Import official C23 code examples for Modern C (Jens Gustedt, 2024)HEADupstream-importmain

- Add official C source files, Makefile, c23-fallback.h, and LICENSE
- Update README.md with study mirror notice

1 files changed, 96 insertions, 0 deletions

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+#include <stdio.h>
+
+/**
+ ** @file
+ **
+ ** This plays with gcc features to define a weird macro that emulates
+ ** anonymous functions that can be used as closures. Don't use that
+ ** seriously, this is only meant to prove that this is syntactically
+ ** not a big deal, and that the ideas necessary for it can be found
+ ** in modern compilers.
+ **
+ **/
+
+/**
+ ** @def ANONYMOUS(RET)
+ ** @brief Use this to define an anonymous function with return type @a RET.
+ **
+ ** The @c ... argument list is the parameter list of the
+ ** function. This the expects the code of the function in <code>({
+ ** })</code> parenthesis. A simple example would be
+ **
+ ** @code
+ ** // Function header
+ ** ANONYMOUS(int) (double a, unsigned b)
+ ** // Function body
+ ** ({
+ **    return printf("we found %g for %u, average %g\n", a, b, a/b);
+ ** })
+ ** // Function arguments
+ ** (x, 23);
+ ** @endcode
+ **
+ ** Of course you would usually put such things inside a macro. See
+ ** MAX() for a more complicated example.
+ **/
+#ifdef __clang__
+# define ANONYMOUS(RET) (^ RET ANON_PART2
+# define ANON_PART2(...) (__VA_ARGS__) ANON_PART3
+# define ANON_PART3(...) __VA_ARGS__)
+#else
+# define ANONYMOUS(RET) ({ RET __anon_func ANON_PART2
+# define ANON_PART2(...) (__VA_ARGS__) ANON_PART3
+# define ANON_PART3(...) __VA_ARGS__ __anon_func; })
+#endif
+
+#define MAXU(A, B)                              \
+ANONYMOUS(uintmax_t) (uintmax_t a, uintmax_t b) \
+({                                              \
+    if (a < b) return b;                        \
+    else return a;                              \
+})                                              \
+((A), (B))
+
+#define MAX(A, B)                               \
+/* function header */                           \
+  /* return type */                             \
+ANONYMOUS(__typeof__(1 ? (A) : (B)))            \
+ /*and then argument list */                    \
+(__typeof__(A) a, __typeof__(B) b)              \
+/* function body */                             \
+({                                              \
+  if (a < b) return b;                          \
+  else return a;                                \
+ })                                             \
+/* function call */                             \
+((A), (B))
+
+/**
+ ** Test to see if that even works when such functions are nested.
+ **/
+
+typedef void df(unsigned);
+
+df* Df[5] = { };
+
+void g(unsigned level, unsigned depth) {
+  Df[depth%5] =
+    ANONYMOUS(void) (register unsigned a)
+    ({
+      printf("from level %u, depth %u found in position %u\n",
+             level, depth,
+             MAX(2*a, depth)
+             );
+      return;
+    });
+  if (level)
+    g(level-1, depth+1);
+  else
+    for (unsigned i = 0; i < 5; ++i)
+      if (Df[i]) Df[i](i);
+}
+
+
+int main(int argc, [[maybe_unused]] char* argv[]) {
+  g(argc, 0);
+}