Add ISR print functions PrintFromISR() and VPrintFromISR()

The functions PrintFromISR() and VPrintFromISR() perform formatted
output from ISRs. Don't use them outside of an ISR. The functions do
not synchronize with the output of the regular print functions of any
task.

Author: tdz

src/FormattedIO.c

@@ -64,3 +64,47 @@ VPrint(const char* fmt, va_list ap)
   }
   return PrintIPC(res, buf);
 }
+
+int
+_Print(const char* fmt, ...)
+{
+  va_list ap;
+
+  va_start(ap, fmt);
+  int res = _VPrint(fmt, ap);
+  va_end(ap);
+
+  return res;
+}
+
+int
+_VPrint(const char* fmt, va_list ap)
+{
+  char buf[128];
+  int res = vsnprintf(buf, sizeof(buf), fmt, ap);
+  if (res < 0) {
+    return -1;
+  }
+  SerialPutString(res, buf);
+  return res;
+}
+
+int
+PrintFromISR(const char* fmt, ...)
+{
+  va_list ap;
+
+  va_start(ap, fmt);
+  int res = VPrintFromISR(fmt, ap);
+  va_end(ap);
+
+  return res;
+}
+
+int
+VPrintFromISR(const char* fmt, va_list ap)
+{
+  /* TODO: ISRs might be limited on stack memory. Output each
+   *       character individually; instead of a string buffer. */
+  return _VPrint(fmt, ap);
+}

src/FormattedIO.h

@@ -11,3 +11,17 @@ Print(const char* fmt, ...);
 
 int
 VPrint(const char* fmt, va_list ap);
+
+int
+PrintFromISR(const char* fmt, ...);
+
+int
+VPrintFromISR(const char* fmt, va_list ap);
+
+/* Internal functions for debugging; don't use in production code. */
+
+int
+_Print(const char* fmt, ...);
+
+int
+_VPrint(const char* fmt, va_list ap);