newlib

Newlib is a C standard library implementation designed for use in embedded systems and as the runtime library for cross-compilers. It provides most of the ISO C library interfaces (such as stdio, stdlib, string, math, and time) along with a broad set of POSIX-like facilities, while remaining portable across a wide range of architectures and environments. Newlib is not tied to a single operating system; it uses a retargetable system interface in which the core C library functions rely on a small set of low-level, target-specific calls. This arrangement allows Newlib to run on bare-metal systems as well as on top of various operating systems or kernels.

The project supports multiple targets and architectures, including ARM, MIPS, PowerPC, x86, and RISC-V, and is widely used as the C library component of GNU toolchains, especially in GCC-based cross-compilers. It also appears in other contexts such as emscripten, which compiles to JavaScript, and various embedded Linux and RTOS environments. Newlib provides reentrant versions of many functions and includes facilities for dynamic memory management, while its modular design enables minimal footprints for constrained devices.

Newlib is distributed as part of the GNU project and is licensed under the GNU Lesser General Public License (LGPL), which facilitates use in both open-source and proprietary software. It has been maintained by Rich Felker and a community of contributors, with ongoing development aimed at expanding portability, performance, and API coverage across platforms.