Specifically, I'm talking more about C++ and Rust than others. I don't understand how C++ has a "runtime" in the sense that Java and C# have a runtime--while Java and C# run on top of a virtual machine with its own encapsulated abstractions and such, I don't get how C++ might have one.
Take virtual tables for C++, for example. Do we consider dynamic_cast<type> a part of C++'s runtime functionality or are we talking about C++'s structure for vtables in general? Can we consider new and delete a part of the C++ runtime environment? What exactly constitutes a runtime?
For example, here we have a Rust article on its own runtime, which describes it as :
The Rust runtime can be viewed as a collection of code which enables
services like I/O, task spawning, TLS, etc. It's essentially an
ephemeral collection of objects which enable programs to perform
common tasks more easily.
But is this not the function of a standard library or language features, not an actual runtime? What constitutes this very thin but existent runtime? Even Bjarne expresses his thoughts that C++ has "zero-overhead abstraction", but if C++ has a runtime, does this not imply that C++ does indeed have some sort of "backend" code to orchestrate its own very light but still existent abstractions?
TL;DR: What is a runtime and/or runtime environment in the context of languages like C++ and Rust that have supposedly "zero-overhead" and don't have "heavy" runtimes like Java or C#?
Edit: I suspect that I'm just missing something about semantics here...
C++ requires a few things that aren't required in something like C.
For example, it typically involves some overhead for exception handling. Although it may not be strictly required, most systems have at least a tiny bit of a top-level exception handler to tell you that the program shut down if an exception was thrown but not caught anywhere.
It's open to question whether it qualifies as "runtime environment", but the compiler also generates code to search up the stack and find a handler for a particular exception when one is thrown.
On one hand, this is exceptionally tiny (bordering on negligible) compared to something like a complete JVM. On the other hand, it's quite large and complex relative to what happens by default in something like a JVM or Microsoft's CLR.
As to zero overhead...well, it depends a bit on your viewpoint. Exception handling code can normally be moved out of the main stream of the code, so it doesn't impose any overhead in terms of execution speed as long as no exception is thrown. It does, however, require extra code so there can be (often is) quite a bit of overhead if you look at executable sizes. Just for example, doing a quick look at a "hello world" program, it looks like turning off exception handling reduces the executable size by about 2 kilobytes with VC++.
Admittedly, 2K isn't a whole lot of extra code--on the other hand, that's just what's added to essentially the most trivial program humanly possible. For a program that actually does something, it's undoubtedly more.
In the end, it's not enough that most people really have a reason to care, but it does exist nonetheless.
As to how this is handled, it involves a combination of code that's linked in from the standard library and code generated by the compiler (but the exact details vary with the implementation--for example, most 32-bit Windows compilers used Microsoft's Structured Exception Handling (in which case the operating system provides part of the code) but for 64-bit Windows, I believe all of them deal with exception handling on their own (which increases executable sizes more, but reduces overhead in terms of speed).
I am trying to use the MinGW GCC toolchain on XP with some vendor code from an embedded project that accesses high memory (>0xFFFF0000) which is, I believe, beyond the virtual mem address space allowed in 'civilian' processes in XP.
I want to handle the memory access exceptions myself in some way that will permit execution to continue at the instruction following the exception, ie ignore it. Is there some way to do it with MinGW? Or with MS toolchain?
The vastly simplified picture is thus:
/////////////
// MyFile.c
MyFunc(){
VendorFunc_A();
}
/////////////////
// VendorFile.c
VendorFunc_A(){
VendorFunc_DoSomeDesirableSideEffect();
VendorFunc_B();
VendorFunc_DoSomeMoreGoodStuff();
}
VendorFunc_B(){
int *pHW_Reg = 0xFFFF0000;
*pHW_Reg = 1; // Mem Access EXCEPTION HERE
return(0); // I want to continue here
}
More detail:
I am developing an embedded project on an Atmel AVR32 platform with freeRTOS using the AVR32-gcc toolchain. It is desirable to develop/debug high level application code independent of the hardware (and the slow avr32 simulator). Various gcc, makefile and macro tricks permit me to build my Avr32/freeRTOS project in the MinGW/Win32 freeRTOS port enviroment and I can debug in eclipse/gdb. But the high-mem HW access in the (vendor supplied) Avr32 code crashes the MinGW exe (due to the mem access exception).
I am contemplating some combination of these approaches:
1) Manage the access exceptions in SW. Ideally I'd be creating a kind of HW simulator but that'd be difficult and involve some gnarly assembly code, I think. Alot of the exceptions can likely just be ignored.
2) Creating a modified copy of the Avr32 header files so as to relocate the HW register #defines into user process address space (and create some structs and linker sections that commmit those areas of virtual memory space)
3) Conditional compilation of function calls that result in highMem/HW access, or alernatively more macro tricks, so as to minimize code cruft in the 'real' HW target code. (There are other developers on this project.)
Any suggestions or helpful links would be appreciated.
This page is on the right track, but seems overly complicated, and is C++ which I'd like to avoid. But I may try it yet, absent other suggestions.
http://www.programmingunlimited.net/siteexec/content.cgi?page=mingw-seh
You need to figure out why the vendor code wants to write 1 to address 0xFFFF0000 in the first place, and then write a custom VendorFunc_B() function that emulates this behavior. It is likely that 0xFFFF0000 is a hardware register that will do something special when written to (eg. change baud rate on a serial port or power up the laser or ...). When you know what will happen when you write to this register on the target hardware, you can rewrite the vendor code to do something appropriate in the windows code (eg. write the string "Starting laser" to a log file). It is safe to assume that writing 1 to address 0xFFFF0000 on Windows XP will not be the right thing to do, and the Windows XP memory protection system detects this and terminates your program.
I had a similar issue recently, and this is the solution i settled on:
Trap memory accesses inside a standard executable built with MinGW
First of all, you need to find a way to remap those address ranges (maybe some undef/define combos) to some usable memory. If you can't do this, maybe you can hook through a seg-fault and handle the write yourself.
I also use this to "simulate" some specific HW behavior inside a single executable, for some already written code. However, in my case, i found a way to redefine early all the register access macros.
This is what wikipedia says:
In computer software, an application
binary interface (ABI) describes the
low-level interface between an
application (or any type of) program
and the operating system or another
application.
ABIs cover details such as data type,
size, and alignment; the calling
convention, which controls how
functions' arguments are passed and
return values retrieved; the system
call numbers and how an application
should make system calls to the
operating system; and in the case of a
complete operating system ABI, the
binary format of object files, program
libraries and so on. A complete ABI,
such as the Intel Binary Compatibility
Standard (iBCS), allows a program
from one operating system supporting
that ABI to run without modifications
on any other such system, provided
that necessary shared libraries are
present, and similar prerequisites are
fulfilled.
I guess that an ABI is a convention or standard, and compilers/linkers use this convention to produce object codes. Is that right? If so who made these conventions(companies or some organization)? What was it like when there was no ABIs? Is there documents about these ABIs that we can refer to?
You're correct about the definition of an ABI, up to a point. The classic example is the syscall interface in Linux (and other UNIXes).
They are a standard way for code to request the operating system to carry out certain duties.
As such, they're decided by the people that wrote the OS or, in the case where the syscalls have been added later, by whoever added them (in cases where the OS allows this). For example, the Linux syscall interface on x86 states that you load the syscall number into eax, with other parameters placed in ebx, ecx and so on, depending on the syscall you're making (eax).
Typically, it's not the compiler or linker which do the work of interfacing, rather it's the libraries provided for the language you're using.
Returning to Linux, the GNU C libraries contain code for fopen (for example) which eventually call the relevant syscall to perform the lower level tasks (syscall number 5, open). A list of the syscalls can be found in this PDF file.
Specification is more suitable term than convention, as convention is loose term for widely accepted practice whereas specification is well-defined.
You are right. The specification is made by standardization body. Take a look at POSIX specification which is supported by Windows and compiler/build tool-chains such as gcc assume OS's to adhere by it, and even Linux kernel partially (almost exactly) adheres to it.
Before ABIs? Even today, firmware is hand-crafted as new chips come along for set-top boxes and such other devices having embedded systems.
The documentation is digital logic content in the data-sheet for the chips to be programmed by assembly language and for higher-level language, the cross-compiler tool-chain documentation gives away the assumptions that should be part of ABI.
Well, the concept of ABI was presumably conceived to support the binary compatibility of your program on other operating systems and machine architectures. So, lets suppose that you wrote a program on some operating system distribution running on x86 architecture. Now, for a programmer the most important thing is that this program that you wrote on your machine should be able to run exactly the same on any other machine running on same or different architecture lets say for the sake of discussion that the other machine is running on i386 architecture and this is where the concept of ABI or Application Binary Interfaces comes in. As every machine architecture defines its own way in which the operating system kernal talks to the outside world i.e user-space programs, hence every architecture defines a different set of system calls, machine registers, how those registers are used, how are software interrupts handled by the kernal and so on. ABI is the thing that handles these things for you like compiling, linking, byte ordering and so on. System programmers have had hard luck defining a uniform ABI for same operating systems running on different architectures and that is why every machine architecture has its own and you need to compile your programs in order to confirm to the format those machines have.
What is the difference between run-time and compile-time?
The difference between compile time and run time is an example of what pointy-headed theorists call the phase distinction. It is one of the hardest concepts to learn, especially for people without much background in programming languages. To approach this problem, I find it helpful to ask
What invariants does the program satisfy?
What can go wrong in this phase?
If the phase succeeds, what are the postconditions (what do we know)?
What are the inputs and outputs, if any?
Compile time
The program need not satisfy any invariants. In fact, it needn't be a well-formed program at all. You could feed this HTML to the compiler and watch it barf...
What can go wrong at compile time:
Syntax errors
Typechecking errors
(Rarely) compiler crashes
If the compiler succeeds, what do we know?
The program was well formed---a meaningful program in whatever language.
It's possible to start running the program. (The program might fail immediately, but at least we can try.)
What are the inputs and outputs?
Input was the program being compiled, plus any header files, interfaces, libraries, or other voodoo that it needed to import in order to get compiled.
Output is hopefully assembly code or relocatable object code or even an executable program. Or if something goes wrong, output is a bunch of error messages.
Run time
We know nothing about the program's invariants---they are whatever the programmer put in. Run-time invariants are rarely enforced by the compiler alone; it needs help from the programmer.
What can go wrong are run-time errors:
Division by zero
Dereferencing a null pointer
Running out of memory
Also there can be errors that are detected by the program itself:
Trying to open a file that isn't there
Trying find a web page and discovering that an alleged URL is not well formed
If run-time succeeds, the program finishes (or keeps going) without crashing.
Inputs and outputs are entirely up to the programmer. Files, windows on the screen, network packets, jobs sent to the printer, you name it. If the program launches missiles, that's an output, and it happens only at run time :-)
I think of it in terms of errors, and when they can be caught.
Compile time:
string my_value = Console.ReadLine();
int i = my_value;
A string value can't be assigned a variable of type int, so the compiler knows for sure at compile time that this code has a problem
Run time:
string my_value = Console.ReadLine();
int i = int.Parse(my_value);
Here the outcome depends on what string was returned by ReadLine(). Some values can be parsed to an int, others can't. This can only be determined at run time
Compile-time: the time period in which you, the developer, are compiling your code.
Run-time: the time period which a user is running your piece of software.
Do you need any clearer definition?
(edit: the following applies to C# and similar, strongly-typed programming languages. I'm not sure if this helps you).
For example, the following error will be detected by the compiler (at compile time) before you run a program and will result in a compilation error:
int i = "string"; --> error at compile-time
On the other hand, an error like the following can not be detected by the compiler. You will receive an error/exception at run-time (when the program is run).
Hashtable ht = new Hashtable();
ht.Add("key", "string");
// the compiler does not know what is stored in the hashtable
// under the key "key"
int i = (int)ht["key"]; // --> exception at run-time
Translation of source code into stuff-happening-on-the-[screen|disk|network] can occur in (roughly) two ways; call them compiling and interpreting.
In a compiled program (examples are c and fortran):
The source code is fed into another program (usually called a compiler--go figure), which produces an executable program (or an error).
The executable is run (by double clicking it, or typing it's name on the command line)
Things that happen in the first step are said to happen at "compile time", things that happen in the second step are said to happen at "run time".
In an interpreted program (example MicroSoft basic (on dos) and python (I think)):
The source code is fed into another program (usually called an interpreter) which "runs" it directly. Here the interpreter serves as an intermediate layer between your program and the operating system (or the hardware in really simple computers).
In this case the difference between compile time and run time is rather harder to pin down, and much less relevant to the programmer or user.
Java is a sort of hybrid, where the code is compiled to bytecode, which then runs on a virtual machine which is usually an interpreter for the bytecode.
There is also an intermediate case in which the program is compiled to bytecode and run immediately (as in awk or perl).
Basically if your compiler can work out what you mean or what a value is "at compile time" it can hardcode this into the runtime code. Obviously if your runtime code has to do a calculation every time it will run slower, so if you can determine something at compile time it is much better.
Eg.
Constant folding:
If I write:
int i = 2;
i += MY_CONSTANT;
The compiler can perform this calulation at compile time because it knows what 2 is, and what MY_CONSTANT is. As such it saves itself from performing a calculation every single execution.
Hmm, ok well, runtime is used to describe something that occurs when a program is running.
Compile time is used to describe something that occurs when a program is being built (usually, by a compiler).
Compile Time:
Things that are done at compile time incur (almost) no cost when the resulting program is run, but might incur a large cost when you build the program.
Run-Time:
More or less the exact opposite. Little cost when you build, more cost when the program is run.
From the other side; If something is done at compile time, it runs only on your machine and if something is run-time, it run on your users machine.
Relevance
An example of where this is important would be a unit carrying type. A compile time version (like Boost.Units or my version in D) ends up being just as fast as solving the problem with native floating point code while a run-time version ends up having to pack around information about the units that a value are in and perform checks in them along side every operation. On the other hand, the compile time versions requiter that the units of the values be known at compile time and can't deal with the case where they come from run-time input.
As an add-on to the other answers, here's how I'd explain it to a layman:
Your source code is like the blueprint of a ship. It defines how the ship should be made.
If you hand off your blueprint to the shipyard, and they find a defect while building the ship, they'll stop building and report it to you immediately, before the ship has ever left the drydock or touched water. This is a compile-time error. The ship was never even actually floating or using its engines. The error was found because it prevented the ship even being made.
When your code compiles, it's like the ship being completed. Built and ready to go. When you execute your code, that's like launching the ship on a voyage. The passengers are boarded, the engines are running and the hull is on the water, so this is runtime. If your ship has a fatal flaw that sinks it on its maiden voyage (or maybe some voyage after for extra headaches) then it suffered a runtime error.
Following from previous similar answer of question What is the difference between run-time error and compiler error?
Compilation/Compile time/Syntax/Semantic errors: Compilation or compile time errors are error occurred due to typing mistake, if we do not follow the proper syntax and semantics of any programming language then compile time errors are thrown by the compiler. They wont let your program to execute a single line until you remove all the syntax errors or until you debug the compile time errors.
Example: Missing a semicolon in C or mistyping int as Int.
Runtime errors: Runtime errors are the errors that are generated when the program is in running state. These types of errors will cause your program to behave unexpectedly or may even kill your program. They are often referred as Exceptions.
Example: Suppose you are reading a file that doesn't exist, will result in a runtime error.
Read more about all programming errors here
Here is a quote from Daniel Liang, author of 'Introduction to JAVA programming', on the subject of compilation:
"A program written in a high-level language is called a source program or source code. Because a computer cannot execute a source program, a source program must be translated into machine code for execution. The translation can be done using another programming tool called an interpreter or a compiler." (Daniel Liang, "Introduction to JAVA programming", p8).
...He Continues...
"A compiler translates the entire source code into a machine-code file, and the machine-code file is then executed"
When we punch in high-level/human-readable code this is, at first, useless! It must be translated into a sequence of 'electronic happenings' in your tiny little CPU! The first step towards this is compilation.
Simply put: a compile-time error happens during this phase, while a run-time error occurs later.
Remember: Just because a program is compiled without error does not mean it will run without error.
A Run-time error will occur in the ready, running or waiting part of a programs life-cycle while a compile-time error will occur prior to the 'New' stage of the life cycle.
Example of a Compile-time error:
A Syntax Error - how can your code be compiled into machine level instructions if they are ambiguous?? Your code needs to conform 100% to the syntactical rules of the language otherwise it cannot be compiled into working machine code.
Example of a run-time error:
Running out of memory - A call to a recursive function for example might lead to stack overflow given a variable of a particular degree! How can this be anticipated by the compiler!? it cannot.
And that is the difference between a compile-time error and a run-time error
For example: In a strongly typed language, a type could be checked at compile time or at runtime. At compile time it means, that the compiler complains if the types are not compatible. At runtime means, that you can compile your program just fine but at runtime, it throws an exception.
In simply word difference b/w Compile time & Run time.
compile time:Developer writes the program in .java format & converts in to the Bytecode which is a class file,during this compilation any error occurs can be defined as compile time error.
Run time:The generated .class file is use by the application for its additional functionality & the logic turns out be wrong and throws an error which is a run time error
Compile time:
Time taken to convert the source code into a machine code so that it becomes an executable is called compile time.
Run time:
When an application is running, it is called run time.
Compile time errors are those syntax errors, missing file reference errors.
Runtime errors happen after the source code has been compiled into an executable program and while the program is running. Examples are program crashes, unexpected program behavior or features don't work.
Run time means something happens when you run the program.
Compile time means something happens when you compile the program.
Imagine that you are a boss and you have an assistant and a maid, and you give them a list of tasks to do, the assistant (compile time) will grab this list and make a checkup to see if the tasks are understandable and that you didn't write in any awkward language or syntax, so he understands that you want to assign someone for a Job so he assign him for you and he understand that you want some coffee, so his role is over and the maid (run time)starts to run those tasks so she goes to make you some coffee but in sudden she doesn’t find any coffee to make so she stops making it or she acts differently and make you some tea (when the program acts differently because he found an error).
Compile Time:
Things that are done at compile time incur (almost) no cost when the resulting program is run, but might incur a large cost when you build the program.
Run-Time:
More or less the exact opposite. Little cost when you build, more cost when the program is run.
From the other side; If something is done at compile time, it runs only on your machine and if something is run-time, it run on your users machine.
I have always thought of it relative to program processing overhead and how it affects preformance as previously stated. A simple example would be, either defining the absolute memory required for my object in code or not.
A defined boolean takes x memory this is then in the compiled program and cannot be changed. When the program runs it knows exactly how much memory to allocate for x.
On the other hand if I just define a generic object type (i.e. kind of a undefined place holder or maybe a pointer to some giant blob) the actual memory required for my object is not known until the program is run and I assign something to it, thus it then must be evaluated and memory allocation, etc. will be then handled dynamically at run time (more run time overhead).
How it is dynamically handled would then depend on the language, the compiler, the OS, your code, etc.
On that note however it would really depends on the context in which you are using run time vs compile time.
Here is an extension to the Answer to the question "difference between run-time and compile-time?" -- Differences in overheads associated with run-time and compile-time?
The run-time performance of the product contributes to its quality by delivering results faster. The compile-time performance of the product contributes to its timeliness by shortening the edit-compile-debug cycle. However, both run-time performance and compile-time performance are secondary factors in achieving timely quality. Therefore, one should consider run-time and compile-time performance improvements only when justified by improvements in overall product quality and timeliness.
A great source for further reading here:
we can classify these under different two broad groups static binding and dynamic binding. It is based on when the binding is done with the corresponding values. If the references are resolved at compile time, then it is static binding and if the references are resolved at runtime then it is dynamic binding. Static binding and dynamic binding also called as early binding and late binding. Sometimes they are also referred as static polymorphism and dynamic polymorphism.
Joseph Kulandai.
The major difference between run-time and compile time is:
If there are any syntax errors and type checks in your code,then it throws compile time error, where-as run-time:it checks after executing the code.
For example:
int a = 1
int b = a/0;
here first line doesn't have a semi-colon at the end---> compile time error after executing the program while performing operation b, result is infinite---> run-time error.
Compile time doesn't look for output of functionality provided by your code, whereas run-time does.
here's a very simple answer:
Runtime and compile time are programming terms that refer to different stages of software program development.
In order to create a program, a developer first writes source code, which defines how the program will function. Small programs may only contain a few hundred lines of source code, while large programs may contain hundreds of thousands of lines of source code. The source code must be compiled into machine code in order to become and executable program. This compilation process is referred to as compile time.(think of a compiler as a translator)
A compiled program can be opened and run by a user. When an application is running, it is called runtime.
The terms "runtime" and "compile time" are often used by programmers to refer to different types of errors. A compile time error is a problem such as a syntax error or missing file reference that prevents the program from successfully compiling. The compiler produces compile time errors and usually indicates what line of the source code is causing the problem.
If a program's source code has already been compiled into an executable program, it may still have bugs that occur while the program is running. Examples include features that don't work, unexpected program behavior, or program crashes. These types of problems are called runtime errors since they occur at runtime.
The reference
Look into this example:
public class Test {
public static void main(String[] args) {
int[] x=new int[-5];//compile time no error
System.out.println(x.length);
}}
The above code is compiled successfully, there is no syntax error, it is perfectly valid.
But at the run time, it throws following error.
Exception in thread "main" java.lang.NegativeArraySizeException
at Test.main(Test.java:5)
Like when in compile time certain cases has been checked, after that run time certain cases has been checked once the program satisfies all the condition you will get an output.
Otherwise, you will get compile time or run time error.
You can understand the code compile structure from reading the actual code. Run-time structure are not clear unless you understand the pattern that was used.
public class RuntimeVsCompileTime {
public static void main(String[] args) {
//test(new D()); COMPILETIME ERROR
/**
* Compiler knows that B is not an instance of A
*/
test(new B());
}
/**
* compiler has no hint whether the actual type is A, B or C
* C c = (C)a; will be checked during runtime
* #param a
*/
public static void test(A a) {
C c = (C)a;//RUNTIME ERROR
}
}
class A{
}
class B extends A{
}
class C extends A{
}
class D{
}
It's not a good question for S.O. (it's not a specific programming question), but it's not a bad question in general.
If you think it's trivial: what about read-time vs compile-time, and when is this a useful distinction to make? What about languages where the compiler is available at runtime? Guy Steele (no dummy, he) wrote 7 pages in CLTL2 about EVAL-WHEN, which CL programmers can use to control this. 2 sentences are barely enough for a definition, which itself is far short of an explanation.
In general, it's a tough problem that language designers have seemed to try to avoid.
They often just say "here's a compiler, it does compile-time things; everything after that is run-time, have fun". C is designed to be simple to implement, not the most flexible environment for computation. When you don't have the compiler available at runtime, or the ability to easily control when an expression is evaluated, you tend to end up with hacks in the language to fake common uses of macros, or users come up with Design Patterns to simulate having more powerful constructs. A simple-to-implement language can definitely be a worthwhile goal, but that doesn't mean it's the end-all-be-all of programming language design. (I don't use EVAL-WHEN much, but I can't imagine life without it.)
And the problemspace around compile-time and run-time is huge and still largely unexplored. That's not to say S.O. is the right place to have the discussion, but I encourage people to explore this territory further, especially those who have no preconceived notions of what it should be. The question is neither simple nor silly, and we could at least point the inquisitor in the right direction.
Unfortunately, I don't know any good references on this. CLTL2 talks about it a bit, but it's not great for learning about it.