So, to write a portable C program, no values other than NULL should be used as an invalid memory address? On 5/18/21, Peter Bergner <bergner@xxxxxxxxxxxxx> wrote: > On 5/18/21 10:55 AM, Peng Yu via Gcc-help wrote: >> NULL (0) is a commonly used value for an invalid memory address. I >> also see -1 is used for an illegal memory address. >> >> Besides those values, I'd suspect that other values that are close to >> 0 (both negatives and positives) won't appear in real programs. >> >> Is there a limit on the safe range that can be assumed to not be used >> by user level code as valid addresses? > > There is no such list that defines which virtual addresses are valid and > which > are not. It depends on your OS, target, system config, etc. I've heard of > some OS systems where 0 was ok to access and NULL was defined to be some > other > non-zero value. > > Also, -1 is sometimes valid too. If you're running on a ppc64 (ie, > big-endian) > Linux kernel and execute a 32-bit application and your system has enough > memory, > the kernel will give you the full 4G virtual address space to your > application. > > Peter > > -- Regards, Peng
