[libc] Add proper alloca to OpenWatcom C library

libc/include/alloca.h

@@ -0,0 +1,25 @@
+#ifndef __ALLOCA_H
+#define __ALLOCA_H
+
+// void *alloca(size_t);
+
+int __stackavail(unsigned int size);
+#define __ALLOCA_ALIGN(s)   (((s)+(sizeof(int)-1))&~(sizeof(int)-1))
+
+#define alloca(s)           (__stackavail(__ALLOCA_ALIGN(s))?   \
+                                 __alloca(__ALLOCA_ALIGN(s)): (void *)0)
+
+#ifdef __WATCOMC__
+#pragma aux __stackavail "*" __modify __nomemory
+
+extern void __based(__segname("_STACK")) *__alloca(unsigned int __size);
+#pragma aux __alloca =                  \
+    "sub sp,ax"                         \
+    __parm __nomemory [__ax]            \
+    __value [__sp]                      \
+    __modify __exact __nomemory [__sp]
+
+#endif
+
+
+#endif

libc/include/malloc.h

@@ -7,7 +7,6 @@ void *malloc(size_t);
 void free(void *);
 void *realloc(void *, size_t);
 void *calloc(size_t elm, size_t sz);
-void *alloca(size_t);
 
 #ifdef __LIBC__
 /*

libc/include/stdlib.h

@@ -5,6 +5,7 @@
 #include <features.h>
 #include <sys/types.h>
 #include <malloc.h>
+#include <alloca.h>
 #include <arch/divmod.h>
 
 /* Don't overwrite user definitions of NULL */

libc/watcom/syscall/crt0.c

@@ -44,17 +44,8 @@ int __argc;
 char **__argv;
 char *__program_filename;
 char **environ;
-unsigned int __stacklow;
-unsigned char _HShift = 12;      /* huge pointer support required by pia.asm */
-
-static unsigned int _SP(void);
-#pragma aux _SP = __value [__sp]
-
-/* called by alloca() to check stack available */
-unsigned int stackavail(void)
-{
-    return (_SP() - __stacklow);
-}
+unsigned int __stacklow;        /* lowest valid SP value */
+unsigned char _HShift = 12;     /* huge pointer support required by pia.asm */
 
 #if defined(__SMALL__) || defined(__MEDIUM__)   /* near data models */
 /* no argv/environ rewrite */

libc/watcom/syscall/stackcheck.c

@@ -0,0 +1,69 @@
+/*
+ * Open Watcom C stack checking helper routines
+ *
+ * 4 Dec 2024 Greg Haerr
+ */
+
+#include <sys/cdefs.h>
+#include <alloca.h>
+#include <unistd.h>
+
+extern unsigned int __stacklow;
+
+#define errmsg(str)     write(STDERR_FILENO, str, sizeof(str) - 1)
+
+static unsigned int __SP(void);
+#pragma aux __SP = __value [__sp]
+
+static void stack_alloca_warning(void)
+{
+    errmsg("ALLOCA FAIL, INCREASE STACK\n");
+}
+
+/* 
+ * Return true if stack can be extended by size bytes,
+ * called by alloca() to check stack available.
+ */
+#pragma aux __stackavail "*"
+int __stackavail(unsigned int size)
+{
+    unsigned int remaining = __SP() - __stacklow;
+
+    if ((int)remaining >= 0 && remaining >= size)
+        return 1;
+    stack_alloca_warning();
+    return 0;
+}
+
+#if LATER
+static void stack_overflow(void)
+{
+    errmsg("STACK OVERFLOW\n");
+}
+
+static void stack_overlimit(void)
+{
+    errmsg("STACK OVER LIMIT\n");
+}
+#endif
+
+/* 
+ * Check if size bytes can be allocated from stack,
+ * called from function prologue when -fstack-check set.
+ */
+#pragma aux __STK "*"
+void __STK(unsigned int size)
+{
+    unsigned int remaining = __SP() - __stacklow;
+    unsigned int curbreak;
+
+    if ((int)remaining >= 0 && remaining >= size)
+        return;
+    curbreak = (unsigned int)sbrk(0);   /* NOTE syscall here will cause SIGSEGV sent */
+#if LATER
+    if (__SP() < curbreak)
+        stack_overflow();
+    else
+        stack_overlimit();
+#endif
+}