[yuubi]

Standard C provides only a few ways to obtain valid pointers. implementations can define behaviors in cases that the standard leaves undefined, such as allowing more cases of casts of integers to pointers than the standard defines (common in embedded-land), or functions like mmap or sbrk. so you or your chip vendor could define FOO_REG as (uint32_t )0x80001234 and use it as if it were a variable.

Olde C used to just let you use integers as struct pointers, and there was only one struct member namespace. so code like this was valid and did an integer-size write to address 0177770. old unix did this for device register access; see the lions book.

struct { int integ; };

f() { 0177770->integ = 012345; }