Require C99

This commit is a compilation of many old patches of mine.
It removes obsolete code and enforces C99 as a build
requirement.

Changes:
- Remove many unused tests and useless feature tests.
  (This was a bit more involved for <sys/types.h>, which
  had annoying special casing.) This included the
  'check for global headers' in the iffe script.
- Assume fchmod is present and use it instead of chmod in
  shcomp for slightly better performance.
- dotpaths_lib(): Use fstat rather than stat for the PWD
  for better performance.
- Scrap pre-C99 fallbacks for iswblank(3).
- Replaced usage of the obsolescent timeofday() macro with the
  more portable tvgettime() function in libast, which itself uses
  the modern clock_gettime internally.
- Replaced usage of strcopy with either strcpy or stpcpy in
  cases where no buffer overlap occurs. There are perhaps
  more places where replacement is possible (ksh93v- and ksh2020
  do this far more liberally), but I'm trying to be conservative
  with these changes to avoid accidentally introducing bugs.
- Replaced an instance of strncopy with strlcpy in keytrap()
  because the buffers don't overlap.
  - Also replaced some instances of strncpy with strlcpy
    (vide att#266 and f63ebd0).
- Add pertinent documentation as it relates to str*copy's
  usage with overlapping buffers.
- Backported a modified version of ksh93v-'s stpcpy fallback.
- Introduced usage of C99 restrict, primarily for standards
  compliance in compatibility functions (though some regular
  functions like sfopen() do also use restrict now).
- Assume C99 __func__ is available and drop now unneeded
  C90 feature tests.
- Added a few casts to fix some compiler warnings.
- Remove basic iffe probes for various C99 multibyte functions
  (excluding those that still need tests due to platform-specific
  bugs). Also removed other unnecessary probes for C99 and POSIX
  functionality.
- Remove unnecessary mktime.c, wc.c and wordexp.c compat files
  (the operating system should provide these functions).
- Switched to using C99-style struct init throughout the codebase.
- Removed usage of bzero, bcopy, memclear and memzero in favor
  of ISO C memset and memcpy.
- make.probe: Automatically use the latest C standard available
  (i.e. C23) because GCC/Clang may use something older by default
  whilst having support for newer standards.

I have additional changes for using C99-style for loops and bools,
but those have been excluded because this patch is big enough as it is.

Progresses #777
(This commit doesn't include or fix the removed _c99_in_the_wild code.)

Author: JohnoKing

NEWS

@@ -30,6 +30,8 @@ Uppercase BUG_* IDs are shell bug IDs as used by the Modernish shell library.
 - Fixed an issue, introduced on 2022-06-13, causing a subshell to fork
   unnecessarily if an EXIT pseudosignal trap is set within it.
 
+- The ksh93 codebase now requires a C99 compiler to build.
+
 2026-03-16:
 
 - Fixed a bug in the join(1) built-in command (bound to /opt/ast/bin) that

src/cmd/INIT/README-mamake.md

@@ -4,7 +4,7 @@ MAM (Make Abstract Machine) is a simple rule-based make language
 that is implemented in just ten four-letter commands and four attributes,
 yet allows unlimited flexibility as it can execute arbitrary shell code.
 The program implementing MAM, `mamake`,
-is a portable C90 program written in a single file, `mamake.c`.
+is a portable C99 program written in a single file, `mamake.c`.
 This allows ksh 93u+m,
 or other programs using this build system,
 to be built using only a standard C compiler and utilities installation

src/cmd/INIT/iffe.sh

@@ -82,10 +82,10 @@ is_hdr() # [ - ] [ file.c ] hdr
 	esac
 	is hdr $1
 	case $1 in
-	sys/types.h | limits.h | stdio.h | unistd.h)
-		# These are often tested for repeatedly, especially sys/types.h.
-		# But POSIX has specified these since issue 1 (1988). It's 2023.
-		# Skip the compile to save time, but act like a positive test.
+	sys/types.h | limits.h | stddef.h | stdio.h | stdlib.h | unistd.h)
+		# These are often tested for repeatedly, but POSIX has specified
+		# these since issue 1 (1988). It's 2026. Skip the compile to save time,
+		# but act like a positive test.
 		: 2>$tmp.e
 		;;
 	*)	compile $cc -c $_is_hdr_file <&$nullin >&$nullout 2>$tmp.e
@@ -689,7 +689,7 @@ case $( (getopts '[-][123:xyz]' opt --xyz; echo 0$opt) 2>/dev/null ) in
 	}
 [+?\abegin\a\b{\b ... \b}end\b delimit multiline code blocks that override
 	or augment the default code provided by \biffe\b. User supplied code
-	blocks should be compatible with the C89/C90 C language
+	blocks should be compatible with the C99 C language
 	standard for maximal portability. Test code may call the function
 	\bNOTE("...")\b to emit short text in \b--verbose\b output; only one
 	\bNOTE()\b should be called per test for readability. In addition to
@@ -1162,7 +1162,6 @@ can=
 cansep=
 cctest=
 file=
-hdrtest=
 ifelse=NONE
 ifstack=
 ini=
@@ -2263,51 +2262,6 @@ int x;
 
 		cc="$cc $mac"
 
-		# check for global default headers (some cc -E insist on compiling)
-
-		case $hdrtest in
-		'')	hdrtest=1
-			allinc=
-			for x in types
-			do	case $config in
-				0)	c=_sys_${x}
-					;;
-				1)	case $shell in
-					ksh)	typeset -u u=$x ;;
-					*)	u=$(echo $x | tr abcdefghijklmnopqrstuvwxyz ABCDEFGHIJKLMNOPQRSTUVWXYZ) ;;
-					esac
-					c=HAVE_SYS_${u}_H
-					;;
-				esac
-				x=sys/$x.h
-				echo "${allinc}#include <$x>" > $tmp.c
-				if	is_hdr $x
-				then	gothdr="$gothdr + $x"
-					case $explicit in
-					0)	can="$can$cansep#define $c	1	/* #include <$x> ok */"
-						nan="$nan$cansep$c=1"
-						cansep=$nl
-						;;
-					esac
-					eval $c=1
-					allinc="${allinc}#include <$x>$nl"
-				else	gothdr="$gothdr - $x"
-					case $explicit$all$config$undef in
-					0?1?|0??1)
-						can="$can$cansep#undef	$c		/* #include <$x> not ok */"
-						nan="$nan$cansep$c="
-						cansep=$nl
-						;;
-					01??)	can="$can$cansep#define $c	0	/* #include <$x> not ok */"
-						nan="$nan$cansep$c=0"
-						cansep=$nl
-						;;
-					esac
-				fi
-			done
-			;;
-		esac
-
 		# add implicit headers/libraries before the checks
 
 		case $op in