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Newlib is a complete C library implementation written for the embedded systems. It is a separate open source project and is not included in source code form with Zephyr. Instead, the Zephyr SDK includes a precompiled library for each supported architecture (libc.a and libm.a).


Other 3rd-party toolchains, such as GNU Arm Embedded, also bundle the Newlib as a precompiled library.

Zephyr implements the “API hook” functions that are invoked by the C standard library functions in the Newlib. These hook functions are implemented in lib/libc/newlib/libc-hooks.c and translate the library internal system calls to the equivalent Zephyr API calls.

Types of Newlib

The Newlib included in the Zephyr SDK comes in two versions: ‘full’ and ‘nano’ variants.

Full Newlib

The Newlib full variant (libc.a and libm.a) is the most capable variant of the Newlib available in the Zephyr SDK, and supports almost all standard C library features. It is optimized for performance (prefers performance over code size) and its footprint is significantly larger than the nano variant.

This variant can be enabled by selecting the CONFIG_NEWLIB_LIBC and de-selecting the CONFIG_NEWLIB_LIBC_NANO in the application configuration file.

Nano Newlib

The Newlib nano variant (libc_nano.a and libm_nano.a) is the size-optimized version of the Newlib, and supports all features that the full variant supports except the new format specifiers introduced in C99, such as the char, long long type format specifiers (i.e. %hhX and %llX).

This variant can be enabled by selecting the CONFIG_NEWLIB_LIBC and CONFIG_NEWLIB_LIBC_NANO in the application configuration file.

Note that the Newlib nano variant is not available for all architectures. The availability of the nano variant is specified by the CONFIG_HAS_NEWLIB_LIBC_NANO.

Formatted Output

Newlib supports all standard C formatted input and output functions, including printf, fprintf, sprintf and sscanf.

The Newlib formatted input and output function implementation supports all format specifiers defined by the C standard with the following exceptions:

Dynamic Memory Management

Newlib implements an internal heap allocator to manage the memory blocks used by the standard dynamic memory management interface functions (for example, malloc() and free()).

The internal heap allocator implemented by the Newlib may vary across the different types of the Newlib used. For example, the heap allocator implemented in the Full Newlib (libc.a and libm.a) of the Zephyr SDK requests larger memory chunks to the operating system and has a significantly higher minimum memory requirement compared to that of the Nano Newlib (libc_nano.a and libm_nano.a).

The only interface between the Newlib dynamic memory management functions and the Zephyr-side libc hooks is the sbrk() function, which is used by the Newlib to manage the size of the memory pool reserved for its internal heap allocator.

The _sbrk() hook function, implemented in libc-hooks.c, handles the memory pool size change requests from the Newlib and ensures that the Newlib internal heap allocator memory pool size does not exceed the amount of available memory space by returning an error when the system is out of memory.

When userspace is enabled, the Newlib internal heap allocator memory pool is placed in a dedicated memory partition called z_malloc_partition, which can be accessed from the user mode threads.

The amount of memory space available for the Newlib heap depends on the system configurations:

The standard dynamic memory management interface functions implemented by the Newlib are thread safe and may be simultaneously called by multiple threads.