For some months I've been working on a "home-made" operating system.
Currently, it boots and goes into 32-bit protected mode.
I've loaded the interrupt table, but haven't set up the pagination (yet).
Now while writing my exception routines I've noticed that when an instruction throws an exception, the exception routine is executed, but then the CPU jumps back to the instruction which threw the exception! This does not apply to every exception (for example, a div by zero exception will jump back to the instruction AFTER the division instruction), but let's consider the following general protection exception:
MOV EAX, 0x8
MOV CS, EAX
My routine is simple: it calls a function that displays a red error message.
The result: MOV CS, EAX fails -> My error message is displayed -> CPU jumps back to MOV CS -> infinite loop spamming the error message.
I've talked about this issue with a teacher in operating systems and unix security.
He told me he knows Linux has a way around it, but he doesn't know which one.
The naive solution would be to parse the throwing instruction from within the routine, in order to get the length of that instruction.
That solution is pretty complex, and I feel a bit uncomfortable adding a call to a relatively heavy function in every affected exception routine...
Therefore, I was wondering if the is another way around the problem. Maybe there's a "magic" register that contains a bit that can change this behaviour?
--
Thank you very much in advance for any suggestion/information.
--
EDIT: It seems many people wonder why I want to skip over the problematic instruction and resume normal execution.
I have two reasons for this:
First of all, killing a process would be a possible solution, but not a clean one. That's not how it's done in Linux, for example, where (AFAIK) the kernel sends a signal (I think SIGSEGV) but does not immediately break execution. It makes sense, since the application can block or ignore the signal and resume its own execution. It's a very elegant way to tell the application it did something wrong IMO.
Another reason: what if the kernel itself performs an illegal operation? Could be due to a bug, but could also be due to a kernel extension. As I've stated in a comment: what should I do in that case? Shall I just kill the kernel and display a nice blue screen with a smiley?
That's why I would like to be able to jump over the instruction. "Guessing" the instruction size is obviously not an option, and parsing the instruction seems fairly complex (not that I mind implementing such a routine, but I need to be sure there is no better way).
Different exceptions have different causes. Some exceptions are normal, and the exception only tells the kernel what it needs to do before allowing the software to continue running. Examples of this include a page fault telling the kernel it needs to load data from swap space, an undefined instruction exception telling the kernel it needs to emulate an instruction that the CPU doesn't support, or a debug/breakpoint exception telling the kernel it needs to notify a debugger. For these it's normal for the kernel to fix things up and silently continue.
Some exceptions indicate abnormal conditions (e.g. that the software crashed). The only sane way of handling these types of exceptions is to stop running the software. You may save information (e.g. core dump) or display information (e.g. "blue screen of death") to help with debugging, but in the end the software stops (either the process is terminated, or the kernel goes into a "do nothing until user resets computer" state).
Ignoring abnormal conditions just makes it harder for people to figure out what went wrong. For example, imagine instructions to go to the toilet:
enter bathroom
remove pants
sit
start generating output
Now imagine that step 2 fails because you're wearing shorts (a "can't find pants" exception). Do you want to stop at that point (with a nice easy to understand error message or something), or ignore that step and attempt to figure out what went wrong later on, after all the useful diagnostic information has gone?
If I understand correctly, you want to skip the instruction that caused the exception (e.g. mov cs, eax) and continue executing the program at the next instruction.
Why would you want to do this? Normally, shouldn't the rest of the program depend on the effects of that instruction being successfully executed?
Generally speaking, there are three approaches to exception handling:
Treat the exception as an unrepairable condition and kill the process. For example, division by zero is usually handled this way.
Repair the environment and then execute the instruction again. For example, page faults are sometimes handled this way.
Emulate the instruction using software and skip over it in the instruction stream. For example, complicated arithmetic instructions are sometimes handled this way.
What you're seeing is the characteristic of the General Protection Exception. The Intel System Programming Guide clearly states that (6.15 Exception and Interrupt Reference / Interrupt 13 - General Protection Exception (#GP)) :
Saved Instruction Pointer
The saved contents of CS and EIP registers point to the instruction that generated the
exception.
Therefore, you need to write an exception handler that will skip over that instruction (which would be kind of weird), or just simply kill the offending process with "General Protection Exception at $SAVED_EIP" or a similar message.
I can imagine a few situations in which one would want to respond to a GPF by parsing the failed instruction, emulating its operation, and then returning to the instruction after. The normal pattern would be to set things up so that the instruction, if retried, would succeed, but one might e.g. have some code that expects to access some hardware at addresses 0x000A0000-0x000AFFFF and wish to run it on a machine that lacks such hardware. In such a situation, one might not want to ever bank in "real" memory in that space, since every single access must be trapped and dealt with separately. I'm not sure whether there's any way to handle that without having to decode whatever instruction was trying to access that memory, although I do know that some virtual-PC programs seem to manage it pretty well.
Otherwise, I would suggest that you should have for each thread a jump vector which should be used when the system encounters a GPF. Normally that vector should point to a thread-exit routine, but code which was about to do something "suspicious" with pointers could set it to an error handler that was suitable for that code (the code should unset the vector when laving the region where the error handler would have been appropriate).
I can imagine situations where one might want to emulate an instruction without executing it, and cases where one might want to transfer control to an error-handler routine, but I can't imagine any where one would want to simply skip over an instruction that would have caused a GPF.
what will the program behave when they have two exceptions.
And none of them have been caught yet.
what type of handler will be called .
lets say both the exceptions were of different type.
i apologize if i am not clear but i feel i have made myself clear enough.
thank you!!!
what if the try block throws an exception and try block is exited which destroyes all the automatic variables.Lets say one was an automatic object and its destructor again threw an exception.Now we have two uncaught exception.My question is based on this fact.
thank you!!
It depends entirely on the language. However, in all the languages I know there can't ever be multiple exceptions at the same time (in the same thread). If an exception has been thrown, it travels up the call stack until it's caught, with no code executing during this time. If the exception is not caught, the program crashes before another can be thrown. If it is caught, the exception is no longer "active" and if the handler throws a new exception, the old one is forgotten.
At the CPU level (on x86), there is a situation called a double fault:
On the x86 architecture, a double fault exception occurs if the processor encounters a problem while trying to service a pending interrupt or exception.
However, this kind of "double fault" is a very low level situation and is only of concern to the operating system kernel.
Few languages or frameworks can do a good job of handling an exception that occurs while cleaning up from an earlier exception. Effective handling of such situations would require the cleanup code to know what exception, if any, occurred on the "main line". Conceptually it would not be at all difficult for such information to be provided to cleanup code, but frameworks generally don't provide it.
Otherwise, normal behavior in C++ is to hard-crash the system when an exception occurs during the cleanup from another exception; Java and .NET languages generally abandon any pending exception if a cleanup exception occurs. Newer versions of Java, however, include a feature which (if used) will handles such things much better. In a try-with-resources block, an exception which occurs while cleaning up resources when no other exceptions are pending will be treated normally; if, however, an exception was pending, the pending exception will remain pending but have the new exception added to its list of "suppressed exceptions". It would be nice if there were a way of specifying that a particular finally block should behave the same way, but I don't know of any feature for that.
When Exception is occurred then complier unwind stack(closing current functions and going to back, caller function). If no exception is caught in main than complier called abort function. By this program terminate abnormally.
But during unwinding stack if another exception is occurred(for your case in destructor) than at this time/point without reaching main function, compiler called abort function which terminate program abnormally.
If you know that exception could be occurred in destructor than you must handled in destructor .Means in destructor you should have catch block to catch that exception. By this way second exception generated is to be handled within a destructor and this exception is not out from the destructor and program is to saved from crash due to two exception generated at a time
Complier handle only one exception at a time. If compiler find more than one exception call abort function by which program terminate abnormally.
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.