Set the C standard to 11 (#4975)

Ignores some of the older Autotools platform files, since the Autotools
will be dropped in the next major release (and we don't have
compilers on which to test, anyway).

Also drops support for the old, non-compliant MSVC
preprocessor.

config/clang-flags

@@ -116,14 +116,8 @@ if test "X-clang" = "X-$cc_vendor" -o "X-Apple LLVM" = "X-$cc_vendor"; then
     esac
 
     case "$host_os-$host_cpu" in
-        # cygwin needs the "-std=c99" flag removed, so make
-        # a specific case for Cygwin without the flag and a default
-        # case to add the flag everywhere else
-        cygwin-*)
-            ;;
-
         *)
-            H5_CFLAGS="$H5_CFLAGS -std=c99"
+            H5_CFLAGS="$H5_CFLAGS -std=c11"
             ;;
     esac
 

config/cmake/HDFCompilerFlags.cmake

@@ -9,10 +9,10 @@
 # If you do not have access to either file, you may request a copy from
 # [email protected].
 #
-set (CMAKE_C_STANDARD 99)
+set (CMAKE_C_STANDARD 11)
 set (CMAKE_C_STANDARD_REQUIRED TRUE)
 
-set (CMAKE_C_FLAGS "${CMAKE_C99_STANDARD_COMPILE_OPTION} ${CMAKE_C_FLAGS}")
+set (CMAKE_C_FLAGS "${CMAKE_C11_STANDARD_COMPILE_OPTION} ${CMAKE_C_FLAGS}")
 set (CMAKE_C_FLAGS "${CMAKE_C_SANITIZER_FLAGS} ${CMAKE_C_FLAGS}")
 message (VERBOSE "Warnings Configuration: C default: ${CMAKE_C_FLAGS}")
 #-----------------------------------------------------------------------------

config/gnu-flags

@@ -117,18 +117,15 @@ if test "X-gcc" = "X-$cc_vendor"; then
     esac
 
     case "$host_os-$host_cpu" in
-        # Cygwin needs the "-std=c99" flag removed.
-        cygwin-*)
-            ;;
-        # On Solaris, gcc needs the gnu99 standard to pick up certain POSIX
+        # On Solaris, gcc needs the gnu11 standard to pick up certain POSIX
         # things. Do NOT use this as the gcc norm as this encourages the use
         # of non-standard gcc extensions.
         *solaris*)
-            H5_CFLAGS="$H5_CFLAGS -std=gnu99"
+            H5_CFLAGS="$H5_CFLAGS -std=gnu11"
             ;;
-        # Everybody else gets c99 as the standard.
+        # Everybody else gets C11 as the standard.
         *)
-            H5_CFLAGS="$H5_CFLAGS -std=c99"
+            H5_CFLAGS="$H5_CFLAGS -std=c11"
             ;;
     esac
 

config/nvidia-flags

@@ -68,8 +68,8 @@ if test "X-nvc" = "X-$cc_vendor" -o "X-nvcc" = "X-$cc_vendor"; then
     # General #
     ###########
 
-    # Default to C99 standard.
-    H5_CFLAGS="$H5_CFLAGS $arch -c99 -Minform=warn"
+    # Default to C11 standard.
+    H5_CFLAGS="$H5_CFLAGS $arch -std=c11 -Minform=warn"
 
     ##############
     # Production #

config/oneapi-flags

@@ -80,8 +80,8 @@ if test "X-icx" = "X-$cc_vendor"; then
     # General #
     ###########
 
-    # Default to C99 standard.
-    H5_CFLAGS="$H5_CFLAGS $arch -std=c99"
+    # Default to C11 standard.
+    H5_CFLAGS="$H5_CFLAGS $arch -std=c11"
 
     ##############
     # Production #

release_docs/RELEASE.txt

@@ -47,6 +47,21 @@ New Features
 
     Configuration:
     -------------
+    - Dropped support for the traditional MSVC preprocessor
+
+      Visual Studio has recently started using a standards-compliant
+      preprocessor (In VS2019+) and this is the default in C11.
+
+      https://learn.microsoft.com/en-us/cpp/preprocessor/preprocessor-experimental-overview?view=msvc-170
+
+      Because of this, we've dropped support for the traditional
+      MSVC preprocessor.
+
+    - The standard for building the library is now C11
+
+      We have updated the build files to set the C standard to C11, though
+      some platforms use gnu11 to get some GNU things to work.
+
     - Added configuration option for internal threading/concurrency support:
 
           CMake:      HDF5_ENABLE_THREADS (ON/OFF) (Default: ON)

src/H5win32defs.h

@@ -43,20 +43,8 @@ struct timezone {
 #define HDlstat(S, B)        _lstati64(S, B)
 #define HDmkdir(S, M)        _mkdir(S)
 
-/* Note that with the traditional MSVC preprocessor, the variadic
- * HDopen macro uses an MSVC-specific extension where the comma
- * is dropped if nothing is passed to the ellipsis.
- *
- * MinGW and the newer, conforming MSVC preprocessor do not exhibit this
- * behavior.
- */
-#if (defined(_MSC_VER) && !defined(_MSVC_TRADITIONAL)) || defined(_MSVC_TRADITIONAL)
-/* Using the MSVC traditional preprocessor */
-#define HDopen(S, F, ...) Wopen(S, F, __VA_ARGS__)
-#else
-/* Using a standards conformant preprocessor */
+/* We only support the standards conformant preprocessor */
 #define HDopen(S, F, ...) Wopen(S, F, ##__VA_ARGS__)
-#endif
 
 #define HDremove(S)           Wremove(S)
 #define HDsetenv(N, V, O)     Wsetenv(N, V, O)