# Minimizing Binary Size¶

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 (for example, 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 firmware binary size and memory usage it’s necessary to measure statically allocated RAM (“data”, “bss”), code (“text”) and read-only data (“rodata”) in your project.

Using the idf.py sub-commands size, size-components and size-files provides a summary of memory used by the project:

 $idf.py size [...] Total sizes: DRAM .data size: 11584 bytes DRAM .bss size: 19624 bytes Used static DRAM: 0 bytes ( 0 available, nan% used) Used static IRAM: 0 bytes ( 0 available, nan% used) Used stat D/IRAM: 136276 bytes ( 519084 available, 20.8% used) Flash code: 630508 bytes Flash rodata: 177048 bytes Total image size:~ 924208 bytes (.bin may be padded larger)  This output breaks down the size of all static memory regions in the firmware binary: • DRAM .data size is statically allocated RAM that is assigned to non-zero values at startup. This uses RAM (DRAM) at runtime and also uses space in the binary file. • DRAM .bss size is statically allocated RAM that is assigned zero at startup. This uses RAM (DRAM) at runtime but doesn’t use any space in the binary file. • Used static DRAM, Used static IRAM - these options are kept for compatibility with ESP32 target, and currently read 0. • Used stat D/IRAM - This is total internal RAM usage, the sum of static DRAM .data + .bss, and also static IRAM (Instruction RAM) used by the application for executable code. The available size is the estimated amount of DRAM which will be available as heap memory at runtime (due to metadata overhead and implementation constraints, and heap allocations done by ESP-IDF during startup, the actual free heap at startup will be lower than this). • Flash code is the total size of executable code executed from flash cache (IROM). This uses space in the binary file. • Flash rodata is the total size of read-only data loaded from flash cache (DROM). This uses space in the binary file. • Total image size is the estimated total binary file size, which is the total of all the used memory types except for .bss. ### Component Usage Summary (idf.py size-components)¶ The summary output provided by idf.py size does not give enough detail to find the main contributor to excessive binary size. To analyze in more detail, use idf.py size-components $ idf.py size-components
[...]
Total sizes:
DRAM .data size:   14956 bytes
DRAM .bss  size:   15808 bytes
Used static DRAM:   30764 bytes ( 149972 available, 17.0% used)
Used static IRAM:   83918 bytes (  47154 available, 64.0% used)
Flash code:  559943 bytes
Flash rodata:  176736 bytes
Total image size:~ 835553 bytes (.bin may be padded larger)
Per-archive contributions to ELF file:
Archive File DRAM .data & .bss & other   IRAM   D/IRAM Flash code & rodata   Total
libnet80211.a       1267   6044       0   5490        0     107445    18484  138730
liblwip.a         21   3838       0      0        0      97465    16116  117440
libmbedtls.a         60    524       0      0        0      27655    69907   98146
libmbedcrypto.a         64     81       0     30        0      76645    11661   88481
libpp.a       2427   1292       0  20851        0      37208     4708   66486
libc.a          4      0       0      0        0      57056     6455   63515
libphy.a       1439    715       0   7798        0      33074        0   43026
libwpa_supplicant.a         12    848       0      0        0      35505     1446   37811
libfreertos.a       3104    740       0  15711        0        367     4228   24150
libnvs_flash.a          0     24       0      0        0      14347     2924   17295
libspi_flash.a       1562    294       0   8851        0       1840     1913   14460
libesp_system.a        245    206       0   3078        0       5990     3817   13336
libesp-tls.a          0      4       0      0        0       5637     3524    9165
[... removed some lines here ...]
libtcpip_adapter.a          0     17       0      0        0        216        0     233
libesp_rom.a          0      0       0    112        0          0        0     112
libcxx.a          0      0       0      0        0         47        0      47
(exe)          0      0       0      3        0          3       12      18
libesp_pm.a          0      0       0      0        0          8        0       8
libesp_eth.a          0      0       0      0        0          0        0       0
libmesh.a          0      0       0      0        0          0        0       0


The first lines of output from idf.py size-components are the same as idf.py size. After this a table is printed of “per-archive contributions to ELF file”. This means how much each static library archive has contributed to the final binary size.

Generally, one static library archive is built per component, although some are binary libraries included by a particular component (for example, libnet80211.a is included by esp_wifi component). There are also toolchain libraries such as libc.a and libgcc.a listed here, these provide Standard C/C++ Library and toolchain built-in functionality.

If your project is simple and only has a “main” component, then all of the project’s code will be shown under libmain.a. If your project includes its own components (see Build System), then they will each be shown on a separate line.

The table is sorted in descending order of the total contribution to the binary size.

The columns are as follows:

• DRAM .data & .bss & other - .data and .bss are the same as for the totals shown above (static variables, these both reduce total available RAM at runtime but .bss doesn’t contribute to the binary file size). “other” is a column for any custom section types that also contribute to RAM size (usually this value is 0).

• IRAM - is the same as for the totals shown above (code linked to execute from IRAM, uses space in the binary file and also reduces DRAM available as heap at runtime.

• Flash code & rodata - these are the same as the totals above, IROM and DROM space accessed from flash cache that contribute to the binary size.

### Source File Usage Summary (idf.py size-files)¶

For even more detail, run idf.py size-files to get a summary of the contribution each object file has made to the final binary size. Each object file corresponds to a single source file.

$idf.py size-files [...] Total sizes: DRAM .data size: 14956 bytes DRAM .bss size: 15808 bytes Used static DRAM: 30764 bytes ( 149972 available, 17.0% used) Used static IRAM: 83918 bytes ( 47154 available, 64.0% used) Flash code: 559943 bytes Flash rodata: 176736 bytes Total image size:~ 835553 bytes (.bin may be padded larger) Per-file contributions to ELF file: Object File DRAM .data & .bss & other IRAM D/IRAM Flash code & rodata Total x509_crt_bundle.S.o 0 0 0 0 0 0 64212 64212 wl_cnx.o 2 3183 0 221 0 13119 3286 19811 phy_chip_v7.o 721 614 0 1642 0 16820 0 19797 ieee80211_ioctl.o 740 96 0 437 0 15325 2627 19225 pp.o 1142 45 0 8871 0 5030 537 15625 ieee80211_output.o 2 20 0 2118 0 11617 914 14671 ieee80211_sta.o 1 41 0 1498 0 10858 2218 14616 lib_a-vfprintf.o 0 0 0 0 0 13829 752 14581 lib_a-svfprintf.o 0 0 0 0 0 13251 752 14003 ssl_tls.c.o 60 0 0 0 0 12769 463 13292 sockets.c.o 0 648 0 0 0 11096 1030 12774 nd6.c.o 8 932 0 0 0 11515 314 12769 phy_chip_v7_cal.o 477 53 0 3499 0 8561 0 12590 pm.o 32 364 0 2673 0 7788 782 11639 ieee80211_scan.o 18 288 0 0 0 8889 1921 11116 lib_a-svfiprintf.o 0 0 0 0 0 9654 1206 10860 lib_a-vfiprintf.o 0 0 0 0 0 10069 734 10803 ieee80211_ht.o 0 4 0 1186 0 8628 898 10716 phy_chip_v7_ana.o 241 48 0 2657 0 7677 0 10623 bignum.c.o 0 4 0 0 0 9652 752 10408 tcp_in.c.o 0 52 0 0 0 8750 1282 10084 trc.o 664 88 0 1726 0 6245 1108 9831 tasks.c.o 8 704 0 7594 0 0 1475 9781 ecp_curves.c.o 28 0 0 0 0 7384 2325 9737 ecp.c.o 0 64 0 0 0 8864 286 9214 ieee80211_hostap.o 1 41 0 0 0 8578 585 9205 wdev.o 121 125 0 4499 0 3684 580 9009 tcp_out.c.o 0 0 0 0 0 5686 2161 7847 tcp.c.o 2 26 0 0 0 6161 1617 7806 ieee80211_input.o 0 0 0 0 0 6797 973 7770 wpa.c.o 0 656 0 0 0 6828 55 7539 [... additional lines removed ...]  After the summary of total sizes, a table of “Per-file contributions to ELF file” is printed. The columns are the same as shown above for idy.py size-components, but this time the granularity is the contribution of each individual object file to the binary size. For example, we can see that the file x509_crt_bundle.S.o contributed 64212 bytes to the total firmware size, all as .rodata in flash. Therefore we can guess that this application is using the ESP x509 Certificate Bundle feature and not using this feature would save at last this many bytes from the firmware size. Some of the object files are linked from binary libraries and therefore you won’t find a corresponding source file. To locate which component a source file belongs to, it’s generally possible to search in the ESP-IDF source tree or look in the Linker Map File for the full path. ### Comparing Two Binaries¶ If making some changes that affect binary size, it’s possible to use an ESP-IDF tool to break down the exact differences in size. This operation isn’t part of idf.py, it’s necessary to run the idf-size.py Python tool directly. To do so, first locate the linker map file in the build directory. It will have the name PROJECTNAME.map. The idf-size.py tool performs its analysis based on the output of the linker map file. To compare with another binary, you will also need its corresponding .map file saved from the build directory. For example, to compare two builds: one with the default CONFIG_COMPILER_OPTIMIZATION setting “Debug (-Og)” configuration and one with “Optimize for size (-Os)”: $ $IDF_PATH/tools/idf_size.py --diff build_Og/https_request.map build_Os/https_request.map <CURRENT> MAP file: build_Os/https_request.map <REFERENCE> MAP file: build_Og/https_request.map Difference is counted as <CURRENT> - <REFERENCE>, i.e. a positive number means that <CURRENT> is larger. Total sizes of <CURRENT>: <REFERENCE> Difference DRAM .data size: 14516 bytes 14956 -440 DRAM .bss size: 15792 bytes 15808 -16 Used static DRAM: 30308 bytes ( 150428 available, 16.8% used) 30764 -456 ( +456 available, +0 total) Used static IRAM: 78498 bytes ( 52574 available, 59.9% used) 83918 -5420 ( +5420 available, +0 total) Flash code: 509183 bytes 559943 -50760 Flash rodata: 170592 bytes 176736 -6144 Total image size:~ 772789 bytes (.bin may be padded larger) 835553 -62764  We can see from the “Difference” column that changing this one setting caused the whole binary to be over 60 KB smaller and over 5 KB more RAM is available. It’s also possible to use the “diff” mode to output a table of component-level (static library archive) differences: $IDF_PATH/tools/idf_size.py --archives --diff build_Og/https_request.map build_Oshttps_request.map


Also at the individual source file level:

$IDF_PATH/tools/idf_size.py --files --diff build_Og/https_request.map build_Oshttps_request.map  Other options (like writing the output to a file) are available, pass --help to see the full list. ### Showing Size When Linker Fails¶ If too much static memory is used, then the linker will fail with an error such as DRAM segment data does not fit, region iram0_0_seg' overflowed by 44 bytes, or similar. In these cases, idf.py size will not succeed either. However it is possible to run idf_size.py manually in order to view the partial static memory usage (the memory usage will miss the variables which could not be linked, so there still appears to be some free space.) The map file argument is <projectname>.map in the build directory $IDF_PATH/tools/idf_size.py build/project_name.map


It is also possible to view the equivalent of size-components or size-files output:

$IDF_PATH/tools/idf_size.py --archives build/project_name.map$IDF_PATH/tools/idf_size.py --files build/project_name.map


This is an advanced analysis method, but it can be very useful. Feel free to skip ahead to :ref: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 (“linked”) the final firmware binary file

Linker map files themselves are plain text files, so it’s possible to read them and find out exactly what the linker did. However, they are also very complex and long - often 100,000 or more 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’re 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 that 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 doesn’t 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 the 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 (ELF sections .debug_*, etc.). These don’t contribute to the final binary size. You’ll notice the address of these symbols is a very low number (starting from 0x0000000000000000 and counting up).

• Cross Reference Table. This table shows for each symbol (function or static variable), the list of object file(s) that referred to it. If you’re wondering why a particular thing is included in the binary, this will help determine what included it.

Note

Unfortunately, the Cross Reference Table doesn’t only include symbols that made it into the final binary. It also includes symbols in discarded sections. Therefore, just because something is shown here doesn’t 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 will reduce the final binary size of almost any ESP-IDF project:

• Set CONFIG_COMPILER_OPTIMIZATION to “Optimize for size (-Os)”. In some cases, “Optimize for performance (-O2)” will also reduce the binary size compared to the default. Note that if your code contains C or C++ Undefined Behaviour then increasing the compiler optimization level may expose bugs that otherwise don’t happen.

• Reduce the compiled-in log output by lowering the app CONFIG_LOG_DEFAULT_LEVEL. If the CONFIG_LOG_MAXIMUM_LEVEL is changed from the default then this setting controls the binary size instead. Reducing compiled-in logging reduces the number of strings in the binary, and also the code size of the calls to logging functions.

• Set the CONFIG_COMPILER_OPTIMIZATION_ASSERTION_LEVEL to “Silent”. This avoids compiling in a dedicated assertion string and source file name for each assert that may fail. It’s still possible to find the failed assert in the code by looking at the memory address where the assertion failed.

• Besides the CONFIG_COMPILER_OPTIMIZATION_ASSERTION_LEVEL, you can disable or silent the assertion for HAL component separately by setting CONFIG_HAL_DEFAULT_ASSERTION_LEVEL.

• Set CONFIG_COMPILER_OPTIMIZATION_CHECKS_SILENT. This removes specific error messages for particular internal ESP-IDF error check macros. This may make it harder to debug some error conditions by reading the log output.

• Don’t enable CONFIG_COMPILER_CXX_EXCEPTIONS, CONFIG_COMPILER_CXX_RTTI, or set the CONFIG_COMPILER_STACK_CHECK_MODE to Overall. All of these options are already disabled by default, but they have a large impact on binary size.

• Disabling CONFIG_ESP_ERR_TO_NAME_LOOKUP will remove the lookup table to translate user-friendly names for error values (see Error Handling) in error logs, etc. This saves some binary size, but error values will be printed as integers only.

• Setting CONFIG_ESP_SYSTEM_PANIC to “Silent reboot” will save a small amount of binary size, however this is only recommended if no one will use UART output to debug the device.

Note

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

### Targeted Optimizations¶

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

#### 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 coap and ASIO port, These components will not be available if IPV6 is disabled.

#### Newlib nano formatting¶

By default, ESP-IDF uses newlib “full” formating for I/O (printf, scanf, etc.)

Enabling the config option CONFIG_NEWLIB_NANO_FORMAT will switch newlib to the “nano” formatting mode. This both smaller in code size and a large part of the implementation is compiled into the ESP32-S2 ROM, so it doesn’t 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 also reduces the stack usage of each function that calls printf() or another string formatting function, see Reducing Stack Sizes.

“Nano” formatting doesn’t 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 which are enabled by default but can be disabled if not needed to save code size.

These include:

Important

It is strongly not recommended to disable all these mbedTLS options. Only disable options where 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, recommend ensuring 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-S2 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.

#### FreeModBus¶

If using Modbus, enable or disable CONFIG_FMB_COMM_MODE_TCP_EN, CONFIG_FMB_COMM_MODE_RTU_EN, CONFIG_FMB_COMM_MODE_ASCII_EN as applicable for the necessary functionality.

#### VFS¶

Virtual filesystem 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 doesn’t 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 doesn’t use 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 doesn’t use directory related functions, such as readdir` (see the description of this option for the complete list). Applications which only open, read and write specific files and don’t 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 doesn’t 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’t 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.