Undefined behaviour? Accessing s twice in a statement.

[Kobu]

Hello,

Wonder if the below double access to s is undefined behaviour? Works
fine, but I know "works" can still be a consequence of undefined
behaviour.

#include <string.h>

void mystrcat(char *s, const char *t)
{
/* is the below undefined behaviour ? */
s += strlen(s);

while(*s++ = *t++)
; /* empty statement */
}

S is accessed twice (which is a no no), yet the statement "feels" right
to me.

 

[Walter Roberson]

:Wonder if the below double access to s is undefined behaviour?

: s += strlen(s);

That is well defined. It is equivilent to s = s + strlen(s)
and you can see that on the right-hand side there are only
accesses to s without modifications. The restrictions
are against modifying the same value twice within the same
sequence point.

Consider that you would undoubtedly consider this valid:
y = 3 * x * x + x * 7 + 2;
(with x being a numeric variable): you don't worry about
pure accesses on the right hand side.

Similarily, you know that this is valid:
i = i + 1;
(with x being a numeric variable or a pointer): we'd have
some serious problems with programs if that didn't work.

So you can access the same variable twice on the RHS and
you can make an assignment on the LHS to a variable which
is mentioned on the RHS. Together, we can see that
s += strlen(s);
is valid.

What -would- be a problem is something like:

y = 3 * x * x++ + x++ * 7 + 2;

 

[Chris Williams]

Wonder if the below double access to s is undefined behaviour? Works
fine, but I know "works" can still be a consequence of undefined
behaviour.

void mystrcat(char *s, const char *t)
{
/* is the below undefined behaviour ? */
s += strlen(s);

while(*s++ = *t++)
; /* empty statement */
}

It looks correct to me.

-Chris

 

[Dave Vandervies]

Hello,

Wonder if the below double access to s is undefined behaviour? Works
fine, but I know "works" can still be a consequence of undefined
behaviour.

#include <string.h>

void mystrcat(char *s, const char *t)
{
/* is the below undefined behaviour ? */
s += strlen(s);

while(*s++ = *t++)
; /* empty statement */
}

S is accessed twice (which is a no no), yet the statement "feels" right
to me.

Accessing s twice isn't a problem, if none of those accesses modifies it
(consider doing "i+i") or of only one of the accesses modifies it and
all the others are required to determine the new value to be stored
(as in your code).

The restrictions on multiple access are when an attempt is made to modify
an object twice between sequence points (i=i++) or when an object is
modified and an unrelated access to it is made (which is hard to come
up with a simple example for; aliasing and multiple side-effects are
usually involved - consider a[++*i]=a[++*j], if i or j point at the
element of a[] that gets modified).


dave