Defect Report #001
Submission Date: 10 Dec 92
Submittor: WG14
Source: X3J11/90-009 (Paul Eggert)
Question 1
Do functions return values by copying?
The C Standard is clear (in subclause 6.3.2.2) that function arguments
are copied, but is not clear (in subclause 6.6.6.4) whether a function's
returned value is also copied. This question becomes an issue in the
assignment statement s=f(); where f yields a str
ucture:
is the result defined when the structure s overlaps the structur
e
that f obtained the returned value from?
I ask this question because the GNU C compiler does not copy the structure
in this case. When I filed the enclosed bug report [omitted from this
document], Richard Stallman, the author of GNU C, replied that he
didn't think that Standard C required the extra copy. I sympathize
with Stallman's desire for efficient code, and I also would prefer
that the C Standard did not require the extra copy here, but the point
should be made clear in the C Standard.
Correction
In subclause 6.6.6.4, page 80, lines 30-32, replace:
If the expression has a type different from that of the function in
which it appears, it is converted as if it were assigned to an object
of that type.
with:
If the expression has a type different from the return type of the
function in which it appears, the value is converted as if by assignment
toan object having the return type of the function.*
[Footnote *: The return statement is not an assignment. The
overlap restriction in subclause 6.3.16.1 does not apply to the case
of function return.]
Add to subclause 6.6.6.4, page 80:
Example
In:
struct s {double i;} f(void);
union {struct {int f1;
struct s f2;} u1;
struct {struct s f3;
int f4;} u2;
} g;
struct s f(void)
{
return g.u1.f2;
}
/* ... */
g.u2.f3 = f();
the behavior is defined.
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