Add typedef for putc1, fn_putc1_t.

Replaced relevent usage of `(void *)` with `fn_putc1_t`.
Correct usage of `ets_putc()`, returning 0, in libc_replacement.cpp
This PR assumes PR esp8266#6489 (comment) has merged and removes `uart_buff_switch` from `umm_performance.cpp`
Updated method of defining `_rom_putc1` to be more acceptable (I hope) to the new compiler.

Author: mhightower83

cores/esp8266/core_esp8266_postmortem.cpp

@@ -109,7 +109,7 @@ void __wrap_system_restart_local() {
         rst_info.reason = s_user_reset_reason;
 
     // TODO:  ets_install_putc1 definition is wrong in ets_sys.h, need cast
-    ets_install_putc1((void *)&uart_write_char_d);
+    ets_install_putc1(&uart_write_char_d);
 
     if (s_panic_line) {
         ets_printf_P(PSTR("\nPanic %S:%d %S"), s_panic_file, s_panic_line, s_panic_func);

cores/esp8266/libc_replacements.cpp

@@ -100,7 +100,8 @@ int ICACHE_RAM_ATTR _putc_r(struct _reent* r, int c, FILE* file) __attribute__((
 int ICACHE_RAM_ATTR _putc_r(struct _reent* r, int c, FILE* file) {
     (void) r;
     if (file->_file == STDOUT_FILENO) {
-        return ets_putc(c);
+      ets_putc(c);
+      return c;
     }
     return EOF;
 }

cores/esp8266/uart.cpp

@@ -899,7 +899,7 @@ void
 uart_set_debug(int uart_nr)
 {
     s_uart_debug_nr = uart_nr;
-    void (*func)(char) = NULL;
+    fp_putc_t func = NULL;
     switch(s_uart_debug_nr) 
     {
     case UART0:
@@ -921,7 +921,7 @@ uart_set_debug(int uart_nr)
         } else {
             system_set_os_print(0);
         }
-        ets_install_putc1((void *) func);
+        ets_install_putc1(func);
     }
 }
 

cores/esp8266/umm_malloc/umm_performance.cpp

@@ -40,24 +40,16 @@ bool ICACHE_FLASH_ATTR get_umm_get_perf_data(struct _UMM_TIME_STATS *p, size_t s
   when interrupts are disabled and w/o availability of heap resources.
 */
 
-// ROM _putc1, ignores CRs and sends CR/LF for LF, newline.
-// Always returns character sent.
-int constexpr (*_rom_putc1)(int) = (int (*)(int))0x40001dcc;
-void uart_buff_switch(uint8_t);
+/*
+ * ROM builtin "print character function" _putc1, ignores CRs and sends CR/LF
+ * for LF, newline. This function is used internally by ets_uart_printf. It is
+ * also the function that gets installed by ets_uart_install_printf through a
+ * call to ets_install_putc1.
+ */
+#define _rom_putc1 ((fp_putc_t)0x40001dcc)
 
 int _isr_safe_printf_P(const char *fmt, ...) __attribute__((format(printf, 1, 2)));
 int ICACHE_RAM_ATTR _isr_safe_printf_P(const char *fmt, ...) {
-#ifdef DEBUG_ESP_PORT
-#define VALUE(x) __STRINGIFY(x)
-  // Preprocessor and compiler together will optimize away the if.
-  if (strcmp("Serial1", VALUE(DEBUG_ESP_PORT)) == 0) {
-    uart_buff_switch(1U);
-  } else {
-    uart_buff_switch(0U);
-  }
-#else
-  uart_buff_switch(0U); // Side effect, clears RX FIFO
-#endif
   /*
     To use ets_strlen() and ets_memcpy() safely with PROGMEM, flash storage,
     the PROGMEM address must be word (4 bytes) aligned. The destination

tools/sdk/include/ets_sys.h

@@ -33,6 +33,17 @@
 extern "C" {
 #endif
 
+/*
+ * This "print character function" prototype is modeled after the argument for
+ * ets_install_putc1() found in "ESP8266_NONOS_SDK/include/osapi.h". This
+ * deviates away from the familiar C library definition of putchar; however, it
+ * agrees with the code we are working with. Note, in the ROM some "character
+ * print functions" always return 0 (uart_tx_one_char and ets_putc), some return
+ * last character printed (buildin _putc1), and some return nothing
+ * (ets_write_char). Using a void return type safely represents them all.
+ */
+typedef void (*fp_putc_t)(char);
+
 typedef uint32_t ETSSignal;
 typedef uint32_t ETSParam;
 
@@ -205,15 +216,20 @@ char *ets_strstr(const char *haystack, const char *needle);
 int ets_sprintf(char *str, const char *format, ...)  __attribute__ ((format (printf, 2, 3)));
 int os_snprintf(char *str, size_t size, const char *format, ...) __attribute__ ((format (printf, 3, 4)));
 int ets_printf(const char *format, ...)  __attribute__ ((format (printf, 1, 2)));
-void ets_install_putc1(void* routine);
+void ets_install_putc1(fp_putc_t routine);
 void ets_isr_mask(int intr);
 void ets_isr_unmask(int intr);
 void ets_isr_attach(int intr, int_handler_t handler, void *arg);
 void ets_intr_lock();
 void ets_intr_unlock();
 int ets_vsnprintf(char * s, size_t n, const char * format, va_list arg)  __attribute__ ((format (printf, 3, 0)));
-int ets_vprintf(int (*print_function)(int), const char * format, va_list arg) __attribute__ ((format (printf, 2, 0)));
-int ets_putc(int);
+int ets_vprintf(fp_putc_t print_function, const char * format, va_list arg) __attribute__ ((format (printf, 2, 0)));
+/*
+ * ets_putc(), a "print character function" in ROM, prints a character to a
+ * UART. It always returns 0. The use of this function requires a prior setup
+ * call to uart_buff_switch() to select the UART.
+ */
+int ets_putc(char);
 bool ets_task(ETSTask task, uint8 prio, ETSEvent *queue, uint8 qlen);
 bool ets_post(uint8 prio, ETSSignal sig, ETSParam par);
 void ets_update_cpu_frequency(uint32_t ticks_per_us);