<sys/param.h>: Add nitems()

'nitems()' calculates the length of an array in number of items. It is safe: if a pointer is passed to the macro (or function, in C++), the compilation is broken due to:
- In >= C11: _Static_assert()
- In C89, C99: Negative anonymous bitfield
- In C++: The template requires an array

Some BSDs already provide a macro nitems() in <sys/param.h>, although it usually doesn't provide safety against pointers.

This patch uses the same name for compatibility reasons, and to be the least disruptive with existing code.

This patch also adds some other macros, which are required by 'nitems()':

__is_same_type(a, b):
Returns non-zero if the two input arguments are of the same type.

__is_array(arr):
Returns non-zero if the input argument is of an array type.

__must_be(expr, msg):
Allows using _Static_assert() everywhere an expression can be used. It evaluates '(int)0' or breaks the compilation.

__must_be_array(arr):
It evaluates to '(int)0' if the argument is of an array type. Else, it breaks compilation.

__nitems(arr):
It implements the basic sizeof division needed to calculate the array length.

P.S.: I'd like to put this patch in the public domain.

Signed-off-by Alejandro Colomar <[email protected]>

---

A few changes since v3:

- Macros don't need reserved names in their parameters, so I simplified those names.

- I fixed some wrong indentation levels.

- Renamed __array_len() to __nitems() for consistency.

  misc/sys/param.h | 47 +++++++++++++++++++++++++++++++++++++++++++++++
  1 file changed, 47 insertions(+)

misc/sys/param.h +47 -0

index d7c319b157..08d4093961 100644
@@ -102,5 +102,52 @@
  #define MIN(a,b) (((a)<(b))?(a):(b))
  #define MAX(a,b) (((a)>(b))?(a):(b))
 
+/* Macros related to the types of variables */
+#define __is_same_type(a, b)                                                  \
+	__builtin_types_compatible_p(__typeof__(a), __typeof__(b))
+#define __is_array(arr)	(!__is_same_type((arr), &(arr)[0]))
+
+/* Macros for embedding _Static_assert() in expressions */
+#if __STDC_VERSION__ >= 201112L
+# define __must_be(expr, msg)   (                                             \
+        0 * (int)sizeof(                                                      \
+          struct {                                                            \
+            _Static_assert((expr), msg);                                      \
+            char _ISO_C_forbids_a_struct_with_no_members;                     \
+          }                                                                   \
+        )                                                                     \
+)
+#else
+# define __must_be(expr, msg)   (                                             \
+        0 * (int)sizeof(                                                      \
+          struct {                                                            \
+            int  : (-!(expr));                                                \
+            char _ISO_C_forbids_a_struct_with_no_members;                     \
+          }                                                                   \
+        )                                                                     \
+)
+#endif
+#define __must_be_array(arr)	__must_be(__is_array(arr), "Must be an array!")
+
+/* Macros for array sizes */
+#if defined(__cplusplus)
+# if __cplusplus >= 201103L
+template<typename _Tp, std::size_t _Len>
+  constexpr inline std::size_t
+  nitems(const _Tp(&)[_Len]) __THROW
+  {
+    return _Len;
+  }
+# else /* __cplusplus < 201103L */
+template<typename _Tp, std::size_t _Len>
+  char
+  (&__nitems_chararr(const _Tp(&)[_Len]))[_Len];
+#  define nitems(arr)	(sizeof(__nitems_chararr(arr)))
+# endif /* __cplusplus < 201103L */
+#else /* !defined(__cplusplus) */
+# define __nitems(arr)	(sizeof((arr)) / sizeof((arr)[0]))
+# define nitems(arr)	(__nitems(arr) + __must_be_array(arr))
+#endif /* !defined(__cplusplus) */
+
 
  #endif  /* sys/param.h */

'nitems()' calculates the length of an array in number of items. It is safe: if a pointer is passed to the macro (or function, in C++), the compilation is broken due to:
- In >= C11: _Static_assert()
- In C89, C99: Negative anonymous bitfield
- In C++: The template requires an array

Some BSDs already provide a macro nitems() in <sys/param.h>, although it usually doesn't provide safety against pointers.

This patch uses the same name for compatibility reasons, and to be the least disruptive with existing code.

This patch also adds some other macros, which are required by 'nitems()':

__is_same_type(a, b):
Returns non-zero if the two input arguments are of the same type.

__is_array(arr):
Returns non-zero if the input argument is of an array type.

__must_be(expr, msg):
Allows using _Static_assert() everywhere an expression can be used. It evaluates '(int)0' or breaks the compilation.

__must_be_array(arr):
It evaluates to '(int)0' if the argument is of an array type. Else, it breaks compilation.

__nitems(arr):
It implements the basic sizeof division needed to calculate the array length.

P.S.: I'd like to put this patch in the public domain.

Signed-off-by Alejandro Colomar <[email protected]>

---

A few changes since v3:

- Macros don't need reserved names in their parameters, so I simplified those names.

- I fixed some wrong indentation levels.

- Renamed __array_len() to __nitems() for consistency.

  misc/sys/param.h | 47 +++++++++++++++++++++++++++++++++++++++++++++++
  1 file changed, 47 insertions(+)

misc/sys/param.h +47 -0

index d7c319b157..08d4093961 100644
@@ -102,5 +102,52 @@
  #define MIN(a,b) (((a)<(b))?(a):(b))
  #define MAX(a,b) (((a)>(b))?(a):(b))
 
+/* Macros related to the types of variables */
+#define __is_same_type(a, b)                                                  \
+	__builtin_types_compatible_p(__typeof__(a), __typeof__(b))
+#define __is_array(arr)	(!__is_same_type((arr), &(arr)[0]))
+
+/* Macros for embedding _Static_assert() in expressions */
+#if __STDC_VERSION__ >= 201112L
+# define __must_be(expr, msg)   (                                             \
+        0 * (int)sizeof(                                                      \
+          struct {                                                            \
+            _Static_assert((expr), msg);                                      \
+            char _ISO_C_forbids_a_struct_with_no_members;                     \
+          }                                                                   \
+        )                                                                     \
+)
+#else
+# define __must_be(expr, msg)   (                                             \
+        0 * (int)sizeof(                                                      \
+          struct {                                                            \
+            int  : (-!(expr));                                                \
+            char _ISO_C_forbids_a_struct_with_no_members;                     \
+          }                                                                   \
+        )                                                                     \
+)
+#endif
+#define __must_be_array(arr)	__must_be(__is_array(arr), "Must be an array!")
+
+/* Macros for array sizes */
+#if defined(__cplusplus)
+# if __cplusplus >= 201103L
+template<typename _Tp, std::size_t _Len>
+  constexpr inline std::size_t
+  nitems(const _Tp(&)[_Len]) __THROW
+  {
+    return _Len;
+  }
+# else /* __cplusplus < 201103L */
+template<typename _Tp, std::size_t _Len>
+  char
+  (&__nitems_chararr(const _Tp(&)[_Len]))[_Len];
+#  define nitems(arr)	(sizeof(__nitems_chararr(arr)))
+# endif /* __cplusplus < 201103L */
+#else /* !defined(__cplusplus) */
+# define __nitems(arr)	(sizeof((arr)) / sizeof((arr)[0]))
+# define nitems(arr)	(__nitems(arr) + __must_be_array(arr))
+#endif /* !defined(__cplusplus) */
+
 
  #endif  /* sys/param.h */

'nitems()' calculates the length of an array in number of items. It is safe: if a pointer is passed to the macro (or function, in C++), the compilation is broken due to:
- In >= C11: _Static_assert()
- In C89, C99: Negative anonymous bitfield
- In C++: The template requires an array

Some BSDs already provide a macro nitems() in <sys/param.h>, although it usually doesn't provide safety against pointers.

This patch uses the same name for compatibility reasons, and to be the least disruptive with existing code.

This patch also adds some other macros, which are required by 'nitems()':

__is_same_type(a, b):
Returns non-zero if the two input arguments are of the same type.

__is_array(arr):
Returns non-zero if the input argument is of an array type.

__must_be(expr, msg):
Allows using _Static_assert() everywhere an expression can be used. It evaluates '(int)0' or breaks the compilation.

__must_be_array(arr):
It evaluates to '(int)0' if the argument is of an array type. Else, it breaks compilation.

__nitems(arr):
It implements the basic sizeof division needed to calculate the array length.

P.S.: I'd like to put this patch in the public domain.

Signed-off-by Alejandro Colomar <[email protected]>

---

A few changes since v3:

- Macros don't need reserved names in their parameters, so I simplified those names.

- I fixed some wrong indentation levels.

- Renamed __array_len() to __nitems() for consistency.

 misc/sys/param.h | 47 +++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 47 insertions(+)

misc/sys/param.h +47 -0

index d7c319b157..08d4093961 100644
@@ -102,5 +102,52 @@
 #define MIN(a,b) (((a)<(b))?(a):(b))
 #define MAX(a,b) (((a)>(b))?(a):(b))
 
+/* Macros related to the types of variables */
+#define __is_same_type(a, b)                                                  \
+	__builtin_types_compatible_p(__typeof__(a), __typeof__(b))
+#define __is_array(arr)	(!__is_same_type((arr), &(arr)[0]))
+
+/* Macros for embedding _Static_assert() in expressions */
+#if __STDC_VERSION__ >= 201112L
+# define __must_be(expr, msg)   (                                             \
+        0 * (int)sizeof(                                                      \
+          struct {                                                            \
+            _Static_assert((expr), msg);                                      \
+            char _ISO_C_forbids_a_struct_with_no_members;                     \
+          }                                                                   \
+        )                                                                     \
+)
+#else
+# define __must_be(expr, msg)   (                                             \
+        0 * (int)sizeof(                                                      \
+          struct {                                                            \
+            int  : (-!(expr));                                                \
+            char _ISO_C_forbids_a_struct_with_no_members;                     \
+          }                                                                   \
+        )                                                                     \
+)
+#endif
+#define __must_be_array(arr)	__must_be(__is_array(arr), "Must be an array!")
+
+/* Macros for array sizes */
+#if defined(__cplusplus)
+# if __cplusplus >= 201103L
+template<typename _Tp, std::size_t _Len>
+  constexpr inline std::size_t
+  nitems(const _Tp(&)[_Len]) __THROW
+  {
+    return _Len;
+  }
+# else /* __cplusplus < 201103L */
+template<typename _Tp, std::size_t _Len>
+  char
+  (&__nitems_chararr(const _Tp(&)[_Len]))[_Len];
+#  define nitems(arr)	(sizeof(__nitems_chararr(arr)))
+# endif /* __cplusplus < 201103L */
+#else /* !defined(__cplusplus) */
+# define __nitems(arr)	(sizeof((arr)) / sizeof((arr)[0]))
+# define nitems(arr)	(__nitems(arr) + __must_be_array(arr))
+#endif /* !defined(__cplusplus) */
+
 
 #endif  /* sys/param.h */

I've asked the WG14 reflector why N2529 (and N2530, though that one's not relevant to this feature) doesn't seem to have made it onto a meeting agenda yet, when there have been two WG14 meetings since that proposal

made and a third one coming up.

Nice!
Please update me on any feedback you receive.

So glibc will basically hold this patch
at least until the WG answers to that, right?

Quoted 2 lines · Alejandro Colomar via Libc-alpha

Nice!
Please update me on any feedback you receive.

Apparently the author is planning new versions of those papers so WG14 discussion is waiting for those.

Quoted 2 lines

So glibc will basically hold this patch
at least until the WG answers to that, right?

I think that whether C2x gets an array-size feature of some kind is relevant to whether such a feature goes in glibc and what it looks like in glibc, but the fact that it will be considered in WG14 doesn't rule out glibc considering such a feature without waiting for WG14.