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Migration guide to Zephyr v3.5.0 (Working Draft)

This document describes the changes required or recommended when migrating your application from Zephyr v3.4.0 to Zephyr v3.5.0.

Required changes

  • The kernel k_mem_slab_free() function has changed its signature, now taking a void *mem pointer instead of a void **mem double-pointer. The new signature will not immediately trigger a compiler error or warning, instead likely causing a invalid memory access at runtime. A new _ASSERT statement, that you can enable with CONFIG_ASSERT, will detect if you pass the function memory not belonging to the memory blocks in the slab.

  • The CONFIG_BOOTLOADER_SRAM_SIZE default value is now 0 (was 16). Bootloaders that use a part of the SRAM should set this value to an appropriate size. GitHub #60371

  • The Kconfig option CONFIG_GPIO_NCT38XX_INTERRUPT has been renamed to CONFIG_GPIO_NCT38XX_ALERT.

  • MCUmgr SMP version 2 error codes entry has changed due to a collision with an existing response in shell_mgmt. Previously, these errors had the entry ret but now have the entry err. smp_add_cmd_ret() is now deprecated and smp_add_cmd_err() should be used instead, MGMT_CB_ERROR_RET is now deprecated and MGMT_CB_ERROR_ERR should be used instead. SMP version 2 error code defines for in-tree modules have been updated to replace the *_RET_RC_* parts with *_ERR_*.

  • MCUmgr SMP version 2 error translation (to legacy MCUmgr error code) is now handled in function handlers by setting the mg_translate_error function pointer of mgmt_group when registering a group. See smp_translate_error_fn for function details. Any SMP version 2 handlers made for Zephyr 3.4 need to be updated to include these translation functions when the groups are registered.

  • zephyr,memory-region-mpu was renamed zephyr,memory-attr and its type moved from ‘enum’ to ‘int’. To have a seamless conversion this is the required change in the DT:

    - "RAM"         -> <( DT_MEM_ARM(ATTR_MPU_RAM) )>
    - "FLASH"       -> <( DT_MEM_ARM(ATTR_MPU_FLASH) )>
    - "PPB"         -> <( DT_MEM_ARM(ATTR_MPU_PPB) )>
    - "IO"          -> <( DT_MEM_ARM(ATTR_MPU_IO) )>
    - "EXTMEM"      -> <( DT_MEM_ARM(ATTR_MPU_EXTMEM) )>
  • A new networking Kconfig option CONFIG_NET_INTERFACE_NAME defaults to y. The option allows user to set a name to a network interface. During system startup a default name is assigned to the network interface like eth0 to the first Ethernet network interface. The option affects the behavior of SO_BINDTODEVICE BSD socket option. If the Kconfig option is set to n, which is how the system worked earlier, then the name of the device assigned to the network interface is used by the SO_BINDTODEVICE socket option. If the Kconfig option is set to y (current default), then the network interface name is used by the SO_BINDTODEVICE socket option.

  • On all STM32 ADC, it is no longer possible to read sensor channels (Vref, Vbat or temperature) using the ADC driver. The dedicated sensor driver should be used instead. This change is due to a limitation on STM32F4 where the channels for temperature and Vbat are identical, and the impossibility of determining what we want to measure using solely the ADC API.

  • The default C library used on most targets has changed from the built-in minimal C library to Picolibc. While both provide standard C library interfaces and shouldn’t cause any behavioral regressions for applications, there are a few side effects to be aware of when migrating to Picolibc.

    • Picolibc enables thread local storage (CONFIG_THREAD_LOCAL_STORAGE) where supported. This changes some internal operations within the kernel that improve performance using some TLS variables. Zephyr places TLS variables in the memory reserved for the stack, so stack usage for every thread will increase by 8-16 bytes.

    • Picolibc uses the same malloc implementation as the minimal C library, but the default heap size depends on which C library is in use. When using the minimal C library, the default heap is zero bytes, which means that malloc will always fail. When using Picolibc, the default is 16kB with CONFIG_MMU or ARCH_POSIX, 2kB with CONFIG_USERSPACE and CONFIG_MPU_REQUIRES_POWER_OF_TWO_ALIGNMENT. For all other targets, the default heap uses all remaining memory on the system. You can change this by adjusting CONFIG_COMMON_LIBC_MALLOC_ARENA_SIZE.

    • Picolibc can either be built as part of the OS build or pulled from the toolchain. When building as part of the OS, the build will increase by approximately 1000 files.

    • When using the standard C++ library with Picolibc, both of those must come from the toolchain as the standard C++ library depends upon the C library ABI.

    • Picolibc removes the -ffreestanding compiler option. This allows significant compiler optimization improvements, but also means that the compiler will now warn about declarations of main which don’t conform to the Zephyr required type – int main(void).

    • Picolibc’s default floating point input/output code is larger than the minimal C library version (this is necessary to conform with the C language “round trip” requirements for these operations). If you use CONFIG_CBPRINTF_FP_SUPPORT, you will see increased memory usage unless you also disable CONFIG_PICOLIBC_IO_FLOAT_EXACT, which switches Picolibc to a smaller, but inexact conversion algorithm. This requires building Picolibc as a module.

  • The CAN controller timing API functions can_set_timing() and can_set_timing_data() no longer fallback to the (Re-)Synchronization Jump Width (SJW) value set in the devicetree properties for the given CAN controller upon encountering an SJW value corresponding to CAN_SJW_NO_CHANGE (which is no longer available). The caller will therefore need to fill in the sjw field in can_timing. To aid in this, the can_calc_timing() and can_calc_timing_data() functions now automatically calculate a suitable SJW. The calculated SJW can be overwritten by the caller if needed. The CAN controller API functions can_set_bitrate() and can_set_bitrate_data() now also automatically calculate a suitable SJW, but their SJW cannot be overwritten by the caller.

  • Ethernet PHY devicetree bindings were updated to use the standard reg property for the PHY address instead of a custom address property. As a result, MDIO controller nodes now require #address-cells and #size-cells properties. Similarly, Ethernet PHY devicetree nodes and corresponding driver were updated to consistently use the node name ethernet-phy instead of phy. Devicetrees and overlays must be updated accordingly:

    mdio {
        compatible = "mdio-controller";
        #address-cells = <1>;
        #size-cells = <0>;
        ethernet-phy@0 {
            compatible = "ethernet-phy";
            reg = <0>;
  • The accept() callback’s signature in bt_l2cap_server has changed to int (*accept)(struct bt_conn *conn, struct bt_l2cap_server *server, struct bt_l2cap_chan **chan), adding a new server parameter pointing to the bt_l2cap_server structure instance the callback relates to. GitHub #60536