Q. What is a "null pointer assignment" error? What are bus errors, memory faults, and core dumps?
These are all serious errors, symptoms of a wild pointer or subscript.
Null pointer assignment is a message you might get when an MS-DOS program finishes executing. Some such programs can arrange for a small amount of memory to be available "where the NULL pointer points to" (so to speak). If the program tries to write to that area, it will overwrite the data put there by the compiler. When the program is done, code generated by the compiler examines that area. If that data has been changed, the compiler-generated code complains with null pointer assignment.
This message carries only enough information to get you worried. There's no way to tell, just from a null pointer assignment message, what part of your program is responsible for the error. Some debuggers, and some compilers, can give you more help in finding the problem.
Bus error: core dumped and Memory fault: core dumped are messages you might see from a program running under UNIX. They're more programmer friendly. Both mean that a pointer or an array subscript was wildly out of bounds. You can get these messages on a read or on a write. They aren't restricted to null pointer problems.
The core dumped part of the message is telling you about a file, called core, that has just been written in your current directory. This is a dump of everything on the stack and in the heap at the time the program was running. With the help of a debugger, you can use the core dump to find where the bad pointer was used.
That might not tell you why the pointer was bad, but it's a step in the right direction. If you don't have write permission in the current directory, you won't get a core file, or the core dumped message.
The same tools that help find memory allocation bugs can help find some wild pointers and subscripts, sometimes. The best such tools can find almost all occurrences of this kind of problem.
Q. How can you determine the size of an allocated portion of memory?
You can't, really. free() can, but there's no way for your program to know the trick free() uses.
Q. How does free() know how much memory to release?
There's no standard way. It can vary from compiler to compiler, even from version to version of the same compiler.free(), malloc(), calloc(), and realloc() are functions; as long as they all work the same way, they can work any way that works.
Most implementations take advantage of the same trick, though. When malloc() (or one of the other allocation functions) allocates a block of memory, it grabs a little more than it was asked to grab. malloc() doesn't return the address of the beginning of this block. Instead, it returns a pointer a little bit after that.
At the very beginning of the block, before the address returned, malloc() stores some information, such as how big the block is. (If this information gets overwritten, you'll have wild pointer problems when you free the memory.)
There's no guarantee free() works this way. It could use a table of allocated addresses and their lengths. It could store the data at the end of the block (beyond the length requested by the call to malloc()). It could store a pointer rather than a count.
Q. Can math operations be performed on a void pointer?
No. Pointer addition and subtraction are based on advancing the pointer by a number of elements. By definition, if you have a void pointer, you don't know what it's pointing to, so you don't know the size of what it's pointing to.
If you want pointer arithmetic to work on raw addresses, use character pointers.
Q. How do you print an address?
The safest way is to use printf() (or fprintf() or sprintf()) with the %P specification. That prints a voidpointer (void*). Different compilers might print a pointer with different formats. Your compiler will pick a format that's right for your environment.
If you have some other kind of pointer (not a void*) and you want to be very safe, cast the pointer to a void*:
printf( "%P\n", (void*) buffer );
There's no guarantee any integer type is big enough to store a pointer. With most compilers, an unsigned long is big enough. The second safest way to print an address (the value of a pointer) is to cast it to an unsigned long, then print that.
No comments:
Post a Comment