Is ((void *) -1) a valid address?

Question

Verbatim from Linux' man shmat:

RETURN VALUE

[...] on error (void *) -1 is returned, and errno is set to indicate the cause of the error.

^{(POSIX tells the same using a slightly different wording.)}

Is there any mandatory rule or definition (standard?) that (void *) -1 may not be a valid address?

Answer 1

0xffffffff is technically a valid address in a 32 bit environment but on most operating systems (Certainly Linux/Windows) will be in the reserved kernel part of the address space. That means that in user mode processes it's safe to use it as a error code since no user mode allocation function would return this as a usable address.

Answer 2

For simplicity, consider a machine with 16 bits of address space. This machine can address memory from 0 to 65535. In this case (void*) -1 would be the same as 0xffff which is 65535. While technically this address is valid, there are few system that would have anything there that they would allow to be accessed.

The other thing to consider is that almost all POSIX system calls returns -1 on error.

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As noted by Benj, it's actually possible to map the address NULL. This can be used for example when you want to see if there is a mapping with a specified shmid, in which case the shmaddr argument is set to NULL and the function return NULL to signify that the shared memory exists.