diff --git a/bootloader_message/Android.mk b/bootloader_message/Android.mk
index 1d5c85f..0d84713 100644
--- a/bootloader_message/Android.mk
+++ b/bootloader_message/Android.mk
@@ -14,32 +14,16 @@
 
 LOCAL_PATH := $(call my-dir)
 
-ifeq ($(shell test $(PLATFORM_SDK_VERSION) -ge 25; echo $$?),0)
-    include $(CLEAR_VARS)
-    LOCAL_CLANG := true
-    LOCAL_SRC_FILES := bootloader_message.cpp
-    LOCAL_MODULE := libbootloader_message
-    LOCAL_STATIC_LIBRARIES := libfs_mgr
-    LOCAL_C_INCLUDES := $(LOCAL_PATH)/include
-    LOCAL_EXPORT_C_INCLUDE_DIRS := $(LOCAL_PATH)/include
-    include $(BUILD_STATIC_LIBRARY)
-endif
-
 include $(CLEAR_VARS)
 LOCAL_CLANG := true
 LOCAL_SRC_FILES := bootloader_message.cpp
 LOCAL_MODULE := libbootloader_message
-LOCAL_C_INCLUDES += bionic $(LOCAL_PATH)/include
-ifeq ($(shell test $(PLATFORM_SDK_VERSION) -lt 23; echo $$?),0)
-    LOCAL_C_INCLUDES += external/stlport/stlport
-    LOCAL_SHARED_LIBRARIES += libstlport
-else
-    LOCAL_SHARED_LIBRARIES += libc++
+LOCAL_STATIC_LIBRARIES := libbase libfs_mgr
+LOCAL_CFLAGS := -Werror
+ifeq ($(shell test $(PLATFORM_SDK_VERSION) -lt 26; echo $$?),0)
+    TARGET_GLOBAL_CFLAGS += -DUSE_OLD_BOOTLOADER_MESSAGE
+    CLANG_TARGET_GLOBAL_CFLAGS += -DUSE_OLD_BOOTLOADER_MESSAGE
 endif
-LOCAL_CFLAGS := -DEXCLUDE_FS_MGR
-# ignore bootloader's factory reset command even when written to /misc
-ifeq ($(TW_IGNORE_MISC_WIPE_DATA), true)
-    LOCAL_CFLAGS += -DIGNORE_MISC_WIPE_DATA
-endif
+LOCAL_C_INCLUDES := $(LOCAL_PATH)/include
 LOCAL_EXPORT_C_INCLUDE_DIRS := $(LOCAL_PATH)/include
-include $(BUILD_SHARED_LIBRARY)
+include $(BUILD_STATIC_LIBRARY)
diff --git a/bootloader_message/bootloader_message.cpp b/bootloader_message/bootloader_message.cpp
index 4d1ce5b..dcaeb79 100644
--- a/bootloader_message/bootloader_message.cpp
+++ b/bootloader_message/bootloader_message.cpp
@@ -19,31 +19,18 @@
 #include <errno.h>
 #include <fcntl.h>
 #include <string.h>
-#include <stdlib.h>
-#include <sys/stat.h>
-#include <unistd.h>
-#include <sys/types.h>
-#include <sys/system_properties.h>
 
 #include <string>
 #include <vector>
 
-/*
 #include <android-base/file.h>
 #include <android-base/stringprintf.h>
 #include <android-base/unique_fd.h>
-*/
-#ifndef EXCLUDE_FS_MGR
 #include <fs_mgr.h>
-#endif
 
-static std::string misc_blkdev;
+#ifdef USE_OLD_BOOTLOADER_MESSAGE
+#include <sys/system_properties.h>
 
-void set_misc_device(std::string name) {
-    misc_blkdev = name;
-}
-
-#ifndef EXCLUDE_FS_MGR
 static struct fstab* read_fstab(std::string* err) {
   // The fstab path is always "/fstab.${ro.hardware}".
   std::string fstab_path = "/fstab.";
@@ -62,23 +49,28 @@
 #endif
 
 static std::string get_misc_blk_device(std::string* err) {
-#ifdef EXCLUDE_FS_MGR
-  return misc_blkdev;
-#else
+#ifdef USE_OLD_BOOTLOADER_MESSAGE
   struct fstab* fstab = read_fstab(err);
-  if (fstab == nullptr) {
+#else
+  std::unique_ptr<fstab, decltype(&fs_mgr_free_fstab)> fstab(fs_mgr_read_fstab_default(),
+                                                             fs_mgr_free_fstab);
+#endif
+  if (!fstab) {
+    *err = "failed to read default fstab";
     return "";
   }
+#ifdef USE_OLD_BOOTLOADER_MESSAGE
   fstab_rec* record = fs_mgr_get_entry_for_mount_point(fstab, "/misc");
+#else
+  fstab_rec* record = fs_mgr_get_entry_for_mount_point(fstab.get(), "/misc");
+#endif
   if (record == nullptr) {
     *err = "failed to find /misc partition";
     return "";
   }
   return record->blk_device;
-#endif
 }
 
-
 // In recovery mode, recovery can get started and try to access the misc
 // device before the kernel has actually created it.
 static bool wait_for_device(const std::string& blk_device, std::string* err) {
@@ -90,127 +82,94 @@
     struct stat buf;
     ret = stat(blk_device.c_str(), &buf);
     if (ret == -1) {
-      char buffer[2048];
-      sprintf(buffer, "failed to stat %s try %d: %s\n",
-                                          blk_device.c_str(), tries, strerror(errno));
-      *err += buffer;
-      /*
       *err += android::base::StringPrintf("failed to stat %s try %d: %s\n",
                                           blk_device.c_str(), tries, strerror(errno));
-      */
       sleep(1);
     }
   } while (ret && tries < 10);
 
   if (ret) {
-    *err += "failed to stat " + blk_device + "\n";
-    /*
     *err += android::base::StringPrintf("failed to stat %s\n", blk_device.c_str());
-    */
   }
   return ret == 0;
 }
 
-static bool read_misc_partition(void* p, size_t size, size_t offset, std::string* err) {
-  std::string misc_blk_device = get_misc_blk_device(err);
-  if (misc_blk_device.empty()) {
-    return false;
-  }
+static bool read_misc_partition(void* p, size_t size, const std::string& misc_blk_device,
+                                size_t offset, std::string* err) {
   if (!wait_for_device(misc_blk_device, err)) {
     return false;
   }
-  int fd(open(misc_blk_device.c_str(), O_RDONLY));
-  if (fd < 0) {
-    *err = "failed to open " + misc_blk_device + ": ";
-    *err += strerror(errno);
-    /*
+  android::base::unique_fd fd(open(misc_blk_device.c_str(), O_RDONLY));
+  if (fd == -1) {
     *err = android::base::StringPrintf("failed to open %s: %s", misc_blk_device.c_str(),
                                        strerror(errno));
-    */
     return false;
   }
   if (lseek(fd, static_cast<off_t>(offset), SEEK_SET) != static_cast<off_t>(offset)) {
-    *err = "failed to lseek " + misc_blk_device + ": ";
-    *err += strerror(errno);
-    close(fd);
-    /*
     *err = android::base::StringPrintf("failed to lseek %s: %s", misc_blk_device.c_str(),
                                        strerror(errno));
-    */
     return false;
   }
-  if (read(fd, p, size) != size) {
-    *err = "failed to read " + misc_blk_device + ": ";
-    *err += strerror(errno);
-    close(fd);
-    /*
+  if (!android::base::ReadFully(fd, p, size)) {
     *err = android::base::StringPrintf("failed to read %s: %s", misc_blk_device.c_str(),
                                        strerror(errno));
-    */
     return false;
   }
-  close(fd);
   return true;
 }
 
-static bool write_misc_partition(const void* p, size_t size, size_t offset, std::string* err) {
-  std::string misc_blk_device = get_misc_blk_device(err);
-  if (misc_blk_device.empty()) {
-    *err = "no misc device set";
-    return false;
-  }
-  int fd = (open(misc_blk_device.c_str(), O_WRONLY | O_SYNC));
+static bool write_misc_partition(const void* p, size_t size, const std::string& misc_blk_device,
+                                 size_t offset, std::string* err) {
+  android::base::unique_fd fd(open(misc_blk_device.c_str(), O_WRONLY));
   if (fd == -1) {
-    *err = "failed to open " + misc_blk_device + ": ";
-    *err += strerror(errno);
-    /*
     *err = android::base::StringPrintf("failed to open %s: %s", misc_blk_device.c_str(),
                                        strerror(errno));
-    */
     return false;
   }
   if (lseek(fd, static_cast<off_t>(offset), SEEK_SET) != static_cast<off_t>(offset)) {
-    *err = "failed to lseek " + misc_blk_device + ": ";
-    *err += strerror(errno);
-    close(fd);
-    /*
     *err = android::base::StringPrintf("failed to lseek %s: %s", misc_blk_device.c_str(),
                                        strerror(errno));
-    */
     return false;
   }
-  if (write(fd, p, size) != size) {
-    *err = "failed to write " + misc_blk_device + ": ";
-    *err += strerror(errno);
-    close(fd);
-    /*
+  if (!android::base::WriteFully(fd, p, size)) {
     *err = android::base::StringPrintf("failed to write %s: %s", misc_blk_device.c_str(),
                                        strerror(errno));
-    */
     return false;
   }
-
-  // TODO: O_SYNC and fsync duplicates each other?
   if (fsync(fd) == -1) {
-    *err = "failed to fsync " + misc_blk_device + ": ";
-    *err += strerror(errno);
-    close(fd);
-    /*
     *err = android::base::StringPrintf("failed to fsync %s: %s", misc_blk_device.c_str(),
                                        strerror(errno));
-    */
     return false;
   }
-  close(fd);
   return true;
 }
 
+bool read_bootloader_message_from(bootloader_message* boot, const std::string& misc_blk_device,
+                                  std::string* err) {
+  return read_misc_partition(boot, sizeof(*boot), misc_blk_device,
+                             BOOTLOADER_MESSAGE_OFFSET_IN_MISC, err);
+}
+
 bool read_bootloader_message(bootloader_message* boot, std::string* err) {
-  return read_misc_partition(boot, sizeof(*boot), BOOTLOADER_MESSAGE_OFFSET_IN_MISC, err);
+  std::string misc_blk_device = get_misc_blk_device(err);
+  if (misc_blk_device.empty()) {
+    return false;
+  }
+  return read_bootloader_message_from(boot, misc_blk_device, err);
+}
+
+bool write_bootloader_message_to(const bootloader_message& boot, const std::string& misc_blk_device,
+                                 std::string* err) {
+  return write_misc_partition(&boot, sizeof(boot), misc_blk_device,
+                              BOOTLOADER_MESSAGE_OFFSET_IN_MISC, err);
 }
 
 bool write_bootloader_message(const bootloader_message& boot, std::string* err) {
-  return write_misc_partition(&boot, sizeof(boot), BOOTLOADER_MESSAGE_OFFSET_IN_MISC, err);
+  std::string misc_blk_device = get_misc_blk_device(err);
+  if (misc_blk_device.empty()) {
+    return false;
+  }
+  return write_bootloader_message_to(boot, misc_blk_device, err);
 }
 
 bool clear_bootloader_message(std::string* err) {
@@ -224,120 +183,72 @@
   strlcpy(boot.recovery, "recovery\n", sizeof(boot.recovery));
   for (const auto& s : options) {
     strlcat(boot.recovery, s.c_str(), sizeof(boot.recovery));
-    if (s.substr(s.size() - 1) != "\n") {
+    if (s.back() != '\n') {
       strlcat(boot.recovery, "\n", sizeof(boot.recovery));
     }
   }
   return write_bootloader_message(boot, err);
 }
 
+bool update_bootloader_message(const std::vector<std::string>& options, std::string* err) {
+  bootloader_message boot;
+  if (!read_bootloader_message(&boot, err)) {
+    return false;
+  }
+
+  // Zero out the entire fields.
+  memset(boot.command, 0, sizeof(boot.command));
+  memset(boot.recovery, 0, sizeof(boot.recovery));
+
+  strlcpy(boot.command, "boot-recovery", sizeof(boot.command));
+  strlcpy(boot.recovery, "recovery\n", sizeof(boot.recovery));
+  for (const auto& s : options) {
+    strlcat(boot.recovery, s.c_str(), sizeof(boot.recovery));
+    if (s.back() != '\n') {
+      strlcat(boot.recovery, "\n", sizeof(boot.recovery));
+    }
+  }
+  return write_bootloader_message(boot, err);
+}
+
+bool write_reboot_bootloader(std::string* err) {
+  bootloader_message boot;
+  if (!read_bootloader_message(&boot, err)) {
+    return false;
+  }
+  if (boot.command[0] != '\0') {
+    *err = "Bootloader command pending.";
+    return false;
+  }
+  strlcpy(boot.command, "bootonce-bootloader", sizeof(boot.command));
+  return write_bootloader_message(boot, err);
+}
+
 bool read_wipe_package(std::string* package_data, size_t size, std::string* err) {
+  std::string misc_blk_device = get_misc_blk_device(err);
+  if (misc_blk_device.empty()) {
+    return false;
+  }
   package_data->resize(size);
-  return read_misc_partition(&(*package_data)[0], size, WIPE_PACKAGE_OFFSET_IN_MISC, err);
+  return read_misc_partition(&(*package_data)[0], size, misc_blk_device,
+                             WIPE_PACKAGE_OFFSET_IN_MISC, err);
 }
 
 bool write_wipe_package(const std::string& package_data, std::string* err) {
-  return write_misc_partition(package_data.data(), package_data.size(),
+  std::string misc_blk_device = get_misc_blk_device(err);
+  if (misc_blk_device.empty()) {
+    return false;
+  }
+  return write_misc_partition(package_data.data(), package_data.size(), misc_blk_device,
                               WIPE_PACKAGE_OFFSET_IN_MISC, err);
 }
 
+extern "C" bool write_reboot_bootloader(void) {
+  std::string err;
+  return write_reboot_bootloader(&err);
+}
+
 extern "C" bool write_bootloader_message(const char* options) {
   std::string err;
-  bootloader_message boot = {};
-  memcpy(&boot, options, sizeof(boot));
-  return write_bootloader_message(boot, &err);
-}
-
-static const char *COMMAND_FILE = "/cache/recovery/command";
-static const int MAX_ARG_LENGTH = 4096;
-static const int MAX_ARGS = 100;
-
-// command line args come from, in decreasing precedence:
-//   - the actual command line
-//   - the bootloader control block (one per line, after "recovery")
-//   - the contents of COMMAND_FILE (one per line)
-void
-get_args(int *argc, char ***argv) {
-    bootloader_message boot = {};
-    std::string err;
-    if (!read_bootloader_message(&boot, &err)) {
-        printf("%s\n", err.c_str());
-        // If fails, leave a zeroed bootloader_message.
-        memset(&boot, 0, sizeof(boot));
-    }
-    //stage = strndup(boot.stage, sizeof(boot.stage));
-
-    if (boot.command[0] != 0 && boot.command[0] != 255) {
-        printf("Boot command: %.*s\n", (int)sizeof(boot.command), boot.command);
-    }
-
-    if (boot.status[0] != 0 && boot.status[0] != 255) {
-        printf("Boot status: %.*s\n", (int)sizeof(boot.status), boot.status);
-    }
-
-    // --- if arguments weren't supplied, look in the bootloader control block
-    if (*argc <= 1) {
-        boot.recovery[sizeof(boot.recovery) - 1] = '\0';  // Ensure termination
-        const char *arg = strtok(boot.recovery, "\n");
-        if (arg != NULL && !strcmp(arg, "recovery")) {
-            *argv = (char **) malloc(sizeof(char *) * MAX_ARGS);
-            (*argv)[0] = strdup(arg);
-            for (*argc = 1; *argc < MAX_ARGS; ++*argc) {
-                if ((arg = strtok(NULL, "\n")) == NULL) break;
-
-// if the device does not have an own recovery key combo we just want to open TWRP after
-// walking through the factory reset screen - without actually doing a factory reset
-#ifdef IGNORE_MISC_WIPE_DATA
-                if (!strcmp(arg, "--wipe_data")) {
-                    (*argv)[*argc] = "";
-                    *argc = *argc -1;
-                    printf("Bootloader arg \"%s\" ignored because TWRP was compiled with TW_IGNORE_MISC_WIPE_DATA\n", arg);
-                    continue;
-                }
-#endif
-                (*argv)[*argc] = strdup(arg);
-            }
-            printf("Got arguments from boot message\n");
-        } else if (boot.recovery[0] != 0 && boot.recovery[0] != 255) {
-            printf("Bad boot message\n\"%.20s\"\n", boot.recovery);
-        }
-    }
-
-    // --- if that doesn't work, try the command file (if we have /cache).
-    if (*argc <= 1/* && has_cache*/) {
-        FILE *fp = fopen(COMMAND_FILE, "r");
-        if (fp != NULL) {
-            char *token;
-            char *argv0 = (*argv)[0];
-            *argv = (char **) malloc(sizeof(char *) * MAX_ARGS);
-            (*argv)[0] = argv0;  // use the same program name
-
-            char buf[MAX_ARG_LENGTH];
-            for (*argc = 1; *argc < MAX_ARGS; ++*argc) {
-                if (!fgets(buf, sizeof(buf), fp)) break;
-                token = strtok(buf, "\r\n");
-                if (token != NULL) {
-                    (*argv)[*argc] = strdup(token);  // Strip newline.
-                } else {
-                    --*argc;
-                }
-            }
-
-            fclose(fp);
-            printf("Got arguments from %s\n", COMMAND_FILE);
-        }
-    }
-
-    // --> write the arguments we have back into the bootloader control block
-    // always boot into recovery after this (until finish_recovery() is called)
-    strlcpy(boot.command, "boot-recovery", sizeof(boot.command));
-    strlcpy(boot.recovery, "recovery\n", sizeof(boot.recovery));
-    int i;
-    for (i = 1; i < *argc; ++i) {
-        strlcat(boot.recovery, (*argv)[i], sizeof(boot.recovery));
-        strlcat(boot.recovery, "\n", sizeof(boot.recovery));
-    }
-    if (!write_bootloader_message(boot, &err)) {
-        printf("%s\n", err.c_str());
-    }
+  return write_bootloader_message({options}, &err);
 }
diff --git a/bootloader_message/include/bootloader_message/bootloader_message.h b/bootloader_message/include/bootloader_message/bootloader_message.h
index e0fc2cd..4ad18f2 100644
--- a/bootloader_message/include/bootloader_message/bootloader_message.h
+++ b/bootloader_message/include/bootloader_message/bootloader_message.h
@@ -17,23 +17,21 @@
 #ifndef _BOOTLOADER_MESSAGE_H
 #define _BOOTLOADER_MESSAGE_H
 
+#include <assert.h>
 #include <stddef.h>
+#include <stdint.h>
 
 // Spaces used by misc partition are as below:
-// 0   - 2K     Bootloader Message
-// 2K  - 16K    Used by Vendor's bootloader
+// 0   - 2K     For bootloader_message
+// 2K  - 16K    Used by Vendor's bootloader (the 2K - 4K range may be optionally used
+//              as bootloader_message_ab struct)
 // 16K - 64K    Used by uncrypt and recovery to store wipe_package for A/B devices
 // Note that these offsets are admitted by bootloader,recovery and uncrypt, so they
 // are not configurable without changing all of them.
-#ifdef BOARD_RECOVERY_BLDRMSG_OFFSET
-static const size_t BOOTLOADER_MESSAGE_OFFSET_IN_MISC = BOARD_RECOVERY_BLDRMSG_OFFSET;
-static const size_t WIPE_PACKAGE_OFFSET_IN_MISC = 16 * 1024 + BOOTLOADER_MESSAGE_OFFSET_IN_MISC;
-#else
 static const size_t BOOTLOADER_MESSAGE_OFFSET_IN_MISC = 0;
 static const size_t WIPE_PACKAGE_OFFSET_IN_MISC = 16 * 1024;
-#endif
 
-/* Bootloader Message
+/* Bootloader Message (2-KiB)
  *
  * This structure describes the content of a block in flash
  * that is used for recovery and the bootloader to talk to
@@ -44,8 +42,9 @@
  * It is also updated by the bootloader when firmware update
  * is complete (to boot into recovery for any final cleanup)
  *
- * The status field is written by the bootloader after the
- * completion of an "update-radio" or "update-hboot" command.
+ * The status field was used by the bootloader after the completion
+ * of an "update-radio" or "update-hboot" command, which has been
+ * deprecated since Froyo.
  *
  * The recovery field is only written by linux and used
  * for the system to send a message to recovery or the
@@ -56,18 +55,17 @@
  * package it is.  If the value is of the format "#/#" (eg, "1/3"),
  * the UI will add a simple indicator of that status.
  *
- * The slot_suffix field is used for A/B implementations where the
- * bootloader does not set the androidboot.ro.boot.slot_suffix kernel
- * commandline parameter. This is used by fs_mgr to mount /system and
- * other partitions with the slotselect flag set in fstab. A/B
- * implementations are free to use all 32 bytes and may store private
- * data past the first NUL-byte in this field.
+ * We used to have slot_suffix field for A/B boot control metadata in
+ * this struct, which gets unintentionally cleared by recovery or
+ * uncrypt. Move it into struct bootloader_message_ab to avoid the
+ * issue.
  */
 struct bootloader_message {
     char command[32];
     char status[32];
     char recovery[768];
 
+#ifdef USE_OLD_BOOTLOADER_MESSAGE
     // The 'recovery' field used to be 1024 bytes.  It has only ever
     // been used to store the recovery command line, so 768 bytes
     // should be plenty.  We carve off the last 256 bytes to store the
@@ -76,23 +74,156 @@
     char stage[32];
     char slot_suffix[32];
     char reserved[192];
+#else
+    // The 'recovery' field used to be 1024 bytes.  It has only ever
+    // been used to store the recovery command line, so 768 bytes
+    // should be plenty.  We carve off the last 256 bytes to store the
+    // stage string (for multistage packages) and possible future
+    // expansion.
+    char stage[32];
+
+    // The 'reserved' field used to be 224 bytes when it was initially
+    // carved off from the 1024-byte recovery field. Bump it up to
+    // 1184-byte so that the entire bootloader_message struct rounds up
+    // to 2048-byte.
+    char reserved[1184];
+#endif
 };
 
+/**
+ * We must be cautious when changing the bootloader_message struct size,
+ * because A/B-specific fields may end up with different offsets.
+ */
+#if !defined(USE_OLD_BOOTLOADER_MESSAGE) && ((__STDC_VERSION__ >= 201112L) || defined(__cplusplus))
+static_assert(sizeof(struct bootloader_message) == 2048,
+              "struct bootloader_message size changes, which may break A/B devices");
+#endif
+
+/**
+ * The A/B-specific bootloader message structure (4-KiB).
+ *
+ * We separate A/B boot control metadata from the regular bootloader
+ * message struct and keep it here. Everything that's A/B-specific
+ * stays after struct bootloader_message, which should be managed by
+ * the A/B-bootloader or boot control HAL.
+ *
+ * The slot_suffix field is used for A/B implementations where the
+ * bootloader does not set the androidboot.ro.boot.slot_suffix kernel
+ * commandline parameter. This is used by fs_mgr to mount /system and
+ * other partitions with the slotselect flag set in fstab. A/B
+ * implementations are free to use all 32 bytes and may store private
+ * data past the first NUL-byte in this field. It is encouraged, but
+ * not mandatory, to use 'struct bootloader_control' described below.
+ */
+struct bootloader_message_ab {
+    struct bootloader_message message;
+    char slot_suffix[32];
+
+    // Round up the entire struct to 4096-byte.
+    char reserved[2016];
+};
+
+/**
+ * Be cautious about the struct size change, in case we put anything post
+ * bootloader_message_ab struct (b/29159185).
+ */
+#if !defined(USE_OLD_BOOTLOADER_MESSAGE) && ((__STDC_VERSION__ >= 201112L) || defined(__cplusplus))
+static_assert(sizeof(struct bootloader_message_ab) == 4096,
+              "struct bootloader_message_ab size changes");
+#endif
+
+#define BOOT_CTRL_MAGIC   0x42414342 /* Bootloader Control AB */
+#define BOOT_CTRL_VERSION 1
+
+struct slot_metadata {
+    // Slot priority with 15 meaning highest priority, 1 lowest
+    // priority and 0 the slot is unbootable.
+    uint8_t priority : 4;
+    // Number of times left attempting to boot this slot.
+    uint8_t tries_remaining : 3;
+    // 1 if this slot has booted successfully, 0 otherwise.
+    uint8_t successful_boot : 1;
+    // 1 if this slot is corrupted from a dm-verity corruption, 0
+    // otherwise.
+    uint8_t verity_corrupted : 1;
+    // Reserved for further use.
+    uint8_t reserved : 7;
+} __attribute__((packed));
+
+/* Bootloader Control AB
+ *
+ * This struct can be used to manage A/B metadata. It is designed to
+ * be put in the 'slot_suffix' field of the 'bootloader_message'
+ * structure described above. It is encouraged to use the
+ * 'bootloader_control' structure to store the A/B metadata, but not
+ * mandatory.
+ */
+struct bootloader_control {
+    // NUL terminated active slot suffix.
+    char slot_suffix[4];
+    // Bootloader Control AB magic number (see BOOT_CTRL_MAGIC).
+    uint32_t magic;
+    // Version of struct being used (see BOOT_CTRL_VERSION).
+    uint8_t version;
+    // Number of slots being managed.
+    uint8_t nb_slot : 3;
+    // Number of times left attempting to boot recovery.
+    uint8_t recovery_tries_remaining : 3;
+    // Ensure 4-bytes alignment for slot_info field.
+    uint8_t reserved0[2];
+    // Per-slot information.  Up to 4 slots.
+    struct slot_metadata slot_info[4];
+    // Reserved for further use.
+    uint8_t reserved1[8];
+    // CRC32 of all 28 bytes preceding this field (little endian
+    // format).
+    uint32_t crc32_le;
+} __attribute__((packed));
+
+#if (__STDC_VERSION__ >= 201112L) || defined(__cplusplus)
+static_assert(sizeof(struct bootloader_control) ==
+              sizeof(((struct bootloader_message_ab *)0)->slot_suffix),
+              "struct bootloader_control has wrong size");
+#endif
+
 #ifdef __cplusplus
 
 #include <string>
 #include <vector>
 
+// Read bootloader message into boot. Error message will be set in err.
 bool read_bootloader_message(bootloader_message* boot, std::string* err);
+
+// Read bootloader message from the specified misc device into boot.
+bool read_bootloader_message_from(bootloader_message* boot, const std::string& misc_blk_device,
+                                  std::string* err);
+
+// Write bootloader message to BCB.
 bool write_bootloader_message(const bootloader_message& boot, std::string* err);
+
+// Write bootloader message to the specified BCB device.
+bool write_bootloader_message_to(const bootloader_message& boot,
+                                 const std::string& misc_blk_device, std::string* err);
+
+// Write bootloader message (boots into recovery with the options) to BCB. Will
+// set the command and recovery fields, and reset the rest.
 bool write_bootloader_message(const std::vector<std::string>& options, std::string* err);
+
+// Update bootloader message (boots into recovery with the options) to BCB. Will
+// only update the command and recovery fields.
+bool update_bootloader_message(const std::vector<std::string>& options, std::string* err);
+
+// Clear BCB.
 bool clear_bootloader_message(std::string* err);
 
-bool read_wipe_package(std::string* package_data, size_t size, std::string* err);
-bool write_wipe_package(const std::string& package_data, std::string* err);
+// Writes the reboot-bootloader reboot reason to the bootloader_message.
+bool write_reboot_bootloader(std::string* err);
 
-void set_misc_device(std::string name);
-void get_args(int *argc, char ***argv);
+// Read the wipe package from BCB (from offset WIPE_PACKAGE_OFFSET_IN_MISC).
+bool read_wipe_package(std::string* package_data, size_t size, std::string* err);
+
+// Write the wipe package into BCB (to offset WIPE_PACKAGE_OFFSET_IN_MISC).
+bool write_wipe_package(const std::string& package_data, std::string* err);
 
 #else
 
@@ -100,6 +231,7 @@
 
 // C Interface.
 bool write_bootloader_message(const char* options);
+bool write_reboot_bootloader(void);
 
 #endif  // ifdef __cplusplus