[PATCH v2] Removed the casts within ngx_memcmp()

Alejandro Colomar alx.manpages at gmail.com
Tue Nov 8 10:55:40 UTC 2022

From: Alejandro Colomar <alx at kernel.org>

The casts are unnecessary, since memcmp(3)'s arguments are
'const void *', which allows implicit conversion from any pointer type.
It might have been necessary in the times of K&R C, where 'void *'
didn't exist yet, up until the early 2000s, because some old systems
still had limited or no support for ISO C89.  Those systems passed away
a long time ago, and current systems, even the oldest living ones, have
support for ISO C89.

The cast in this case is (almost) innocuous, because it only hides
warnings for conversions from integer to pointer such as:

    nxt_memcmp(n, "foo", 3);  // no warnings
    memcmp(n, "foo", 3);      // warning: integer to pointer conversion

which is a difficult bug to write, since it's too obvious.  Such code
will probably be caught in a code review, but there's always a small
risk.  Since there's no reason to keep the small risk around, when we
can just avoid it by removing the cast.

In general, it's better to avoid a cast if possible, since casts will
disable many compiler warnings regarding type safety.

Apart from the small risk, there's a bigger concern about readability.
Casts tend to disable compiler warnings, and so are unsafe by nature.
A lot of care needs to be taken when casts are used, and a programmer
reading the cast might wonder what was that specific cast trying to
achieve.  The answer is clear: nothing.  So it's better to just prevent
that moment of "wtf is this doing here".

Signed-off-by: Alejandro Colomar <alx at nginx.com>
 src/core/ngx_string.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/core/ngx_string.h b/src/core/ngx_string.h
index 0fb9be72..6c218e22 100644
--- a/src/core/ngx_string.h
+++ b/src/core/ngx_string.h
@@ -145,7 +145,7 @@ ngx_copy(u_char *dst, u_char *src, size_t len)
 /* msvc and icc7 compile memcmp() to the inline loop */
-#define ngx_memcmp(s1, s2, n)  memcmp((const char *) s1, (const char *) s2, n)
+#define ngx_memcmp(s1, s2, n)  memcmp(s1, s2, n)
 u_char *ngx_cpystrn(u_char *dst, u_char *src, size_t n);

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