, it is often much cheaper in time and space to copy and dereference pointers than it is to copy and access the data to which

1922-487: Is installed (i.e. beyond the range of available memory) or the architecture does not support such addresses. The first case may, in certain platforms such as the Intel x86 architecture, be called a segmentation fault (segfault). The second case is possible in the current implementation of AMD64 , where pointers are 64 bit long and addresses only extend to 48 bits. Pointers must conform to certain rules (canonical addresses), so if

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1984-511: Is necessary to have pointers to help manage how the structure is implemented and controlled. Typical examples of pointers are start pointers, end pointers, and stack pointers. These pointers can either be absolute (the actual physical address or a virtual address in virtual memory ) or relative (an offset from an absolute start address ("base") that typically uses fewer bits than a full address, but will usually require one additional arithmetic operation to resolve). Relative addresses are

2046-416: Is the datum's memory address. More generally, a pointer is a kind of reference , and it is said that a pointer references a datum stored somewhere in memory ; to obtain that datum is to dereference the pointer . The feature that separates pointers from other kinds of reference is that a pointer's value is meant to be interpreted as a memory address, which is a rather low-level concept. References serve as

2108-592: Is then used to change the pointers in the linked list during the insertion of a new node. Only one process can be successful in its CAS; all other processes attempting to add a node at the same time will have to try again. Each process can then keep a local copy of the data structure, and upon traversing the linked list, can perform each operation from the list on its local copy. In addition to hardware-supported solutions, some software solutions exist that use busy waiting to achieve mutual exclusion. Examples include: These algorithms do not work if out-of-order execution

2170-433: Is used and a thread tries to acquire an already acquired lock, the operating system could suspend the thread using a context switch and swap it out with another thread that is ready to be run, or could put that processor into a low power state if there is no other thread that can be run. Therefore, most modern mutual exclusion methods attempt to reduce latency and busy-waits by using queuing and context switches. However, if

2232-429: Is used on the platform that executes them. Programmers have to specify strict ordering on the memory operations within a thread. It is often preferable to use synchronization facilities provided by an operating system 's multithreading library, which will take advantage of hardware solutions if possible but will use software solutions if no hardware solutions exist. For example, when the operating system's lock library

2294-442: The sizeof operator differs. In this example, sizeof(array) will evaluate to 5*sizeof(int) (the size of the array), while sizeof(ptr) will evaluate to sizeof(int*) , the size of the pointer itself. Default values of an array can be declared like: If array is located in memory starting at address 0x1000 on a 32-bit little-endian machine then memory will contain the following (values are in hexadecimal , like

2356-405: The next pointer of node i – 1 to point to node i + 1 , while another thread of execution changes the next pointer of node i to point to node i + 2 . Although both removal operations complete successfully, the desired state of the linked list is not achieved: node i + 1 remains in the list, because the next pointer of node i – 1 points to node i + 1 . This problem (called

2418-532: The 32-bit linear address space through the PAE paging mechanism. Thus, only 1/16 of the possible total memory may be accessed at a time. Another example in the same computer family was the 16-bit protected mode of the 80286 processor, which, though supporting only 16 MB of physical memory, could access up to 1 GB of virtual memory, but the combination of 16-bit address and segment registers made accessing more than 64 KB in one data structure cumbersome. In order to provide

2480-462: The GIL can be a significant barrier to parallelism. Reasons for employing a global interpreter lock include: A way to get around a GIL is creating a separate interpreter per thread, which is too expensive with most languages. Use of a global interpreter lock in a language effectively limits the amount of parallelism reachable through concurrency of a single interpreter process with multiple threads. If

2542-486: The address of a to ptr : yields the following memory values: Then by dereferencing ptr by coding: the computer will take the contents of ptr (which is 0x8130), 'locate' that address, and assign 8 to that location yielding the following memory: Clearly, accessing a will yield the value of 8 because the previous instruction modified the contents of a by way of the pointer ptr . When setting up data structures like lists , queues and trees, it

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2604-403: The aggregate may be called an array ; in a sense, a multi-byte word primitive is an array of bytes, and some programs use words in this way. A pointer is a programming concept used in computer science to reference or point to a memory location that stores a value or an object. It is essentially a variable that stores the memory address of another variable or data structure rather than storing

2666-619: The assumption that no failure occurs while a process is running inside the critical section. However, in reality such failures may be commonplace. For example, a sudden loss of power or faulty interconnect might cause a process in a critical section to experience an unrecoverable error or otherwise be unable to continue. If such a failure occurs, conventional, non-failure-tolerant mutual exclusion algorithms may deadlock or otherwise fail key liveness properties. To deal with this problem, several solutions using crash-recovery mechanisms have been proposed. The solutions explained above can be used to build

2728-665: The concepts appeared in the June 1967 issue of CACM entitled: PL/I List Processing. According to the Oxford English Dictionary , the word pointer first appeared in print as a stack pointer in a technical memorandum by the System Development Corporation . In computer science , a pointer is a kind of reference . A data primitive (or just primitive ) is any datum that can be read from or written to computer memory using one memory access (for instance, both

2790-448: The critical section. Also, if a process halts during its critical section, control will never be returned to another process, effectively halting the entire system. A more elegant method for achieving mutual exclusion is the busy-wait . Busy-waiting is effective for both uniprocessor and multiprocessor systems. The use of shared memory and an atomic test-and-set instruction provide the mutual exclusion. A process can test-and-set on

2852-474: The data itself. Pointers are commonly used in programming languages that support direct memory manipulation, such as C and C++. They allow programmers to work with memory directly, enabling efficient memory management and more complex data structures. By using pointers, you can access and modify data located in memory, pass data efficiently between functions, and create dynamic data structures like linked lists, trees, and graphs. In simpler terms, you can think of

2914-406: The following manner: This allocates a block of five integers and names the block array , which acts as a pointer to the block. Another common use of pointers is to point to dynamically allocated memory from malloc which returns a consecutive block of memory of no less than the requested size that can be used as an array. While most operators on arrays and pointers are equivalent, the result of

2976-440: The head and tail elements of lists of lists; e.g. "taking the car of the cdr of the cdr". By contrast, memory management based on pointer dereferencing in some approximation of an array of memory addresses facilitates treating variables as slots into which data can be assigned imperatively . When dealing with arrays, the critical lookup operation typically involves a stage called address calculation which involves constructing

3038-509: The offset by a factor of 2, 4 or 8, before its addition to the base address). Generally, though, such schemes are a lot of trouble, and for convenience to the programmer absolute addresses (and underlying that, a flat address space ) is preferred. A one byte offset, such as the hexadecimal ASCII value of a character (e.g. X'29') can be used to point to an alternative integer value (or index) in an array (e.g., X'01'). In this way, characters can be very efficiently translated from ' raw data ' to

3100-437: The pointer points. Pointers are a very thin abstraction on top of the addressing capabilities provided by most modern architectures . In the simplest scheme, an address , or a numeric index , is assigned to each unit of memory in the system, where the unit is typically either a byte or a word – depending on whether the architecture is byte-addressable or word-addressable – effectively transforming all of memory into

3162-458: The pointer to be manipulated (arithmetically via pointer arithmetic ) as a memory address, as opposed to a magic cookie or capability which does not allow such. Because pointers allow both protected and unprotected access to memory addresses , there are risks associated with using them, particularly in the latter case. Primitive pointers are often stored in a format similar to an integer ; however, attempting to dereference or "look up" such

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3224-536: The pointer variable contains a value that is both a valid memory address and within the numerical range that the processor is capable of addressing. In 1955, Soviet Ukrainian computer scientist Kateryna Yushchenko created the Address programming language that made possible indirect addressing and addresses of the highest rank – analogous to pointers. This language was widely used on the Soviet Union computers. However, it

3286-720: The pointers point. Pointers are also used to hold the addresses of entry points for called subroutines in procedural programming and for run-time linking to dynamic link libraries (DLLs) . In object-oriented programming , pointers to functions are used for binding methods , often using virtual method tables . A pointer is a simple, more concrete implementation of the more abstract reference data type . Several languages, especially low-level languages , support some type of pointer, although some have more restrictions on their use than others. While "pointer" has been used to refer to references in general, it more properly applies to data structures whose interface explicitly allows

3348-432: The previous node to point to the next node (in other words, if node i is being removed, then the next pointer of node i – 1 is changed to point to node i + 1 , thereby removing from the linked list any reference to node i ). When such a linked list is being shared between multiple threads of execution, two threads of execution may attempt to remove two different nodes simultaneously, one thread of execution changing

3410-450: The process is almost purely made up of interpreted code and does not make calls outside of the interpreter which block for long periods of time (allowing the GIL to be released by that thread while they process), there is likely to be very little increase in speed when running the process on a multiprocessor machine. Due to signaling with a CPU-bound thread, it can cause a significant slowdown, even on single processors. More seriously, when

3472-417: The resource would be used. A successful solution to this problem must have at least these two properties: Deadlock freedom can be expanded to implement one or both of these properties: Every process's program can be partitioned into four sections, resulting in four states. Program execution cycles through these four states in order: If a process wishes to enter the critical section, it must first execute

3534-612: The single native thread calls a blocking OS process (such as disk access), the entire process is blocked, even though other application threads may be waiting. Some language implementations that implement a global interpreter lock are CPython , the most widely-used implementation of Python , and Ruby MRI , the reference implementation of Ruby (where it is called Global VM Lock). JVM -based equivalents of these languages ( Jython and JRuby ) do not use global interpreter locks. IronPython and IronRuby are implemented on top of Microsoft 's Dynamic Language Runtime and also avoid using

3596-435: The synchronization primitives below: Many forms of mutual exclusion have side-effects. For example, classic semaphores permit deadlocks , in which one process gets a semaphore, another process gets a second semaphore, and then both wait till the other semaphore to be released. Other common side-effects include starvation , in which a process never gets sufficient resources to run to completion; priority inversion , in which

3658-411: The system to continue to function—even if a process halts while holding the lock. Several other atomic operations can be used to provide mutual exclusion of data structures; most notable of these is compare-and-swap (CAS). CAS can be used to achieve wait-free mutual exclusion for any shared data structure by creating a linked list where each node represents the desired operation to be performed. CAS

3720-411: The time that is spent suspending a thread and then restoring it can be proven to be always more than the time that must be waited for a thread to become ready to run after being blocked in a particular situation, then spinlocks are an acceptable solution (for that situation only). One binary test&set register is sufficient to provide the deadlock-free solution to the mutual exclusion problem. But

3782-419: The trying section and wait until it acquires access to the critical section. After the process has executed its critical section and is finished with the shared resources, it needs to execute the exit section to release them for other processes' use. The process then returns to its non-critical section. On uni-processor systems, the simplest solution to achieve mutual exclusion is to disable interrupts during

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3844-528: Was unknown outside the Soviet Union and usually Harold Lawson is credited with the invention, in 1964, of the pointer. In 2000, Lawson was presented the Computer Pioneer Award by the IEEE "[f]or inventing the pointer variable and introducing this concept into PL/I, thus providing for the first time, the capability to flexibly treat linked lists in a general-purpose high-level language". His seminal paper on

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