Spurious warning for zero-sized array parameters to a function

[Alejandro Colomar <alx.manpages@gmail.com>]

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Hi!

In the following function, past_end is a pointer to one-past-the-end of the 
array.  Holding such a pointer is legal in C.  I use it as a sentinel value that 
helps (1) avoid overrunning the buffer, and (2) detect truncation.  I mark it as 
having a size of [0], which clearly states that it can't be dereferenced (and as 
you can see, I don't).

/*
  * This function copies an unterminated string into a string.
  * -  It never overruns the dest buffer.
  * -  It can be chained, to concatenate strings.
  * -  It detects truncation.
  * -  Truncation only needs to be tested once after all concatenations.
  * -  The name is self-documenting, compared to its alternative: strncat(3).
  */
char *
ustr2stpe(char *dst, const char *restrict src, size_t n, char past_end[0])
{
	bool       trunc;
	char       *end;
	ptrdiff_t  len;

	if (dst == past_end)
		return past_end;

	trunc = false;
	len = strnlen(src, n);
	if (len > past_end - dst - 1) {
		len = past_end - dst - 1;
		trunc = true;
	}

	end = mempcpy(dst, src, len);
	*end = '\0';

	return trunc ? past_end : end;
}


If I compile the code above, GCC considers the function definition to be fine. 
However, at call site, it always warns:


#define nitems(arr)  (sizeof((arr)) / sizeof((arr)[0]))

int
main(void)
{
	char pre[4] = "pre.";
	char *post = ".post";
	char *src = "some-long-body.post";
	char dest[100];
         char *p, *past_end;

	past_end = dest + nitems(dest);
	p = dest;
	p = ustr2stpe(p, pre, nitems(pre), past_end);
	p = ustr2stpe(p, src, strlen(src) - strlen(post), past_end);
	p = ustr2stpe(p, "", 0, past_end);
	if (p == past_end)
		fprintf(stderr, "truncation\n");

	puts(dest);  // "pre.some-long-body"
}



$ cc -Wall -Wextra ustr2stpe.c
ustr2stpe.c: In function ‘main’:
ustr2stpe.c:43:13: warning: ‘ustr2stpe’ accessing 1 byte in a region of size 0 
[-Wstringop-overflow=]
    43 |         p = ustr2stpe(p, pre, nitems(pre), past_end);
       |             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
ustr2stpe.c:43:13: note: referencing argument 4 of type ‘char[0]’
ustr2stpe.c:10:1: note: in a call to function ‘ustr2stpe’
    10 | ustr2stpe(char *dst, const char *restrict src, size_t n, char past_end[0])
       | ^~~~~~~~~
ustr2stpe.c:44:13: warning: ‘ustr2stpe’ accessing 1 byte in a region of size 0 
[-Wstringop-overflow=]
    44 |         p = ustr2stpe(p, src, strlen(src) - strlen(post), past_end);
       |             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
ustr2stpe.c:44:13: note: referencing argument 4 of type ‘char[0]’
ustr2stpe.c:10:1: note: in a call to function ‘ustr2stpe’
    10 | ustr2stpe(char *dst, const char *restrict src, size_t n, char past_end[0])
       | ^~~~~~~~~
ustr2stpe.c:45:13: warning: ‘ustr2stpe’ accessing 1 byte in a region of size 0 
[-Wstringop-overflow=]
    45 |         p = ustr2stpe(p, "", 0, past_end);
       |             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
ustr2stpe.c:45:13: note: referencing argument 4 of type ‘char[0]’
ustr2stpe.c:10:1: note: in a call to function ‘ustr2stpe’
    10 | ustr2stpe(char *dst, const char *restrict src, size_t n, char past_end[0])
       | ^~~~~~~~~
ustr2stpe.c:43:13: warning: ‘ustr2stpe’ accessing 1 byte in a region of size 0 
[-Wstringop-overflow=]
    43 |         p = ustr2stpe(p, pre, nitems(pre), past_end);
       |             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
ustr2stpe.c:43:13: note: referencing argument 4 of type ‘char[0]’
ustr2stpe.c:10:1: note: in a call to function ‘ustr2stpe’
    10 | ustr2stpe(char *dst, const char *restrict src, size_t n, char past_end[0])
       | ^~~~~~~~~
ustr2stpe.c:44:13: warning: ‘ustr2stpe’ accessing 1 byte in a region of size 0 
[-Wstringop-overflow=]
    44 |         p = ustr2stpe(p, src, strlen(src) - strlen(post), past_end);
       |             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
ustr2stpe.c:44:13: note: referencing argument 4 of type ‘char[0]’
ustr2stpe.c:10:1: note: in a call to function ‘ustr2stpe’
    10 | ustr2stpe(char *dst, const char *restrict src, size_t n, char past_end[0])
       | ^~~~~~~~~
ustr2stpe.c:45:13: warning: ‘ustr2stpe’ accessing 1 byte in a region of size 0 
[-Wstringop-overflow=]
    45 |         p = ustr2stpe(p, "", 0, past_end);
       |             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
ustr2stpe.c:45:13: note: referencing argument 4 of type ‘char[0]’
ustr2stpe.c:10:1: note: in a call to function ‘ustr2stpe’
    10 | ustr2stpe(char *dst, const char *restrict src, size_t n, char past_end[0])
       | ^~~~~~~~~


The warnings are invalid.  While it's true that I'm referencing a pointer of 
size 0, it's false that I'm "accessing 1 byte" in that region.  I guess this is 
all about the bogus design of 'static' in ISO C, where you can have an array 
parameter of size 0, which is very useful in cases like this one.

Cheers,

Alex


-- 
<http://www.alejandro-colomar.es/>

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