[esp8266] Wrap printf/vprintf/fprintf to eliminate _vfiprintf_r (~1.6 KB flash) (#14621)

esphome/components/esp8266/__init__.py

@@ -23,6 +23,7 @@ from esphome.helpers import copy_file_if_changed
 from .boards import BOARDS, ESP8266_LD_SCRIPTS
 from .const import (
     CONF_EARLY_PIN_INIT,
+    CONF_ENABLE_FULL_PRINTF,
     CONF_ENABLE_SERIAL,
     CONF_ENABLE_SERIAL1,
     CONF_RESTORE_FROM_FLASH,
@@ -179,6 +180,7 @@ CONFIG_SCHEMA = cv.All(
             ),
             cv.Optional(CONF_ENABLE_SERIAL): cv.boolean,
             cv.Optional(CONF_ENABLE_SERIAL1): cv.boolean,
+            cv.Optional(CONF_ENABLE_FULL_PRINTF, default=False): cv.boolean,
         }
     ),
     set_core_data,
@@ -260,6 +262,14 @@ async def to_code(config):
     if CORE.testing_mode:
         cg.add_build_flag("-DESPHOME_TESTING_MODE")
 
+    # Wrap FILE*-based printf functions to eliminate newlib's _vfiprintf_r
+    # (~1.6 KB). See printf_stubs.cpp for implementation.
+    if config.get(CONF_ENABLE_FULL_PRINTF):
+        cg.add_define("USE_FULL_PRINTF")
+    else:
+        for symbol in ("vprintf", "printf", "fprintf"):
+            cg.add_build_flag(f"-Wl,--wrap={symbol}")
+
     cg.add_platformio_option("board_build.flash_mode", config[CONF_BOARD_FLASH_MODE])
 
     ver: cv.Version = CORE.data[KEY_CORE][KEY_FRAMEWORK_VERSION]

esphome/components/esp8266/const.py

@@ -6,6 +6,7 @@ KEY_BOARD = "board"
 KEY_PIN_INITIAL_STATES = "pin_initial_states"
 CONF_RESTORE_FROM_FLASH = "restore_from_flash"
 CONF_EARLY_PIN_INIT = "early_pin_init"
+CONF_ENABLE_FULL_PRINTF = "enable_full_printf"
 CONF_ENABLE_SERIAL = "enable_serial"
 CONF_ENABLE_SERIAL1 = "enable_serial1"
 KEY_FLASH_SIZE = "flash_size"

esphome/components/esp8266/printf_stubs.cpp

@@ -0,0 +1,71 @@
+/*
+ * Linker wrap stubs for FILE*-based printf functions.
+ *
+ * The ESP8266 Arduino framework and libraries may reference printf(),
+ * vprintf(), and fprintf() which pull in newlib's _vfprintf_r (~900 bytes).
+ * ESPHome never uses these — all logging writes directly to the UART via
+ * Arduino's Serial, so the libc FILE*-based printf path is dead code.
+ *
+ * These stubs redirect through vsnprintf() (which is already in the binary
+ * for ESPHome's logging) and fwrite(), allowing the linker to dead-code
+ * eliminate _vfprintf_r.
+ *
+ * Saves ~1.6 KB of flash.
+ */
+
+#if defined(USE_ESP8266) && !defined(USE_FULL_PRINTF)
+#include <cstdarg>
+#include <cstdio>
+#include <cstdlib>
+
+namespace esphome::esp8266 {}
+
+static constexpr size_t PRINTF_BUFFER_SIZE = 512;
+
+// These stubs are essentially dead code at runtime — ESPHome writes directly
+// to the UART via Arduino's Serial, and Serial.printf() has its own implementation.
+// The buffer overflow check is purely defensive and should never trigger.
+static int write_printf_buffer(FILE *stream, char *buf, int len) {
+  if (len < 0) {
+    return len;
+  }
+  size_t write_len = len;
+  if (write_len >= PRINTF_BUFFER_SIZE) {
+    fwrite(buf, 1, PRINTF_BUFFER_SIZE - 1, stream);
+    abort();
+  }
+  if (fwrite(buf, 1, write_len, stream) < write_len || ferror(stream)) {
+    return -1;
+  }
+  return len;
+}
+
+// NOLINTBEGIN(bugprone-reserved-identifier,cert-dcl37-c,cert-dcl51-cpp,readability-identifier-naming)
+extern "C" {
+
+int __wrap_vprintf(const char *fmt, va_list ap) {
+  char buf[PRINTF_BUFFER_SIZE];
+  return write_printf_buffer(stdout, buf, vsnprintf(buf, sizeof(buf), fmt, ap));
+}
+
+int __wrap_printf(const char *fmt, ...) {
+  va_list ap;
+  va_start(ap, fmt);
+  int len = __wrap_vprintf(fmt, ap);
+  va_end(ap);
+  return len;
+}
+
+int __wrap_fprintf(FILE *stream, const char *fmt, ...) {
+  va_list ap;
+  va_start(ap, fmt);
+  char buf[PRINTF_BUFFER_SIZE];
+  int len = write_printf_buffer(stream, buf, vsnprintf(buf, sizeof(buf), fmt, ap));
+  va_end(ap);
+  return len;
+}
+
+}  // extern "C"
+// NOLINTEND(bugprone-reserved-identifier,cert-dcl37-c,cert-dcl51-cpp,readability-identifier-naming)
+
+#endif  // USE_ESP8266 && !USE_FULL_PRINTF

tests/components/esp8266/test.esp8266-ard.yaml

@@ -1,3 +1,6 @@
+esp8266:
+  enable_full_printf: false
+
 logger:
   level: VERBOSE