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.Dd April 27, 2024 .Dt __BUILTIN_OBJECT_SIZE 3 .Os .Sh NAME .Nm __builtin_object_size .Nd return the size of the given object .Sh SYNOPSIS .Ft size_t .Fn __builtin_object_size "void *ptr" "int type" .Sh DESCRIPTION The .Fn __builtin_object_size function is a .Xr clang 1 and .Xr gcc 1 built-in function that returns the size of the object referenced by .Fa ptr if known at compile time. If the .Fa ptr expression has any side effects, then they will not be evaluated. .Sh RETURN VALUES If the size of the object is not known or the .Fa ptr expression has side effects, the .Fn __builtin_object_size function returns: l -tag -width (size_t)-1 -offset indent t Dv (size_t)-1 for .Fa type .Dv 0 and .Dv 1 . t Dv (size_t)0 for .Fa type .Dv 2 and .Dv 3 . .El
p If the size of the object is known, then the .Fn __builtin_object_size function returns the maximum size of all the objects that the compiler knows can be pointed to by .Fa ptr when .Fa type .Dv & 2 == 0 , and the minimum size when .Fa type .Dv & 2 != 0 . .Sh SEE ALSO .Xr clang 1 , .Xr gcc 1 , .Xr __builtin_return_address 3 , .Xr attribute 3 , .Xr ssp 3 .Sh HISTORY The .Fn __builtin_object_size appeared in .Tn GCC 4.1 . .Sh CAVEATS This is a non-standard, compiler-specific extension.
p 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.
p 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.
p For that reason code fortification (size-checked replacement functions) is currently disabled when optimization is off.