On Tue, 27 Apr 2004, Vijay Kumar R Zanvar wrote:
Are the following inferences of mine correct?
1. #include <string.h>
char *strcpy(char * restrict s1,
const char * restrict s2);
a. s1 != s2
Not necessarily. Yes, if we're to assume that when you
wrote 'strcpy' above you really meant 'strcpy', the function
that copies the target of 's2' into the target of 's1'. But
generally, no, we could have the pathological case
int ptreq(const char * restrict s1, const char * restrict s2)
{
return (s1 == s2);
}
that would be perfectly conforming and unable to generate
undefined behavior, no matter the values of 's1' and 's2'.
b. That means,
char s[10] = "vijoeyz";
(void) strcpy ( s, s ); /* though useless */
is not allowed!
Naturally. In the case of 'strcpy', the Standard explicitly
rules this sort of thing out (see N869 7.21.2.3 #2). But
again, in the general case, you *could* pass 's' twice to the
'ptrcmp' function I defined above, because that function never
tries to access the targets of those pointers --- only their
values.
2. #include <string.h>
size_t strlen(const char *s);
a. If there is only one parameter of type "pointer to T", for any type
T, then the `restrict' qualifier need not be used.
No. 'restrict' guarantees that the pointer will have a sort of
a "lock" on its target for its entire lifetime. This means that
the following program exhibits undefined behavior:
char g[] = "hello world";
void foo(const char * restrict s) {
g[0] = 'J';
puts(s);
}
int main(void) {
foo(g);
}
because the target of 's' is modified through 'g' during the
lifetime of 's'. I'm almost positive that by writing
char ga[] = "hello world";
char * restrict g = ga;
at file scope, you would be "preventing" anyone's tampering with
that string except through 'g', for the entire run of the program.
Quotes around "preventing" because it's not something the compiler
can catch; it just means anyone who tampers with that array invokes
undefined behavior on himself.
N.B.: You can't define
char restrict ga[] = "hello world";
by itself; 'restrict' can only be applied to pointers, according to
N869. I don't see why it can't apply to array objects too; perhaps
someone will enlighten me.
-Arthur
