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How do I know if a capability is valid?
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Every memory location in the system has an associated tag bit, which
indicates whether or not the value in this location is a valid capability.
This tag bit is often called the '129th bit'. It is generally stored
out-of-band, in a separate metadata block.

If the tag bit is set, this means the corresponding memory location
does contain a valid capability value so can be dereferenced. On the
other hand, if the tag bit is not set then the value in the corresponding
memory location is not a valid capability.

We can query tag bits with the ``cheri_tag_get(void *capability)`` function.

.. code-block:: C
   :emphasize-lines: 8,11

   #include <cheriintrin.h>
   #include <stdio.h>
   #include <stdlib.h>
   
   int main() {
       void *p = (void *)0xbeef;      // not valid!
       void *q = malloc(sizeof(int)); // should be ok
       if (cheri_tag_get(p)) {
         printf("p is valid\n");
       }
       if (cheri_tag_get(q)) {
         printf("q is valid\n");
       }
       return 0;
   }