How to Eliminate Newlib .impure_data Overhead in Embedded Builds

by

Summary

The impure_data section appears because Newlib’s standard library pulls in global state for error handling, locale, and other runtime services. -fno-math-errno only suppresses the errno side‑effect of math functions; it does not eliminate the whole impure_data region. Switching to nano.specs swaps the full Newlib implementation for a slimmer “newlib‑nano” variant that removes many optional globals, drastically shrinking the section.

Root Cause

  • sqrt() links against libg.a, which depends on libc objects that define the _impure_ptr structure.
  • The structure resides in the .impure_data subsection and contains:
    • errno field
    • __locale data
    • __atexit handler tables
  • -fno-math-errno disables the errno side‑effect inside the math function, but the library still expects the global errno variable to exist, so the linker still pulls in the object that defines it.
  • The default Newlib (libc.a) pulls many additional globals (stdio buffers, reentrancy structures) that inflate the section.

Why This Happens in Real Systems

  • Embedded toolchains ship with full Newlib for compatibility.
  • Many C library functions are compiled with __attribute__((weak)) and resolve to the “full” implementations unless the nano variant is explicitly requested.
  • The linker pulls in whole object files, not individual symbols; any object that defines _impure_ptr brings the whole .impure_data segment.

Real-World Impact

  • Memory budget overflow on devices with < 2 KiB RAM (e.g., MSPM0C).
  • Longer startup time because the C runtime has to zero‑initialize a larger data segment.
  • Increased flash usage: the full Newlib objects are larger than the nano equivalents.
  • Potential runtime failures if the RAM overrun corrupts other variables.

Example or Code (if necessary and relevant)

#include 
#include 

float compute(float x)
{
    return sqrtf(x) + (float)abs((int)x);
}

How Senior Engineers Fix It

  • Prefer nano libraries: add --specs=nano.specs (and optionally --specs=nosys.specs for stub syscalls).
  • Link only required objects: use -Wl,--gc-sections (already present) and --specs=nosys.specs to avoid pulling syscalls that introduce extra globals.
  • Replace heavy functions with lightweight hand‑rolled alternatives when feasible (e.g., an integer square‑root approximation).
  • Enable -ffreestanding for bare‑metal builds to suppress automatic inclusion of the full C library.
  • Audit library dependencies: run arm-none-eabi-nm -C libg.a | grep impure to see which objects introduce the section and replace them with nano counterparts.
  • Use -u _printf_float only when needed; otherwise avoid pulling floating‑point support altogether.

Why Juniors Miss It

  • They assume -fno-math-errno removes all errno handling, not just the math‑specific side‑effect.
  • They overlook that the C runtime (Newlib) introduces a global reentrancy structure independent of the math flags.
  • They often don’t inspect the linker map closely enough to see which object file contributes to .impure_data.
  • Lack of familiarity with the distinction between full Newlib and newlib‑nano specs leads to reliance on default settings that are too heavyweight for a 1 KiB RAM target.

Leave a Comment