Revert "CMake: Change our configuration to enable C17"

This reverts commit 0781007df4.

CMakeLists.txt

@@ -567,18 +567,6 @@ if( CMAKE_C_COMPILER_ID MATCHES "MSVC")
 		list(APPEND LOCAL_CFLAGS /analyze:WX-)
 	endif()
 
-	# The /std:c17 option enables ISO C17 conformance. It's available
-	# starting in Visual Studio 2019 version 16.8.
-	#
-	# XXX Do we need to split this into LOCAL_CFLAGS + LOCAL_CXXFLAGS?
-	#
-	list(APPEND LOCAL_CFLAGS /std:c17)
-
-	# MSVC 14.28 enables C5105, but the Windows SDK 10.0.18362.0 triggers it.
-	if(CMAKE_C_COMPILER_ID STREQUAL "MSVC" AND NOT CMAKE_C_COMPILER_VERSION VERSION_LESS 19.28)
-		list(APPEND LOCAL_CFLAGS -wd5105)
-	endif()
-
 	# Additional compiler warnings to be treated as "Level 3"
 	# when compiling Wireshark sources. (Selected from "level 4" warnings).
 	## 4295: array is too small to include a terminating null character
@@ -609,20 +597,44 @@ else() # ! MSVC
 	endif()
 
 	#
-	# Do whatever is necessary to enable as much C17 support as
+	# Do whatever is necessary to enable as much C99 support as
 	# possible in the C compiler.  Newer versions of compilers
-	# might default to supporting C17, but older versions may
+	# might default to supporting C99, but older versions may
 	# require a special flag.
 	#
-	# We do not want strict C17 support, as we may also want to
+	# We do not want strict C99 support, as we may also want to
 	# use compiler extensions.
 	#
-	# We cannot use CMAKE_C_STANDARD to request C17 because
-	# it requires CMake 3.21.
+	# Prior to CMake 3.1, setting CMAKE_C_STANDARD will not have
+	# any effect, so, unless and until we require CMake 3.1 or
+	# later, we have to do it ourselves on pre-3.1 CMake, so we
+	# just do it ourselves on all versions of CMake.
+	#
+	# Note: with CMake 3.1 through 3.5, the only compilers for
+	# which CMake handles CMAKE_C_STANDARD are GCC and Clang.
+	# 3.6 adds support only for Intel C; 3.9 adds support for
+	# PGI C, Sun C, and IBM XL C, and 3.10 adds support for
+	# Cray C and IAR C, but no version of CMake has support for
+	# HP C.  Therefore, even if we use CMAKE_C_STANDARD with
+	# compilers for which CMake supports it, we may still have
+	# to do it ourselves on other compilers.
+	#
+	# In addition, CMake 3.5.2 seems to think that GCC versions
+	# less than 4.4 don't support -std=gnu99, which we need in
+	# order to get support for "for (int i = 0; i < n; i++) ;",
+	# which is another reason not to rely on CMake's CMAKE_C_STANDARD
+	# support.
+	#
+	# See the CMake documentation for the CMAKE_<LANG>_COMPILER_ID
+	# variables for a list of compiler IDs.
+	#
+	# We don't worry about MSVC; it doesn't have such a flag -
+	# either it doesn't support the C99 features we need at all,
+	# or it supports them regardless of the compiler flag.
 	#
 	# XXX - we add the flag for a given compiler to CMAKE_C_FLAGS,
 	# so we test whether it works and add it if we do.  We don't
-	# test whether it's necessary in order to get the C17 features
+	# test whether it's necessary in order to get the C99 features
 	# that we use; if we ever have a user who tries to compile with
 	# a compiler that can't be made to support those features, we
 	# can add a test to make sure we actually *have* C99 support.
@@ -630,10 +642,39 @@ else() # ! MSVC
 	if(CMAKE_C_COMPILER_ID MATCHES "GNU" OR
 	   CMAKE_C_COMPILER_ID MATCHES "Clang")
 		#
-		# We use -std=gnu17 rather than -std=c17 because extension
-		# use is evaluated on a case-by-case basis.
+		# We use -std=gnu99 rather than -std=c99 because, for
+		# some older compilers such as GCC 4.4.7, -std=gnu99
+		# is required to avoid errors about C99 constructs
+		# such as "for (int i = 0; i < n; i++) ;".
 		#
-		set(CMAKE_C_FLAGS "-std=gnu17 ${CMAKE_C_FLAGS}")
+		set(CMAKE_C_FLAGS "-std=gnu99 ${CMAKE_C_FLAGS}")
+	elseif(CMAKE_C_COMPILER_ID MATCHES "XL")
+		#
+		# We want support for extensions picked up for
+		# GNU C compatibility, so we use -qlanglvl=extc99.
+		#
+		set(CMAKE_C_FLAGS "-qlanglvl=extc99 ${CMAKE_C_FLAGS}")
+	elseif(CMAKE_C_COMPILER_ID MATCHES "HP")
+		#
+		# We also need to add -Wp,-H200000 to handle some large
+		# #defines we have; that flag is not necessary for the
+		# C++ compiler unless the "legacy" C++ preprocessor is
+		# being used (+legacy_cpp).  We don't want the legacy
+		# preprocessor if it's not the default, so we just add
+		# -Wp,-H200000 to the C flags.  (If there are older
+		# versions of aC++ that only support the legacy
+		# preprocessor, and require that we boost the table
+		# size, we'd have to check whether -Wp,-H200000 is
+		# supported by the C++ compiler and add it only if it is.)
+		#
+		set(CMAKE_C_FLAGS "-AC99 -Wp,-H200000 $WS_CFLAGS ${CMAKE_C_FLAGS}")
+	elseif(CMAKE_C_COMPILER_ID MATCHES "Sun")
+		#
+		# We also crank up the warning level.
+		#
+		set(CMAKE_C_FLAGS "-xc99 -v ${CMAKE_C_FLAGS}")
+	elseif(CMAKE_C_COMPILER_ID MATCHES "Intel")
+		set(CMAKE_C_FLAGS "-c99 ${CMAKE_C_FLAGS}")
 	endif()
 
 	if(CMAKE_C_COMPILER_ID MATCHES "Clang")

docbook/release-notes.adoc

@@ -17,7 +17,6 @@ It is used for troubleshooting, analysis, development and education.
 == What’s New
 
 * The PCRE2 library (https://www.pcre.org/) is now a required dependency to build Wireshark.
-* A compiler with C17 support is required.
 
 Many improvements have been made.
 See the “New and Updated Features” section below for more details.