Any pointer type with the value 0 is called a null pointer. Here is the explanation from the C standard §6.3.2.3:
Answer from medalib on Stack Overflow
- An integer constant expression with the value 0, or such an expression cast to type void *, is called a null pointer constant. If a null pointer constant is converted to a pointer type, the resulting pointer, called a null pointer, is guaranteed to compare unequal to a pointer to any object or function
I have nothing more to say,this is all what i want to know.
what is a null pointer in c - Stack Overflow
How can you handle a "null pointer" exception to prevent crashes in your C program?
c++ - How to catch the null pointer exception? - Stack Overflow
c - What is "null pointer assignment error"? - Stack Overflow
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Any pointer type with the value 0 is called a null pointer. Here is the explanation from the C standard §6.3.2.3:
- An integer constant expression with the value 0, or such an expression cast to type void *, is called a null pointer constant. If a null pointer constant is converted to a pointer type, the resulting pointer, called a null pointer, is guaranteed to compare unequal to a pointer to any object or function
Null pointer is a pointer that points to memory address 0. Most system reserve this address so that no object will ever reside in there. This reservation allows to use it as blank address.
If you try to read or write from null pointer you will get runtime error which is sometimes called segmentation fault, or null pointer exception.
In your example the null pointer is used to indicate end of the list. So this condition (struct entry *) 0, checks if you have reached end of the list and iteration should stop
Usually it considered better form to use constant NULL instead of literal value 0. This makes code more readable, and also covers very rare case when NULL pointer is not a 0
The cast (struct entry *) is just to avoid compiler warning, because literal 0 is of type integer not a pointer. That's another reason to use constant NULL, because it is usually defined as (void*) 0 which compares nicely to any pointer value without a warning from compiler
There's no such thing as "null pointer exception" in C++. The only exceptions you can catch, is the exceptions explicitly thrown by throw expressions (plus, as Pavel noted, some standard C++ exceptions thrown intrinsically by standard operator new, dynamic_cast etc). There are no other exceptions in C++. Dereferencing null pointers, division by zero etc. does not generate exceptions in C++, it produces undefined behavior. If you want exceptions thrown in cases like that it is your own responsibility to manually detect these conditions and do throw explicitly. That's how it works in C++.
Whatever else you seem to be looking for has noting to do with C++ language, but rather a feature of particular implementation. In Visual C++, for example, system/hardware exceptions can be "converted" into C++ exceptions, but there's a price attached to this non-standard functionality, which is not normally worth paying.
You cannot. De-referencing a null-pointer is a system thing.
On Linux, the OS raises signals in your application. Take a look at csignal to see how to handle signals. To "catch" one, you'd hook a function in that will be called in the case of SIGSEGV. Here you could try to print some information before you gracefully terminate the program.
Windows uses structured-exception-handling. You could use the instristics __try/__except, as outlined in the previous link. The way I did it in a certain debug utility I wrote was with the function _set_se_translator (because it closely matches hooks). In Visual Studio, make sure you have SEH enabled. With that function, you can hook in a function to call when the system raises an exception in your application; in your case it would call it with EXCEPTION_ACCESS_VIOLATION. You can then throw an exception and have it propagate back out as if an exception was thrown in the first place.
http://www.faqs.org/qa/qa-3786.html
A NULL pointer assignment is a runtime error It occurs due to various reasons one is that your program has tried to access an illegal memory location. Illegal location means either the location is in the operating systems address space or in the other processes memory space. In stdio.h NULL is defined as 0 So whenever your program tries to access 0th location the operating system kills your program with runtime assignment error because the 0th location is in the operating systems address space and operating system doesn't allow access to its address space by user program .
Example code:
int* ptr = NULL;
*ptr = 3;
Explanation:
On almost every system, address 0 is reserved. System won't allow you to write to that location. If you try, you will get a runtime exception (access violation, segmentation fault, etc.).
I actually can not recall the source, but according to the source, this run time error is restricted to small and medium memory models being put into use by corresponding compiler. You see, as told before, the null pointer actually does not points to zero, in fact different compilers use different but fixed memory location to be used as null pointer.
Lets consider the case of TC compiler, this compiler places four zero bytes at the bottom of the data segment and TC copyright notice. TC also uses DS:0000 location, bottom of the data segment as null pointers location. So, assigning a value to this null pointer, would actully change the four bytes and probably, mess up the copyright notice.
Now, at the program termination, the four zeros and copyright banner are checked for any kind of alteration. If any alterations are found, it generates a Null Pointer Assignment error.
So, I think its not just the null pointer, any pointer that gets wild, if tries to access some key areas, you are greeted with Null Pointer Assignment Error.
Why is it an issue? Why is writing to a null pointer not allowed or considered a bad practice