From some feedback I received on some of the attribute warnings
new in GCC 8 it seems that the manual could stand to be clarified
to explain why it makes no sense for a function declared with
attribute const (and pure) to return void. The attached patch
adds a bit more text to make it clear.
In addition, I took the opportunity to clarify how attributes
on multiple declarations of the same function are treated.
Martin
gcc/ChangeLog:
* doc/extend.texi (Common Function Attributes): Clarify.
(const attribute): Likewise.
(pure attribute): Likewise.
Index: gcc/doc/extend.texi
===================================================================
--- gcc/doc/extend.texi (revision 258899)
+++ gcc/doc/extend.texi (working copy)
@@ -2275,8 +2275,11 @@ on a declaration, followed by an attribute specifi
parentheses. You can specify multiple attributes in a declaration by
separating them by commas within the double parentheses or by
immediately following an attribute declaration with another attribute
-declaration. @xref{Attribute Syntax}, for the exact rules on
-attribute syntax and placement.
+declaration. @xref{Attribute Syntax}, for the exact rules on attribute
+syntax and placement. Compatible attribute specifications on distinct
+declarations of the same function are merged. An attribute specification
+that is not compatible with attributes already applied to a declaration
+of the same function is ignored with a warning.
GCC also supports attributes on
variable declarations (@pxref{Variable Attributes}),
@@ -2499,7 +2502,7 @@ themselves to optimization such as common subexpre
The @code{const} attribute imposes greater restrictions on a function's
definition than the similar @code{pure} attribute below because it prohibits
the function from reading global variables. Consequently, the presence of
-the attribute on a function declarations allows GCC to emit more efficient
+the attribute on a function declaration allows GCC to emit more efficient
code for some calls to the function. Decorating the same function with
both the @code{const} and the @code{pure} attribute is diagnosed.
@@ -2507,8 +2510,9 @@ both the @code{const} and the @code{pure} attribut
Note that a function that has pointer arguments and examines the data
pointed to must @emph{not} be declared @code{const}. Likewise, a
function that calls a non-@code{const} function usually must not be
-@code{const}. It does not make sense for a @code{const} function to
-return @code{void}.
+@code{const}. Because a @code{const} function can have no side-effects
+it does not make sense for such a function to return @code{void}.
+Declaring such functions is diagnosed.
@item constructor
@itemx destructor
@@ -3218,6 +3222,9 @@ The @code{pure} attribute imposes similar but loos
a function's defintion than the @code{const} attribute: it allows the
function to read global variables. Decorating the same function with
both the @code{pure} and the @code{const} attribute is diagnosed.
+Because a @code{pure} function can have no side-effects it does not
+make sense for such a function to return @code{void}. Declaring such
+functions is diagnosed.
@item returns_nonnull
@cindex @code{returns_nonnull} function attribute
