Name 

Description 

NULL #define 

Header File
stddef.h
Description
Null pointer constant that is compatible with any data object pointer. It is not compatible with function pointers. When a pointer is equivalent to NULL it is guaranteed not to point to any data object defined within the program. 

_threadid 

Header File
stddef.h
Syntax
extern long _threadid;
Description
_threadid is a long integer that contains the ID of the currently executing thread. It is implemented as a macro, and should be declared only by including stddef.h. 

offsetof 

Header File
stddef.h
Category
Memory Routines
Prototype
size_t offsetof(struct_type, struct_member);
Description
Gets the byte offset to a structure member.
offsetof is available only as a macro. The argument struct_type is a struct type. struct_member is any element of the struct that can be accessed through the member selection operators or pointers.
If struct_member is a bit field, the result is undefined.
See also sizeof for more information on memory allocation and alignment of structures.
Return Value
offsetof returns the number of bytes from the start of the structure to the start of the named structure member.
Portability