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Minimizing Binary Size

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The ESP-IDF build system compiles all source files in the project and ESP-IDF, but only functions and variables that are actually referenced by the program are linked into the final binary. In some cases, it is necessary to reduce the total size of the firmware binary, e.g., in order to fit it into the available flash partition size.

The first step to reducing the total firmware binary size is measuring what is causing the size to increase.

Measuring Static Sizes

To optimize both the firmware binary size and the memory usage, it is necessary to measure statically-allocated RAM (data, bss), code (text), and read-only data (rodata) in your project. The idf.py sub-commands size, size-components, and size-files can be used to examine statically-allocated RAM usage at different levels of detail. For more information, please see the IDF Size tool.

Linker Map File

Note

This is an advanced analysis method, but it can be very useful. Feel free to skip ahead to Reducing Overall Size and possibly come back to this later.

The idf.py size analysis tools all work by parsing the GNU binutils linker map file, which is a summary of everything the linker did when it created (i.e., linked) the final firmware binary file.

Linker map files themselves are plain text files, so it is possible to read them and find out exactly what the linker did. However, they are also very complex and long, often exceeding 100,000 lines.

The map file itself is broken into parts and each part has a heading. The parts are:

  • Archive member included to satisfy reference by file (symbol)

    • This shows you: for each object file included in the link, what symbol (function or variable) was the linker searching for when it included that object file.

    • If you are wondering why some object file in particular was included in the binary, this part may give a clue. This part can be used in conjunction with the Cross Reference Table at the end of the file.

    Note

    Not every object file shown in this list ends up included in the final binary, some end up in the Discarded input sections list instead.

  • Allocating common symbols

    • This is a list of some global variables along with their sizes. Common symbols have a particular meaning in ELF binary files, but ESP-IDF does not make much use of them.

  • Discarded input sections

    • These sections were read by the linker as part of an object file to be linked into the final binary, but then nothing else referred to them, so they were discarded from the final binary.

    • For ESP-IDF, this list can be very long, as we compile each function and static variable to a unique section in order to minimize the final binary size. Specifically, ESP-IDF uses compiler options -ffunction-sections -fdata-sections and linker option --gc-sections.

    • Items mentioned in this list do not contribute to the final binary.

  • Memory Configuration, Linker script and memory map

    • These two parts go together. Some of the output comes directly from the linker command line and the Linker Script, both provided by Build System. The linker script is partially generated from the ESP-IDF project using the Linker Script Generation feature.

    • As the output of the Linker script and memory map part of the map unfolds, you can see each symbol (function or static variable) linked into the final binary along with its address (as a 16 digit hex number), its length (also in hex), and the library and object file it was linked from (which can be used to determine the component and the source file).

    • Following all of the output sections that take up space in the final .bin file, the memory map also includes some sections in the ELF file that are only used for debugging, e.g., ELF sections .debug_*, etc. These do not contribute to the final binary size. You can notice the address of these symbols is a very small number, starting from 0x0000000000000000 and counting up.

  • Cross Reference Table

    • This table shows the symbol (function or static variable) that the list of object file(s) refers to. If you are wondering why a particular thing is included in the binary, this will help determine what included it.

    Note

    Unfortunately, the Cross Reference Table does not only include symbols that made it into the final binary. It also includes symbols in discarded sections. Therefore, just because something is shown here does not mean that it was included in the final binary - this needs to be checked separately.

Note

Linker map files are generated by the GNU binutils linker ld, not ESP-IDF. You can find additional information online about the linker map file format. This quick summary is written from the perspective of ESP-IDF build system in particular.

Reducing Overall Size

The following configuration options reduces the final binary size of almost any ESP-IDF project:

Note

In addition to the many configuration items shown here, there are a number of configuration options where changing the option from the default increases binary size. These are not noted here. Where the increase is significant is usually noted in the configuration item help text.

Targeted Optimizations

The following binary size optimizations apply to a particular component or a function:

Wi-Fi

Bluetooth NimBLE

If using NimBLE-based Host APIs then the following modifications can reduce binary size:

lwIP IPv6

  • Setting CONFIG_LWIP_IPV6 to false will reduce the size of the lwIP TCP/IP stack, at the cost of only supporting IPv4.

    Note

    IPv6 is required by some components such as ASIO Port. These components will not be available if IPV6 is disabled.

lwIP IPv4

  • If IPv4 connectivity is not required, setting CONFIG_LWIP_IPV4 to false will reduce the size of the lwIP, supporting IPv6-only TCP/IP stack.

    Note

    Before disabling IPv4 support, please note that IPv6 only network environments are not ubiquitous and must be supported in the local network, e.g., by your internet service provider or using constrained local network settings.

Picolibc instead of Newlib

By default, ESP-IDF uses the Newlib C library, and it also has experimental support for the Picolibc C library.

Picolibc C library provides smaller printf family functions and can reduce the binary size by up to 30 KB, depending on your application.

To switch to linking against the Picolibc C library, please enable the configuration options CONFIG_IDF_EXPERIMENTAL_FEATURES and CONFIG_LIBC_PICOLIBC.

Newlib Nano Formatting

By default, ESP-IDF uses Newlib "full" formatting for I/O functions (printf(), scanf(), etc.)

Enabling the config option CONFIG_LIBC_NEWLIB_NANO_FORMAT will switch Newlib to the "Nano" formatting mode. This is smaller in code size, and a large part of the implementation is compiled into the ESP32-C61 ROM, so it does not need to be included in the binary at all.

The exact difference in binary size depends on which features the firmware uses, but 25 KB ~ 50 KB is typical.

Enabling "Nano" formatting reduces the stack usage of each function that calls printf() or another string formatting function, see Determining Stack Size.

"Nano" formatting does not support 64-bit integers, or C99 formatting features. For a full list of restrictions, search for --enable-newlib-nano-formatted-io in the Newlib README file.

MbedTLS Features

Under Component Config > mbedTLS, there are multiple mbedTLS features enabled default, some of which can be disabled if not needed to save code size.

These include:

The help text for each option has some more information for reference.

Important

It is strongly not recommended to disable all these mbedTLS options. Only disable options of which you understand the functionality and are certain that it is not needed in the application. In particular:

  • Ensure that any TLS server(s) the device connects to can still be used. If the server is controlled by a third party or a cloud service, it is recommended to ensure that the firmware supports at least two of the supported cipher suites in case one is disabled in a future update.

  • Ensure that any TLS client(s) that connect to the device can still connect with supported/recommended cipher suites. Note that future versions of client operating systems may remove support for some features, so it is recommended to enable multiple supported cipher suites, or algorithms for redundancy.

If depending on third party clients or servers, always pay attention to announcements about future changes to supported TLS features. If not, the ESP32-C61 device may become inaccessible if support changes.

Note

Not every combination of mbedTLS compile-time config is tested in ESP-IDF. If you find a combination that fails to compile or function as expected, please report the details on GitHub.

VFS

Virtual Filesystem Component feature in ESP-IDF allows multiple filesystem drivers and file-like peripheral drivers to be accessed using standard I/O functions (open, read, write, etc.) and C library functions (fopen, fread, fwrite, etc.). When filesystem or file-like peripheral driver functionality is not used in the application, this feature can be fully or partially disabled. VFS component provides the following configuration options:

  • CONFIG_VFS_SUPPORT_TERMIOS — can be disabled if the application does not use termios family of functions. Currently, these functions are implemented only for UART VFS driver. Most applications can disable this option. Disabling this option reduces the code size by about 1.8 KB.

  • CONFIG_VFS_SUPPORT_SELECT — can be disabled if the application does not use the select function with file descriptors. Currently, only the UART and eventfd VFS drivers implement select support. Note that when this option is disabled, select can still be used for socket file descriptors. Disabling this option reduces the code size by about 2.7 KB.

  • CONFIG_VFS_SUPPORT_DIR — can be disabled if the application does not use directory-related functions, such as readdir (see the description of this option for the complete list). Applications that only open, read and write specific files and do not need to enumerate or create directories can disable this option, reducing the code size by 0.5 KB or more, depending on the filesystem drivers in use.

  • CONFIG_VFS_SUPPORT_IO — can be disabled if the application does not use filesystems or file-like peripheral drivers. This disables all VFS functionality, including the three options mentioned above. When this option is disabled, Console can not be used. Note that the application can still use standard I/O functions with socket file descriptors when this option is disabled. Compared to the default configuration, disabling this option reduces code size by about 9.4 KB.

HAL

Heap

Console

For targets that support USB-Serial-JTAG, both the USB-Serial-JTAG and UART console output are enabled by default. If you only need one console, you can reduce the binary size and RAM usage by doing the following:

  1. Disable the secondary console by setting CONFIG_ESP_CONSOLE_SECONDARY to CONFIG_ESP_CONSOLE_SECONDARY_NONE.

  2. Set CONFIG_ESP_CONSOLE_UART to use one of the following:

    • UART reduces the binary size by around 2.5 KB.

    • USB-Serial-JTAG reduces the binary size by around 10 KB and DRAM usage by around 1.5 KB.

Please note that these size reductions assume the UART/USB-Serial-JTAG driver code is not pulled into the app. If these drivers are already used for other purposes, then the savings will be smaller.

Bootloader Size

This document deals with the size of an ESP-IDF app binary only, and not the ESP-IDF Second Stage Bootloader.

For a discussion of ESP-IDF bootloader binary size, see Bootloader Size.

IRAM Binary Size

If the IRAM section of a binary is too large, this issue can be resolved by reducing IRAM memory usage. See Optimizing IRAM Usage.

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