[esp32_ble] Avoid string allocation when setting BLE device name (#12579)

esphome/components/esp32_ble/ble.cpp

@@ -256,8 +256,11 @@ bool ESP32BLE::ble_setup_() {
   }
 #endif
 
+  // BLE device names are limited to 20 characters
+  // Buffer: 20 chars + null terminator
+  constexpr size_t ble_name_max_len = 21;
+  char name_buffer[ble_name_max_len];
   const char *device_name;
-  std::string name_with_suffix;
 
   if (this->name_ != nullptr) {
     if (App.is_name_add_mac_suffix_enabled()) {
@@ -268,23 +271,28 @@ bool ESP32BLE::ble_setup_() {
       char mac_addr[mac_address_len];
       get_mac_address_into_buffer(mac_addr);
       const char *mac_suffix_ptr = mac_addr + mac_address_suffix_len;
-      name_with_suffix =
-          make_name_with_suffix(this->name_, strlen(this->name_), '-', mac_suffix_ptr, mac_address_suffix_len);
-      device_name = name_with_suffix.c_str();
+      make_name_with_suffix_to(name_buffer, sizeof(name_buffer), this->name_, strlen(this->name_), '-', mac_suffix_ptr,
+                               mac_address_suffix_len);
+      device_name = name_buffer;
     } else {
       device_name = this->name_;
     }
   } else {
-    name_with_suffix = App.get_name();
-    if (name_with_suffix.length() > 20) {
+    const std::string &app_name = App.get_name();
+    size_t name_len = app_name.length();
+    if (name_len > 20) {
       if (App.is_name_add_mac_suffix_enabled()) {
         // Keep first 13 chars and last 7 chars (MAC suffix), remove middle
-        name_with_suffix.erase(13, name_with_suffix.length() - 20);
+        memcpy(name_buffer, app_name.c_str(), 13);
+        memcpy(name_buffer + 13, app_name.c_str() + name_len - 7, 7);
       } else {
-        name_with_suffix.resize(20);
+        memcpy(name_buffer, app_name.c_str(), 20);
       }
+      name_buffer[20] = '\0';
+    } else {
+      memcpy(name_buffer, app_name.c_str(), name_len + 1);  // Include null terminator
     }
-    device_name = name_with_suffix.c_str();
+    device_name = name_buffer;
   }
 
   err = esp_ble_gap_set_device_name(device_name);

esphome/core/helpers.cpp

@@ -244,17 +244,16 @@ std::string str_sprintf(const char *fmt, ...) {
 // Maximum size for name with suffix: 120 (max friendly name) + 1 (separator) + 6 (MAC suffix) + 1 (null term)
 static constexpr size_t MAX_NAME_WITH_SUFFIX_SIZE = 128;
 
-std::string make_name_with_suffix(const char *name, size_t name_len, char sep, const char *suffix_ptr,
-                                  size_t suffix_len) {
-  char buffer[MAX_NAME_WITH_SUFFIX_SIZE];
+size_t make_name_with_suffix_to(char *buffer, size_t buffer_size, const char *name, size_t name_len, char sep,
+                                const char *suffix_ptr, size_t suffix_len) {
   size_t total_len = name_len + 1 + suffix_len;
 
   // Silently truncate if needed: prioritize keeping the full suffix
-  if (total_len >= MAX_NAME_WITH_SUFFIX_SIZE) {
-    // NOTE: This calculation could underflow if suffix_len >= MAX_NAME_WITH_SUFFIX_SIZE - 2,
+  if (total_len >= buffer_size) {
+    // NOTE: This calculation could underflow if suffix_len >= buffer_size - 2,
     // but this is safe because this helper is only called with small suffixes:
     // MAC suffixes (6-12 bytes), ".local" (5 bytes), etc.
-    name_len = MAX_NAME_WITH_SUFFIX_SIZE - suffix_len - 2;  // -2 for separator and null terminator
+    name_len = buffer_size - suffix_len - 2;  // -2 for separator and null terminator
     total_len = name_len + 1 + suffix_len;
   }
 
@@ -262,7 +261,14 @@ std::string make_name_with_suffix(const char *name, size_t name_len, char sep, c
   buffer[name_len] = sep;
   memcpy(buffer + name_len + 1, suffix_ptr, suffix_len);
   buffer[total_len] = '\0';
-  return std::string(buffer, total_len);
+  return total_len;
+}
+
+std::string make_name_with_suffix(const char *name, size_t name_len, char sep, const char *suffix_ptr,
+                                  size_t suffix_len) {
+  char buffer[MAX_NAME_WITH_SUFFIX_SIZE];
+  size_t len = make_name_with_suffix_to(buffer, sizeof(buffer), name, name_len, sep, suffix_ptr, suffix_len);
+  return std::string(buffer, len);
 }
 
 std::string make_name_with_suffix(const std::string &name, char sep, const char *suffix_ptr, size_t suffix_len) {

esphome/core/helpers.h

@@ -549,6 +549,18 @@ std::string make_name_with_suffix(const std::string &name, char sep, const char
 std::string make_name_with_suffix(const char *name, size_t name_len, char sep, const char *suffix_ptr,
                                   size_t suffix_len);
 
+/// Zero-allocation version: format name + separator + suffix directly into buffer.
+/// @param buffer Output buffer (must have space for result + null terminator)
+/// @param buffer_size Size of the output buffer
+/// @param name The base name string
+/// @param name_len Length of the name
+/// @param sep Single character separator
+/// @param suffix_ptr Pointer to the suffix characters
+/// @param suffix_len Length of the suffix
+/// @return Length written (excluding null terminator)
+size_t make_name_with_suffix_to(char *buffer, size_t buffer_size, const char *name, size_t name_len, char sep,
+                                const char *suffix_ptr, size_t suffix_len);
+
 ///@}
 
 /// @name Parsing & formatting