On Tue, 2021-05-18 at 19:21 -0500, Segher Boessenkool wrote: > On Tue, May 18, 2021 at 04:46:13PM -0500, Peter Bergner via Gcc-help > wrote: > > On 5/18/21 4:35 PM, Peng Yu wrote: > > > So, to write a portable C program, no values other than NULL > > > should be > > > used as an invalid memory address? > > > > I guess it depends on your definition of invalid. NULL may be a > > special > > address used as a sentinel, but that doesn't mean every system will > > give > > you a SEGV if you happen to access NULL. Some will happily let you > > do it. > > You can also use the address of an actual object for this. This is > standard for sentinel structs even: every pointer in a sentinel object > points to some (other or same) sentinel object, so that you can always > dereference without having to check for NULL first. No non-sentinel > object will have the same address, so this works fine. > > The only thing special about the address 0 is that it is cheaper to > test for than most other numbers, on most architectures. And there is some (very rarely) conditions where NULL can't be used as a sentinel and another value is used. For example, mmap() returns MAP_FAILED (it's value is (void *) -1) on failure. NULL can't be used here because (on some platform) it's perfectly legal to set a memory mapping on page 0. -- Xi Ruoyao <xry111@xxxxxxxxxxxxxxxx> School of Aerospace Science and Technology, Xidian University
