__builtin_object_size(3) - NetBSD Manual Pages

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__BUILTIN_OBJECT_SIZ... NetBSD Library Functions Manual__BUILTIN_OBJECT_SIZ...

__builtin_object_size -- return the size of the given object
size_t __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 min- imum 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 10.99 July 18, 2012 NetBSD 10.99
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