More syscall functions

teensy4/imxrt.ld

 	_sbss = ADDR(.bss);
 	_ebss = ADDR(.bss) + SIZEOF(.bss);
 
-	_sdmamem = ADDR(.bss.dma);
-	_edmamem = ADDR(.bss.dma) + SIZEOF(.bss.dma);
-	_emallocmem = ORIGIN(RAM) + LENGTH(RAM);
+	_heap_start = ADDR(.bss.dma) + SIZEOF(.bss.dma);
+	_heap_end = ORIGIN(RAM) + LENGTH(RAM);
 
 	_flashimagelen = SIZEOF(.text.progmem) + SIZEOF(.text) + SIZEOF(.data);
 	_teensy_model_identifier = 0x23;

teensy4/libc.c

-#include <errno.h>
-#include "debug/printf.h"
-
-// from the linker script
-extern unsigned long _edmamem;
-extern unsigned long _emallocmem;
-
-char *__brkval = (char *)&_edmamem;
-
-void * _sbrk(int incr)
-{
-	char *prev;
-
-	printf("sbrk incr = %d\n", incr);
-
-	prev = __brkval;
-	if (incr != 0) {
-		if (prev + incr > (char *)&_emallocmem) {
-			errno = ENOMEM;
-			return (void *)-1;
-		}
-		__brkval = prev + incr;
-	}
-	return prev;
-}
-
+// this file no longer used

teensy4/startup.c

 }
 
 
+
+// syscall functions need to be in the same C file as the entry point "ResetVector"
+// otherwise the linker will discard them in some cases.
+
+#include <errno.h>
+
+// from the linker script
+extern unsigned long _heap_start;
+extern unsigned long _heap_end;
+
+char *__brkval = (char *)&_heap_start;
+
+void * _sbrk(int incr)
+{
+        char *prev = __brkval;
+        if (incr != 0) {
+                if (prev + incr > (char *)&_heap_end) {
+                        errno = ENOMEM;
+                        return (void *)-1;
+                }
+                __brkval = prev + incr;
+        }
+        return prev;
+}
+
+__attribute__((weak))
+int _read(int file, char *ptr, int len)
+{
+	return 0;
+}
+
+__attribute__((weak))
+int _close(int fd)
+{
+	return -1;
+}
+
+#include <sys/stat.h>
+
+__attribute__((weak))
+int _fstat(int fd, struct stat *st)
+{
+	st->st_mode = S_IFCHR;
+	return 0;
+}
+
+__attribute__((weak))
+int _isatty(int fd)
+{
+	return 1;
+}
+
+__attribute__((weak))
+int _lseek(int fd, long long offset, int whence)
+{
+	return -1;
+}
+
+__attribute__((weak))
+void _exit(int status)
+{
+	while (1);
+}
+
+__attribute__((weak))
+void __cxa_pure_virtual()
+{
+	while (1);
+}
+
+__attribute__((weak))
+int __cxa_guard_acquire (char *g)
+{
+	return !(*g);
+}
+
+__attribute__((weak))
+void __cxa_guard_release(char *g)
+{
+	*g = 1;
+}
+
+__attribute__((weak))
+void abort(void)
+{
+	while (1) ;
+}
+