4 ms·

In C, there is no concept of null or undefined, only 0.

• dllthomas 12y ago
  That's not quite true, but not a terrible approximation.
  • sdegutis 12y ago
    How is it not true? Besides void expressions (which aren't values) the above poster's statement appears correct.
    • dllthomas 12y ago
      A null pointer does not need to have an all-zero representation, which mattered once on some architectures. I'm not aware of any current architecture that does it that way, but that's still what the standard says, which is to say that C does technically have a notion of null which is not necessarily identical with 0. http://c-faq.com/null/machexamp.html http://c-faq.com/null/machexamp.html
      • spc476 12y ago
        But in C, a 0 in a pointer context is always considered the NULL pointer (even if the physical value of a NULL pointer isn't all zeros).
        • loup-vaillant 12y ago
          Additionally in a boolean context a null pointer is indistinguishable from "false" (meaning, zero). http://c-faq.com/null/ptrtest.html http://c-faq.com/null/ptrtest.html For practical purposes, the null pointer and the integer `0` are one and same.
          • dllthomas 12y ago
            "For practical purposes, the null pointer and the integer `0` are one and same." For most practical purposes, which is why I said that it wasn't a terrible approximation. However, they can be distinguished on some architectures: intptr_t x = 0; void *p = 0; x == *(intptr_t*)(char*)&p; I can easily construct fantasy scenarios (involving more than a bit of Doing It Wrong) where this would be relevant. I'm not convinced it couldn't ever be relevant without Doing It Wrong, if one in fact needed to work on that kind of a system.
        • cperciva 12y ago
          Only a compile-time constant zero. So assert(NULL == (void *)0); is fine, but int x = 0; assert(NULL == (void *)x); is not.