Skip to main content

Enable Compact Logs

This tutorial will cover how to enable the compact log feature in the Memfault Firmware SDK. When Compact logging is enabled, format strings will be replaced with an integer id at compile time. When the log is captured only the arguments and integer id will be serialized and sent to the Memfault cloud where the full format string will be recovered.

This has several key benefits:

  • Reduces storage for logs on device
  • Reduces required bandwidth to send logs to Memfault cloud
  • Reduces CPU overhead for storing logs by offloading printf


Today, once enabled, compact logs will be used when capturing trace events with logs.

Let's take a look at a quick example:

MEMFAULT_TRACE_EVENT_WITH_LOG(flash_error, "QSPI Flash Erase Failure: error code: 0x%x", 0x5);

With a formatted string we need to encode the string length as well as the entire string in our binary message taking up 41 bytes

00000000: 5153 5049 2046 6c61 7368 2045 7261 7365 QSPI Flash Erase
00000010: 2046 6169 6c75 7265 3a20 6572 726f 7220 Failure: error
00000020: 636f 6465 3a20 3078 35 code: 0x5

where as with a compact log we need to encode a integer id and the arguments to be serialized which in this example takes a total of 5 bytes

00000000: 8218 8805 ....

So in this example we are able to reduce the storage occupied by 88%!

Integration Steps#


Add the following to your projects memfault_platform_config.h:

//! @file memfault_platform_config.h

2. Add a log fmt section to your linker script#

Locate the linker script for your project (usually ends with .ld) and add the following to your linker script

log_fmt 0xF0000000 (INFO):


How do I fix "## operator not supported" compiler error?#

Once enabled, if you see the following compiler error:

memfault/core/compiler_gcc.h:73:45: error: static assertion failed: "## operator not supported, enable gnu extensions on your compiler"
# define MEMFAULT_STATIC_ASSERT(cond, msg) _Static_assert(cond, msg)

Compact logging uses a GNU Compiler extension which will remove the leading comma in , ## __VA_ARGS__ if __VA_ARGS__ is empty. This particular extension has been implemented in most other modern compilers including ARM Compiler 5, Clang & iccarm. All you need to do is make sure you have enabled gnu extensions for your compiler of choice.

For GCC, this can be accomplished by adding -std=gnu11 as an argument to your compiler flag list

I'm seeing an incorrect pointer value when trying to use the %p formatter.#

When the %p format is used and the argument is typed as a char * or char [], it must be cast to a void *. If it is not cast, a string will be serialized instead and an incorrect pointer value will be displayed in the Memfault UI.

Here's an example of correct usage:

// No cast necessary for pointer types that are not strings
int i = 0;
uint32_t *int_ptr = &i;
MEMFAULT_TRACE_EVENT_WITH_LOG("String Pointer: %p", int_ptr);
// Cast necessary when pointer type is a string
const char *str = "hello world";
MEMFAULT_TRACE_EVENT_WITH_LOG("String Pointer: %p", (void *)str);