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.Dd December 21, 2010
.Dt __BUILTIN_TYPES_COMPATIBLE_P 3
.Os
.Sh NAME
.Nm __builtin_types_compatible_p
.Nd GNU extension to check equivalent types
.Sh SYNOPSIS
.Ft int
.Fn __builtin_types_compatible_p "type_a" "type_b"
.Sh DESCRIPTION
The
.Fn __builtin_types_compatible_p
is a
.Tn GNU
extension for determining whether two types are equivalent.
If
.Fa type_a
is equivalent to
.Fa type_b ,
a value 1 is returned.
Otherwise
.Fn __builtin_types_compatible_p
returns 0.
.Pp
The following remarks should be taken into account.
.Bl -enum -offset indent
.It
The architecture-specific size of the two types
does not have an impact on the result.
For example,
.Fn sizeof "char *"
and
.Fn sizeof "int"
result the same value on i386, but the types naturally are not equivalent.
.It
Type qualifiers are ignored.
The function returns the same value for
.Vt long
and
.Vt const long .
.It
The amount of pointer indirection affects the result.
For example,
.Vt double *
is not equivalent to
.Vt double ** .
.It
Two types defined with
.Em typedef
are equivalent if and only if their underlying types are equivalent.
.It
The
.Em enum
type is a special case in that two
.Em enum
types are not considered equivalent.
.El
.Sh EXAMPLES
The following example combines
.Fn __builtin_types_compatible_p
and the
.Xr typeof 3
construct:
.Bd -literal -offset indent
#define __COMPARE_TYPES(v, t)      \\
         __builtin_types_compatible_p(__typeof__(v), t)

\&...

if (__COMPARE_TYPES(p, double) != 0)
	err(EX_DATAERR, "invalid type");
.Ed
.Sh SEE ALSO
.Xr gcc 1 ,
.Xr __builtin_constant_p 3 ,
.Xr typeof 3
.Sh CAVEATS
This is a non-standard, compiler-specific extension.