use a common definition of NULL as 0L for C and C++

the historical mess of having different definitions for C and C++
comes from the historical C definition as (void *)0 and the fact that
(void *)0 can't be used in C++ because it does not convert to other
pointer types implicitly. however, using plain 0 in C++ exposed bugs
in C++ programs that call variadic functions with NULL as an argument
and (wrongly; this is UB) expect it to arrive as a null pointer. on
64-bit machines, the high bits end up containing junk. glibc dodges
the issue by using a GCC extension __null to define NULL; this is
observably non-conforming because a conforming application could
observe the definition of NULL via stringizing and see that it is
neither an integer constant expression with value zero nor such an
expression cast to void.

switching to 0L eliminates the issue and provides compatibility with
broken applications, since on all musl targets, long and pointers have
the same size, representation, and argument-passing convention. we
could maintain separate C and C++ definitions of NULL (i.e. just use
0L on C++ and use (void *)0 on C) but after careful analysis, it seems
extremely difficult for a C program to even determine whether NULL has
integer or pointer type, much less depend in subtle, unintentional
ways, on whether it does. C89 seems to have no way to make the
distinction. on C99, the fact that (int)(void *)0 is not an integer
constant expression, along with subtle VLA/sizeof semantics, can be
used to make the distinction, but many compilers are non-conforming
and give the wrong result to this test anyway. on C11, _Generic can
trivially make the distinction, but it seems unlikely that code
targetting C11 would be so backwards in caring which definition of
NULL an implementation uses.

as such, the simplest path of using the same definition for NULL in
both C and C++ was chosen. the #undef directive was also removed so
that the compiler can catch and give a warning or error on
redefinition if buggy programs have defined their own versions of
NULL prior to inclusion of standard headers.

diff --git a/include/locale.h b/include/locale.h
index c6cc1ec75ebda3caf03533c8f211d8e135f4c012..f6a29b2ad014efeb9cdd4965c29f6e1e5f4199d1 100644 (file)
--- a/include/locale.h
+++ b/include/locale.h
@@ -7,12 +7,7 @@ extern "C" {
 
 #include <features.h>
 
-#undef NULL
-#ifdef __cplusplus
-#define NULL 0
-#else
-#define NULL ((void*)0)
-#endif
+#define NULL 0L
 
 #define LC_CTYPE    0
 #define LC_NUMERIC  1

diff --git a/include/stddef.h b/include/stddef.h
index a5bb9ecc2539cdfaec4226dc36d53d59f34cd027..9d522486573fd8237e76baddaa89c4d47c74d347 100644 (file)
--- a/include/stddef.h
+++ b/include/stddef.h
@@ -1,12 +1,7 @@
 #ifndef _STDDEF_H
 #define _STDDEF_H
 
-#undef NULL
-#ifdef __cplusplus
-#define NULL 0
-#else
-#define NULL ((void*)0)
-#endif
+#define NULL 0L
 
 #define __NEED_ptrdiff_t
 #define __NEED_size_t

diff --git a/include/stdio.h b/include/stdio.h
index 645e6221002ee3ff0b58695397b1b305d53f14ff..f64880173d0effbfbadf5f854d80364c3a3ebd33 100644 (file)
--- a/include/stdio.h
+++ b/include/stdio.h
@@ -20,12 +20,7 @@ extern "C" {
 
 #include <bits/alltypes.h>
 
-#undef NULL
-#ifdef __cplusplus
-#define NULL 0
-#else
-#define NULL ((void*)0)
-#endif
+#define NULL 0L
 
 #undef EOF
 #define EOF (-1)

diff --git a/include/stdlib.h b/include/stdlib.h
index 2d1505b51b5ad373b4aab9e068f96955cda9240f..671d188fd90f50bc04eb7bf890bd64f6891d8b2e 100644 (file)
--- a/include/stdlib.h
+++ b/include/stdlib.h
@@ -7,12 +7,7 @@ extern "C" {
 
 #include <features.h>
 
-#undef NULL
-#ifdef __cplusplus
-#define NULL 0
-#else
-#define NULL ((void*)0)
-#endif
+#define NULL 0L
 
 #define __NEED_size_t
 #define __NEED_wchar_t

diff --git a/include/string.h b/include/string.h
index c2f8eb558c502a5b36b6dd896c89c4a6b2f4b67e..d44123332753313f4426c136b5330325552e06e1 100644 (file)
--- a/include/string.h
+++ b/include/string.h
@@ -7,12 +7,7 @@ extern "C" {
 
 #include <features.h>
 
-#undef NULL
-#ifdef __cplusplus
-#define NULL 0
-#else
-#define NULL ((void*)0)
-#endif
+#define NULL 0L
 
 #define __NEED_size_t
 #if defined(_POSIX_SOURCE) || defined(_POSIX_C_SOURCE) \

diff --git a/include/time.h b/include/time.h
index fbc47db90e7cf5f62225f8ecf4d78ad7c46263fa..4a681b4a01b6d0a81d643a8f7d354f80a066ba03 100644 (file)
--- a/include/time.h
+++ b/include/time.h
@@ -7,13 +7,7 @@ extern "C" {
 
 #include <features.h>
 
-#undef NULL
-#ifdef __cplusplus
-#define NULL 0
-#else
-#define NULL ((void*)0)
-#endif
-
+#define NULL 0L
 
 #define __NEED_size_t
 #define __NEED_time_t

diff --git a/include/unistd.h b/include/unistd.h
index 89015ad60b36453d5f24e3f299db8d9fb3a81a79..07e49e97e23dac1c5fa179881ea3352602d0ca70 100644 (file)
--- a/include/unistd.h
+++ b/include/unistd.h
@@ -15,12 +15,7 @@ extern "C" {
 #define SEEK_CUR 1
 #define SEEK_END 2
 
-#undef NULL
-#ifdef __cplusplus
-#define NULL 0
-#else
-#define NULL ((void*)0)
-#endif
+#define NULL 0L
 
 #define __NEED_size_t
 #define __NEED_ssize_t

diff --git a/include/wchar.h b/include/wchar.h
index 35706c7ff7abaf796d047ae13a315a5165105a1e..9ceddd57550fc60c386cd3f68c438f4271256fd6 100644 (file)
--- a/include/wchar.h
+++ b/include/wchar.h
@@ -26,12 +26,7 @@ extern "C" {
 
 #include <bits/wchar.h>
 
-#undef NULL
-#ifdef __cplusplus
-#define NULL 0
-#else
-#define NULL ((void*)0)
-#endif
+#define NULL 0L
 
 #undef WEOF
 #define WEOF (-1)