May I set a pointer to NULL portably (at least according to the POSIX standard) by memsetting it to zero? Or even memsetting the struct that contains it (where it becomes actually useful)?

Edit: POSIX specifies in stddef.h(0posix) that NULL shall be (void *) 0, but that may not be enough to allow memset(). I ignore if there are any other details that POSIX defines regarding this.

CodePudding user response：

In C, memset(&ptr, 0, sizeof(ptr)) is not necessarily the same as ptr=NULL since the ISO/IEC 9899:1999 clause 17.7 paragraph 3 says that NULL

expands to an implementation-defined null pointer constant

Thus not necessarily an all-zero bit pattern.

Posix has not had a different guarantee than the C standard. However, the next revision draft says that

POSIX additionally guarantees that any pointer object whose representation has all bits set to zero, perhaps by memset() to 0 or by calloc(), will be interpreted as a null pointer.

Hence it seems that Posix will guarantee that memset(&ptr, 0, sizeof(ptr)) will have the same effect as ptr=NULL (and this is mostly likely already the case on all existing implementations). However, the statements will not necessarily assign identical bit patterns to ptr.

It is important to notice that the C standard (and Posix, also with the mentioned update) does not guarantee that the bit pattern of a null pointer is unique. Thus, in principle ptr=NULL could assign a different bit pattern to ptr than memset(&ptr, 0, sizeof(ptr)) as long as this bit pattern is also a null pointer.

The standard does guarantee (clause 6.3.2.3 paragraph 4) that

Any two null pointers shall compare equal.

Thus if ptr and ptr2 are both null pointers, then ptr == ptr2 evaluates to true. However, memcmp(&ptr, &ptr2, sizeof(ptr)) may not evaluate to 0.