[esp32] Wrap printf/vprintf/fprintf to eliminate _vfprintf_r (~11 KB flash)

ESP-IDF SDK components reference fprintf(), printf(), and vprintf()
which pull in newlib's _vfprintf_r (~11 KB). This is a separate
implementation from _svfprintf_r (used by snprintf/vsnprintf) that
handles FILE* stream I/O with buffering and locking.

ESPHome replaces the ESP-IDF log handler via esp_log_set_vprintf_(),
so the SDK's vprintf() path is dead code at runtime. The fprintf()
and printf() calls in SDK components are only in debug/assert paths
that are either GC'd or never called.

These linker --wrap stubs redirect through vsnprintf() + fwrite(),
allowing the linker to dead-code eliminate _vfprintf_r.

An escape hatch is provided via enable_full_printf: true in the
esp32 advanced config section for external components that need
full FILE*-based fprintf.

esphome/components/esp32/__init__.py

@@ -952,6 +952,7 @@ CONF_HEAP_IN_IRAM = "heap_in_iram"
 CONF_LOOP_TASK_STACK_SIZE = "loop_task_stack_size"
 CONF_USE_FULL_CERTIFICATE_BUNDLE = "use_full_certificate_bundle"
 CONF_DISABLE_DEBUG_STUBS = "disable_debug_stubs"
+CONF_ENABLE_FULL_PRINTF = "enable_full_printf"
 CONF_DISABLE_OCD_AWARE = "disable_ocd_aware"
 CONF_DISABLE_USB_SERIAL_JTAG_SECONDARY = "disable_usb_serial_jtag_secondary"
 CONF_DISABLE_DEV_NULL_VFS = "disable_dev_null_vfs"
@@ -1126,6 +1127,7 @@ FRAMEWORK_SCHEMA = cv.Schema(
                 cv.Optional(
                     CONF_INCLUDE_BUILTIN_IDF_COMPONENTS, default=[]
                 ): cv.ensure_list(cv.string_strict),
+                cv.Optional(CONF_ENABLE_FULL_PRINTF, default=False): cv.boolean,
                 cv.Optional(CONF_DISABLE_DEBUG_STUBS, default=True): cv.boolean,
                 cv.Optional(CONF_DISABLE_OCD_AWARE, default=True): cv.boolean,
                 cv.Optional(
@@ -1469,6 +1471,14 @@ async def to_code(config):
             "_ZSt25__throw_bad_function_callv",
         ]:
             cg.add_build_flag(f"-Wl,--wrap={mangled}")
+
+        # Wrap FILE*-based printf functions to eliminate newlib's _vfprintf_r
+        # (~11 KB). See printf_stubs.cpp for implementation.
+        if conf[CONF_ADVANCED][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}")
     else:
         cg.add_build_flag("-DUSE_ARDUINO")
         cg.add_build_flag("-DUSE_ESP32_FRAMEWORK_ARDUINO")

esphome/components/esp32/printf_stubs.cpp

@@ -0,0 +1,79 @@
+/*
+ * Linker wrap stubs for FILE*-based printf functions.
+ *
+ * ESP-IDF SDK components (gpio driver, ringbuf, log_write) reference
+ * fprintf(), printf(), and vprintf() which pull in newlib's _vfprintf_r
+ * (~11 KB). This is a separate implementation from _svfprintf_r (used by
+ * snprintf/vsnprintf) that handles FILE* stream I/O with buffering and
+ * locking.
+ *
+ * ESPHome replaces the ESP-IDF log handler via esp_log_set_vprintf_(),
+ * so the SDK's vprintf() path is dead code at runtime. The fprintf()
+ * and printf() calls in SDK components are only in debug/assert paths
+ * (gpio_dump_io_configuration, ringbuf diagnostics) that are either
+ * GC'd or never called.
+ *
+ * These stubs redirect through vsnprintf() (which uses _svfprintf_r
+ * already in the binary) and fwrite(), allowing the linker to
+ * dead-code eliminate _vfprintf_r.
+ *
+ * Saves ~11 KB of flash.
+ *
+ * To disable these wraps, set enable_full_printf: true in the esp32
+ * advanced config section.
+ */
+
+#if defined(USE_ESP_IDF) && !defined(USE_FULL_PRINTF)
+#include <cstdarg>
+#include <cstdio>
+
+#include "esp_system.h"
+
+static constexpr size_t PRINTF_BUFFER_SIZE = 512;
+
+// 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];
+  int len = vsnprintf(buf, sizeof(buf), fmt, ap);
+  if (len < 0) {
+    return len;
+  }
+  if (static_cast<size_t>(len) >= sizeof(buf)) {
+    // Output was truncated — this should not happen in normal operation.
+    // Abort to make the issue visible rather than silently losing output.
+    esp_system_abort("printf buffer overflow; set enable_full_printf: true in esp32 advanced config");
+  }
+  fwrite(buf, 1, len, stdout);
+  return len;
+}
+
+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 = vsnprintf(buf, sizeof(buf), fmt, ap);
+  va_end(ap);
+  if (len < 0) {
+    return len;
+  }
+  if (static_cast<size_t>(len) >= sizeof(buf)) {
+    esp_system_abort("fprintf buffer overflow; set enable_full_printf: true in esp32 advanced config");
+  }
+  fwrite(buf, 1, len, stream);
+  return len;
+}
+
+}  // extern "C"
+// NOLINTEND(bugprone-reserved-identifier,cert-dcl37-c,cert-dcl51-cpp,readability-identifier-naming)
+
+#endif  // USE_ESP_IDF && !USE_FULL_PRINTF