Computing is usually defined as the activity of using and improving computer hardware and software. It is the computer-specific part of information technology...
A computer program is a sequence of instructions written to perform a specified task with a computer. A computer requires programs to function, typically executing the program's instructions in a central processor. The program has an executable form that the computer can use directly to execute...
In computer science, a subroutine is a portion of code within a larger program that performs a specific task and is relatively independent of the remaining code....
is called reentrant if it can be interrupted in the middle of its execution and then safely called again ("re-entered") before its previous invocations complete executing. The interruption could be caused by an internal action such as a jump or call or by an external action such as a hardware interrupt or signal
Signal (computing)
A signal is a limited form of inter-process communication used in Unix, Unix-like, and other POSIX-compliant operating systems. Essentially it is an asynchronous notification sent to a process in order to notify it of an event that occurred. When a signal is sent to a process, the operating system...
. Once the reentered invocation completes, the previous invocations will resume correct execution.
This definition originates from single-threaded programming environments where the flow of control could be interrupted by a hardware interrupt and transferred to an interrupt service routine (ISR). Any subroutine used by the ISR that could potentially have been executing when the interrupt was triggered should be reentrant. Often, subroutines accessible via the operating system kernel are in fact not reentrant. Hence, interrupt service routines are limited in the actions they can perform and usually restricted from accessing the file system or even from allocating memory.
A subroutine that is directly or indirectly recursive should be reentrant. This policy is partially enforced by structured programming languages. However a subroutine can fail to be reentrant if it relies on a global variable to remain unchanged but that variable is modified when the subroutine is recursively invoked.
The definition of reentrancy originated in single-threaded environments and differs from that of thread-safety in multi-threaded environments. A reentrant subroutine can achieve thread-safety, but this condition alone might not be sufficient in all situations. Conversely, thread-safe code does not necessarily have to be reentrant (see below for examples).
Example
This is an example of a swap function which fails to be reentrant (as well as thread-safe). As such, it should not have been used in the interrupt service routine isr:
int t;
swap could be made thread-safe by making t thread-local. It still fails to be reentrant and this will continue to cause problems if isr is called in the same context as a thread already executing swap.
The following, somewhat contrived, modification of the swap function, which is careful to leave the global data in a consistent state at the time it exits is perfectly reentrant, but not thread-safe. because it does not ensure the global data is in a consistent state during execution:
int t;
void swap(int *x, int *y)
{
int s;
s = t; // save global variable
t = *x;
*x = *y;
// hardware interrupt might invoke isr here!
*y = t;
t = s; // restore global variable
}
Idempotence is the property of certain operations in mathematics and computer science, that they can be applied multiple times without changing the result beyond the initial application...
(meaning that the function may be called more than once, yet generate exactly the same output as if it had only been called once). Generally speaking, a function produces output data based on some input data (though both are optional, in general). Shared data could be accessed by anybody at any time. If data can be changed by anybody (and nobody keeps track of those changes) then there's no guarantee for those who share a datum whether that datum is the same as at any time before. Idempotence implies reentrancy, but the converse is not necessarily true.
In computer programming, a global variable is a variable that is accessible in every scope . Interaction mechanisms with global variables are called global environment mechanisms...
(outside the scope of any function and with an indefinite extent) or local (created each time a function is called and destroyed upon exit) scope.
Local data are not shared by any, re-entering or not, routines; therefore they don't affect re-entrance. Global data are either shared by any function, called global variable
Global variable
In computer programming, a global variable is a variable that is accessible in every scope . Interaction mechanisms with global variables are called global environment mechanisms...
s, or shared by all functions of the same name, called static variable
Static variable
In computer programming, a static variable is a variable that has been allocated statically — whose lifetime extends across the entire run of the program...
In computer programming, a static variable is a variable that has been allocated statically — whose lifetime extends across the entire run of the program...
(or global) non-constant data.
Reentrant functions can use global data to work with. For example, a reentrant interrupt service routine could grab a piece of hardware status to work with (e.g. serial port read buffer) which is not only global, but volatile. Still typical use of static variables and global data is not advised, in the sense of no non-atomic-read-modify-write instructions should be used in these variables
In computer science, self-modifying code is code that alters its own instructions while it is executing - usually to reduce the instruction path length and improve performance or simply to reduce otherwise repetitively similar code, thus simplifying maintenance...
.
The operating system might allow a process to modify its code. There are various reasons for this (blitting graphics quickly, ignorance of OS programmers) but the fact is that code might not be the same next time.
It may modify itself if it resides in its own unique memory. That is, if each new invocation uses a different physical machine code location where a copy of the original code is made, it will not affect other invocations even if it then modifies itself during execution of that particular thread).
A computer program is a sequence of instructions written to perform a specified task with a computer. A computer requires programs to function, typically executing the program's instructions in a central processor. The program has an executable form that the computer can use directly to execute...
In computer science, a subroutine is a portion of code within a larger program that performs a specific task and is relatively independent of the remaining code....
A priority queue is an abstract data type in computer programming.It is exactly like a regular queue or stack data structure, but additionally, each element is associated with a "priority"....
Multiprocessing is the use of two or more central processing units within a single computer system. The term also refers to the ability of a system to support more than one processor and/or the ability to allocate tasks between them...
usually complicate the control of reentrant code. It is important to keep track of any access and or side effects that are done inside a routine designed to be reentrant.
In computer science, functional programming is a programming paradigm that treats computation as the evaluation of mathematical functions and avoids state and mutable data. It emphasizes the application of functions, in contrast to the imperative programming style, which emphasizes changes in state...
Recursion in computer science is a method where the solution to a problem depends on solutions to smaller instances of the same problem. The approach can be applied to many types of problems, and is one of the central ideas of computer science....
need to be reentrant.
Also, subroutines that are directly or indirectly called from an interrupt handler must to be reentrant if there is need to service an interrupt before the previous is already served.
An interrupt handler, also known as an interrupt service routine , is a callback subroutine in microcontroller firmware, operating system or device driver whose execution is triggered by the reception of an interrupt...
that re-enables interrupts early in the interrupt handler. This may reduce interrupt latency
Interrupt latency
In real-time operating systems, interrupt latency is the time between the generation of an interrupt by a device and the servicing of the device which generated the interrupt. For many operating systems, devices are serviced as soon as the device's interrupt handler is executed...
.
In general, while programming interrupt service routines, it is recommended to re-enable interrupts as soon as possible in the interrupt handler. This helps to avoid losing interrupts.
C is a general-purpose computer programming language developed between 1969 and 1973 by Dennis Ritchie at the Bell Telephone Laboratories for use with the Unix operating system....
In computer programming, a global variable is a variable that is accessible in every scope . Interaction mechanisms with global variables are called global environment mechanisms...
g_var; thus, if two threads execute it and access g_var concurrently, then the result varies depending on the timing of the execution. Hence, f is not reentrant. Neither is g; it calls f, which is not reentrant.
C is a general-purpose computer programming language developed between 1969 and 1973 by Dennis Ritchie at the Bell Telephone Laboratories for use with the Unix operating system....
code, the function is thread-safe, but not reentrant
int function
{
mutex_lock;
...
function body
...
mutex_unlock;
}
In the above, function can be called by different threads without any problem. But if the function is used in a reentrant interrupt handler and a second interrupt arises inside the function, the second routine will hang forever. As interrupt servicing can disable other interrupts, the whole system could suffer.
Relation to thread safety
This concept is distinct from, but closely related to, thread-safe
Thread-safe
Thread safety is a computer programming concept applicable in the context of multi-threaded programs. A piece of code is thread-safe if it only manipulates shared data structures in a thread-safe manner, which enables safe execution by multiple threads at the same time...
Thread safety is a computer programming concept applicable in the context of multi-threaded programs. A piece of code is thread-safe if it only manipulates shared data structures in a thread-safe manner, which enables safe execution by multiple threads at the same time...
and still not reentrant. For example, a function could be wrapped all around with a mutex (which avoids problems in multi-threading environments), and if that function is used as a reentrant function in an interrupt service routine, it could starve waiting for the first execution to release the mutex. The key for avoiding confusion is that reentrant refers to only ONE thread executing. It is a concept from the time when no multi-tasking operating systems existed.
EE Times is an electronics industry newspaper published in the USA by UBM Electronics, a division of United Business Media. Launched in 1972 by Gerard G. Leeds of CMP Publishing. CMP was acquired by United in 1999...
