I hope not to sound very foolish here, but does the NULL module actually require memory allocation or not when doing this:
TheNull = malloc(sizeof(NULL));
If true, how can something that has no memory allocated actually exist in the ram?
CodePudding user response：
If you use NULL as parameter of
malloc(sizeof()), does it returnNULL?
No, unless out-of-memory, like any other allocation.
NULL, the null pointer constant, does not have a particular type. It may be void *, int, long, or some integer type.
Avoid sizeof(NULL) as its size/type may differ from system to system.
CodePudding user response：
The expression is not in any other way related to any NULL module or NULL object. There is are no such things anyway.
NULL is just a useful definition for a null pointer constant.
Depending on the target, NULL is either defined as #define NULL ((void *)0) or #define NULL 0, or possibly some other zero integer constant.
Hence malloc(sizeof(NULL)) either attempts to allocate the number of bytes in an int or in a void pointer.
CodePudding user response：
TheNull = malloc(sizeof(NULL));
That doesn't allocate 0 bytes (because sizeof(NULL) isn't zero), but malloc(0) might. The standard says:
If size is zero, the behavior of malloc is implementation-defined. For example, a null pointer may be returned. Alternatively, a non-null pointer may be returned; but such a pointer should not be dereferenced, and should be passed to free to avoid memory leaks.
So, maybe a null pointer, maybe not.
If true, how can something that has no memory allocated actually exist in the ram?
The pointer returned from malloc might point to a piece of memory slightly larger than what you asked for. For example, if you ask for 1 or 2 bytes, you might get 8 (or even 16) bytes instead. A memory manager often only provides certain size memory blocks (for efficiency), and might require a minimum size so its own bookkeping can fit in a free'd block.
And if it returns an oversized block for 1 or 2 bytes, it could do that for 0 bytes as well.