Access checking checks whether your program accesses memory correctly by monitoring each read, write, allocate, and free operation.
Programs might incorrectly read or write memory in a variety of ways, which are called memory access errors. For example, the program might reference a block of memory that has been deallocated through a free()call for a heap block. Or a function might return a pointer to a local variable and when that pointer is accessed, an error would result. Access errors might result in wild pointers in the program and can cause incorrect program behavior, including wrong outputs and segmentation violations. Some kinds of memory access errors can be very hard to find.
Runtime checking maintains a table that tracks the state of each block of memory being used by the program. Runtime checking checks each memory operation against the state of the block of memory it involves and then determines whether the operation is valid. The possible memory states are:
Unallocated, initial state. Memory has not been allocated. It is illegal to read, write, or free this memory because it is not owned by the program.
Allocated, but uninitialized. Memory has been allocated to the program but not initialized. It is legal to write to or free this memory, but is illegal to read it because it is uninitialized. For example, upon entering a function, stack memory for local variables is allocated, but uninitialized.
Read-only. It is legal to read, but not write or free, read-only memory.
Allocated and initialized. It is legal to read, write, or free allocated and initialized memory.
Using runtime checking to find memory access errors is not unlike using a compiler to find syntax errors in your program. In both cases, a list of errors is produced, with each error message giving the cause of the error and the location in the program where the error occurred. In both cases, you should fix the errors in your program starting at the top of the error list and working your way down. One error can cause other errors in a chain reaction. The first error in the chain is, therefore, the “first cause” and fixing that error might also fix some subsequent errors.
For example, a read from an uninitialized section of memory can create an incorrect pointer, which when dereferenced can cause another invalid read or write, which can in turn lead to yet another error.