What is "runtime"?

Question

I have heard about things like "C Runtime", "Visual C++ 2008 Runtime", ".NET Common Language Runtime", etc.

• What is "runtime" exactly?
• What is it made of?
• How does it interact with my code? Or maybe more precisely, how is my code controlled by it?

When coding assembly language on Linux, I could use the INT instruction to make the system call. So, is the runtime nothing but a bunch of pre-fabricated functions that wrap the low level function into more abstract and high level functions? But doesn't this seem more like the definition for the library, not for the runtime?

Are "runtime" and "runtime library" two different things?

ADD 1

These days, I am thinking maybe Runtime has something in common with the so called Virtual Machine, such as JVM. Here's the quotation that leads to such thought:

This compilation process is sufficiently complex to be broken into several layers of abstraction, and these usually involve three translators: a compiler, a virtual machine implementation, and an assembler. --- The Elements of Computing Systems (Introduction, The Road Down To Hardware Land)

ADD 2

The book Expert C Programming: Deep C Secrets. Chapter 6 Runtime Data Structures is an useful reference to this question.

ADD 3 - 7:31 AM 2/28/2021

Here's some of my perspective after getting some knowledge about processor design. The whole computer thing is just multiple levels of abstraction. It goes from elementary transistors all the way up to the running program. For any level N of abstraction, its runtime is the immediate level N-1 of abstraction that goes below it. And it is God that give us the level 0 of abstraction.

Sclerotomy

Th runtime contains the runtime library plus some control code and and some state (supplied by the OS). – Marlee

I found another post on a sister site that may be useful: programmers.stackexchange.com/questions/294346/… – Arruda

I always thing of it like a sandbox to lower level infrastructure. You have the Task Manager/Or processes on Unix kit, all the low level GUI libraries etc . All that is part of the runtime. The foundations on which things are built. – Senator

Can't upvote enough that last update. – Simpatico

Answer 1

Runtime describes software/instructions that are executed while your program is running, especially those instructions that you did not write explicitly, but are necessary for the proper execution of your code.

Low-level languages like C have very small (if any) runtime. More complex languages like Objective-C, which allows for dynamic message passing, have a much more extensive runtime.

You are correct that runtime code is library code, but library code is a more general term, describing the code produced by any library. Runtime code is specifically the code required to implement the features of the language itself.

Ironworker

I think "especially those instructions that you did not write explicitly" is not a helpful distinction; whether you yourself wrote a line of code, or the compiler brought it in from a library that someone else wrote, has nothing to do with the distinction of compile time vs runtime. – Germain

@BenWheeler this answer is not about the "run-time"(opposite of compile-time). Note that in the context of the OP's question and all these answers, the 'runtime' and 'run-time' are different, even though people just use the same term 'runtime' for these different concepts. Indeed the user(the developer who uses the language) does not write the runtime, which is a part of the language implementation. – Duroc

Answer 2

Runtime is a general term that refers to any library, framework, or platform that your code runs on.

The C and C++ runtimes are collections of functions.

The .NET runtime contains an intermediate language interpreter, a garbage collector, and more.

Vesiculate

The C++ standard library does not only contain functions, and refering to Matt Ball, the C and C++ "runtime" are runtime libraries, the .NET runtime is a runtime library and a runtime system. – Olvera

My question is that static libraries are runtime too? i guess the term is used mostly for shared objects, isn't it? consider libc in linux. And what about platforms and frameworks? they too are broken down into libraries? – Drawl

But how can my code actually run on any "library, framework, or platform"? It should run on the CPU or other processing unit. Could you provide an additional level of detail for more clarification? – Debutant

The C and C++ runtimes are collections of functions. -> What functions? – Bargeboard

@KorayTugay: The functions in the standard library, like cout or printf. – Vesiculate

as a .NET dev this explanation made more sense to me +1 – Coating

@Vesiculate so is the standard c library is c runtime ? – Kolyma

Is the Python interpreter part of its runtime? – Johnsonian

Answer 3

The runtime or execution environment is the part of a language implementation which executes code and is present at run-time; the compile-time part of the implementation is called the translation environment in the C standard.

Examples:

• the Java runtime consists of the virtual machine and the standard library

• a common C runtime consists of the loader (which is part of the operating system) and the runtime library, which implements the parts of the C language which are not built into the executable by the compiler; in hosted environments, this includes most parts of the standard library

Scriptorium

Thanks, I think this is the best answer in this topic. – Bargeboard

I appreciate naming "execution environment" but I think that using just "runtime" as alias for the former is a bad idea (which is from my observations what many programmers do), "runtime environment" is more accurate. I agree with @MichałTrybus that "runtime" means "the time when the program is run". – Devote

What is a hosted environment? I searched it but only found some web-related stuff. – Duroc

Answering my own question: see this comment. - Actually, the C standards describe two types of C environments - "freestanding" and "hosted" - and, in the hosted environments, the functions described in the standards are defined as available. In embedded systems, the C environment is typically freestanding, so that you may not have the library routines, or you may be able to avoid using some and use your own replacements. – Duroc

Adding compile-time entity in the answer make it perfectly clear. Comparing "runtime" with "compile-time" leaves no doubts what the runtime is, because compilation is obvious. It is a great answer. I even wrote it down on a paper. Thank you a lot. – Ulric

The highlighted words in the first sentence of this answer ("The runtime or execution environment is the part of a language implementation which executes code and is present at run-time") are confusing me. Really, in the case of C language the code is executed by CPU, but not by the C runtime library. And in the case of Java language the code is executed by Java Virtual Machine, but not by the Java standard library. – Rigdon

Answer 4

As per Wikipedia: runtime library/run-time system.

In computer programming, a runtime library is a special program library used by a compiler, to implement functions built into a programming language, during the runtime (execution) of a computer program. This often includes functions for input and output, or for memory management.

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A run-time system (also called runtime system or just runtime) is software designed to support the execution of computer programs written in some computer language. The run-time system contains implementations of basic low-level commands and may also implement higher-level commands and may support type checking, debugging, and even code generation and optimization. Some services of the run-time system are accessible to the programmer through an application programming interface, but other services (such as task scheduling and resource management) may be inaccessible.

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Re: your edit, "runtime" and "runtime library" are two different names for the same thing.

Perigynous

Wikipedia is not a good source for quotes. It is a good location to start looking for authoritative sources but itself should not be considered authoritative and as such quoting from it is not reliable. (As per the statements by Mr Wales the creator of Wikipedia). – Marlee

What does "during the runtime" in the first quote (supposedly about runtime library) refer to? An easy [mis]interpretation could be the following: "while the program is executing, the compiler, in parallel, uses a library (for itself) to implement (ie. generate additional code at runtime) "promised" (by the language spec) functions for the program". This may be false and/or easily bent with def of a run-time system. – Debutant

To continue from where Loki left, especially when neither of the articles provide any reference (that is not a quote, it's just something I thought I'd better say) – Sidelight

@MartinYork, not only is an encyclopedia not authoritative, it also focus on documentation and not explanation. Those can be very different. – Marandamarasca

Answer 5

I'm not crazy about the other answers here; they're too vague and abstract for me. I think more in stories. Here's my attempt at a better answer.

a BASIC example

Let's say it's 1985 and you write a short BASIC program on an Apple II:

] 10 PRINT "HELLO WORLD!"
] 20 GOTO 10

So far, your program is just source code. It's not running, and we would say there is no "runtime" involved with it.

But now I run it:

] RUN

How is it actually running? How does it know how to send the string parameter from PRINT to the physical screen? I certainly didn't provide any system information in my code, and PRINT itself doesn't know anything about my system.

Instead, RUN is actually a program itself -- its code tells it how to parse my code, how to execute it, and how to send any relevant requests to the computer's operating system. The RUN program provides the "runtime" environment that acts as a layer between the operating system and my source code. The operating system itself acts as part of this "runtime", but we usually don't mean to include it when we talk about a "runtime" like the RUN program.

Types of compilation and runtime

Compiled binary languages

In some languages, your source code must be compiled before it can be run. Some languages compile your code into machine language -- it can be run by your operating system directly. This compiled code is often called "binary" (even though every other kind of file is also in binary :).

In this case, there is still a minimal "runtime" involved -- but that runtime is provided by the operating system itself. The compile step means that many statements that would cause your program to crash are detected before the code is ever run.

C is one such language; when you run a C program, it's totally able to send illegal requests to the operating system (like, "give me control of all of the memory on the computer, and erase it all"). If an illegal request is hit, usually the OS will just kill your program and not tell you why, and dump the contents of that program's memory at the time it was killed to a .dump file that's pretty hard to make sense of. But sometimes your code has a command that is a very bad idea, but the OS doesn't consider it illegal, like "erase a random bit of memory this program is using"; that can cause super weird problems that are hard to get to the bottom of.

Bytecode languages

Other languages (e.g. Java, Python) compile your code into a language that the operating system can't read directly, but a specific runtime program can read your compiled code. This compiled code is often called "bytecode".

The more elaborate this runtime program is, the more extra stuff it can do on the side that your code did not include (even in the libraries you use) -- for instance, the Java runtime environment ("JRE") and Python runtime environment can keep track of memory assignments that are no longer needed, and tell the operating system it's safe to reuse that memory for something else, and it can catch situations where your code would try to send an illegal request to the operating system, and instead exit with a readable error.

All of this overhead makes them slower than compiled binary languages, but it makes the runtime powerful and flexible; in some cases, it can even pull in other code after it starts running, without having to start over. The compile step means that many statements that would cause your program to crash are detected before the code is ever run; and the powerful runtime can keep your code from doing stupid things (e.g., you can't "erase a random bit of memory this program is using").

Scripting languages

Still other languages don't precompile your code at all; the runtime does all of the work of reading your code line by line, interpreting it and executing it. This makes them even slower than "bytecode" languages, but also even more flexible; in some cases, you can even fiddle with your source code as it runs! Though it also means that you can have a totally illegal statement in your code, and it could sit there in your production code without drawing attention, until one day it is run and causes a crash.

These are generally called "scripting" languages; they include Javascript, Perl, and PHP. Some of these provide cases where you can choose to compile the code to improve its speed (e.g., Javascript's WebAssembly project). So Javascript can allow users on a website to see the exact code that is running, since their browser is providing the runtime.

This flexibility also allows for innovations in runtime environments, like node.js, which is both a code library and a runtime environment that can run your Javascript code as a server, which involves behaving very differently than if you tried to run the same code on a browser.

Germain

Thanks for providing a perspective from different types of languages. It's very informative and enlightening. – Sclerotomy

Good answer. Just one clarification: Python uses bytecode opensource.com/article/18/4/introduction-python-bytecode – Zirconia

Very informative.. thank tou – Simpatico

Loved this answer—this is what I was really looking for when I googled the question! It's useful to see the interaction of a concept with its context. – Melanesian

Thank you for this answer. Really helped to make sense of what runtime actually does – Casualty

Answer 6

In my understanding runtime is exactly what it means - the time when the program is run. You can say something happens at runtime / run time or at compile time.

I think runtime and runtime library should be (if they aren't) two separate things. "C runtime" doesn't seem right to me. I call it "C runtime library".

Answers to your other questions: I think the term runtime can be extended to include also the environment and the context of the program when it is run, so:

• it consists of everything that can be called "environment" during the time when the program is run, for example other processes, state of the operating system and used libraries, state of other processes, etc
• it doesn't interact with your code in a general sense, it just defines in what circumstances your code works, what is available to it during execution.

This answer is to some extend just my opinion, not a fact or definition.

Torry

Answer 7

Matt Ball answered it correctly. I would say about it with examples.

Consider running a program compiled in Turbo-Borland C/C++ (version 3.1 from the year 1991) compiler and let it run under a 32-bit version of windows like Win 98/2000 etc.

It's a 16-bit compiler. And you will see all your programs have 16-bit pointers. Why is it so when your OS is 32bit? Because your compiler has set up the execution environment of 16 bit and the 32-bit version of OS supported it.

What is commonly called as JRE (Java Runtime Environment) provides a Java program with all the resources it may need to execute.

Actually, runtime environment is brain product of idea of Virtual Machines. A virtual machine implements the raw interface between hardware and what a program may need to execute. The runtime environment adopts these interfaces and presents them for the use of the programmer. A compiler developer would need these facilities to provide an execution environment for its programs.

Courses

A virtual machine implements the "raw" interface between hardware and what a program may need to execute. -> I am not sure about this. – Bargeboard

Answer 8

Run time exactly where your code comes into life and you can see lot of important thing your code do.

Runtime has a responsibility of allocating memory , freeing memory , using operating system's sub system like (File Services, IO Services.. Network Services etc.)

Your code will be called "WORKING IN THEORY" until you practically run your code. and Runtime is a friend which helps in achiving this.

Contract

Answer 9

a runtime could denote the current phase of program life (runtime / compile time / load time / link time) or it could mean a runtime library, which form the basic low level actions that interface with the execution environment. or it could mean a runtime system, which is the same as an execution environment.

in the case of C programs, the runtime is the code that sets up the stack, the heap etc. which a requirement expected by the C environment. it essentially sets up the environment that is promised by the language. (it could have a runtime library component, crt0.lib or something like that in case of C)

Armilla

Answer 10

Runtime basically means when program interacts with the hardware and operating system of a machine. C does not have it's own runtime but instead, it requests runtime from an operating system (which is basically a part of ram) to execute itself.

Gallion

Answer 11

I found that the following folder structure makes a very insightful context for understanding what runtime is:

You can see that there is the 'source', there is the 'SDK' or 'Software Development Kit' and then there is the Runtime, eg. the stuff that gets run - at runtime. It's contents are like:

The win32 zip contains .exe -s and .dll -s.

So eg. the C runtime would be the files like this -- C runtime libraries, .so-s or .dll -s -- you run at runtime, made special by their (or their contents' or purposes') inclusion in the definition of the C language (on 'paper'), then implemented by your C implementation of choice. And then you get the runtime of that implementation, to use it and to build upon it.

That is, with a little polarisation, the runnable files that the users of your new C-based program will need. As a developer of a C-based program, so do you, but you need the C compiler and the C library headers, too; the users don't need those.

Debutant

another one to contextualize runtime, is this unrelated article python myths's section myth 6, where you can see the distinction between runtime and programming language, in the first paragraph of myth 6. – Debutant

Answer 12

If my understanding from reading the above answers is correct, Runtime is basically 'background processes' such as garbage collection, memory-allocation, basically any processes that are invoked indirectly, by the libraries / frameworks that your code is written in, and specifically those processes that occur after compilation, while the application is running.

Sik

Answer 13

The fully qualified name of Runtime seems to be the additional environment to provide programming language-related functions required at run time for non-web application software.

Runtime implements programming language-related functions, which remain the same to any application domain, including math operations, memory operations, messaging, OS or DB abstraction service, etc.

The runtime must in some way be connected with the running applications to be useful, such as being loaded into application memory space as a shared dynamic library, a virtual machine process inside which the application runs, or a service process communicating with the application.

Samsun

Answer 14

Runtime is somewhat opposite to design-time and compile-time/link-time. Historically it comes from slow mainframe environment where machine-time was expensive.

Gretna

This doesn't answer the question & further complicates it. – Frisket