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__BUILTIN_OBJECT_SIZE(3)   Library Functions Manual   __BUILTIN_OBJECT_SIZE(3)

     __builtin_object_size -- return the size of the given object

     __builtin_object_size(void *ptr, int type);

     The __builtin_object_size() function is a gcc(1) built-in function that
     returns the size of the ptr object if known at compile time and the
     object does not have any side effects.

     If the size of the object is not known or it has side effects the
     __builtin_object_size() function returns:

           (size_t)-1  for type 0 and 1.

           (size_t)0   for type 2 and 3.

     If the size of the object is known, then the __builtin_object_size()
     function returns the maximum size of all the objects that the compiler
     knows that they can be pointed to by ptr when type & 2 == 0, and the
     minimum size when type & 2 != 0.

     gcc(1), __builtin_return_address(3), attribute(3), ssp(3)

     The __builtin_object_size() appeared in GCC 4.1.

     This is a non-standard, compiler-specific extension.

     Note that currently the object size calculation pass is only done at -O1
     or above, meaning that this function always returns -1 when the optimizer
     is off.

     There are some discussions about always doing the object size pass, but
     the issue is that without the optimization pass data sizes are not going
     to be correct.

     For that reason currently code fortification (size-checked replacement
     functions) is disabled when optimization is off.

NetBSD 7.1.2                     July 18, 2012                    NetBSD 7.1.2