Bionic is an implementation of the standard C library, developed by Google for its Android operating system. It differs from the GNU C Library (glibc) in being designed for devices with less memory and processor power than a typical Linux system. It is a combination of new code and code from FreeBSD, NetBSD, and OpenBSD released under a BSD license, rather than glibc, which uses the GNU Lesser General Public License. This difference was important in the early days of Android, when static linking was common, and is still helpful in introducing Android to software companies used to proprietary operating systems, who can be wary of the LGPL, and unclear about the differences between it and the full GNU General Public License (GPL).
Bionic is a C library for use with the Linux kernel, and provides libc, libdl, and libm (libpthread functionality is part of libc, not a separate library as on some other systems). This differs from the BSD C libraries that bionic shares code with, because they require a BSD kernel.
Original goals
The original publicly stated goals for Bionic were the following:
BSD-licensed: Google wanted to isolate Android applications from the effect of copyleft licenses to create a proprietary user-space and application ecosystem, but: Android is based on the Linux kernel, which is subject to the copyleft GNU General Public License (GPL) version 2. The most widespread standard C library for the Linux kernel is the GNU C Library (glibc), which is subject to the GNU Lesser General Public License (LGPL), also a copyleft license. In contrast to the GPL, the LGPL explicitly allows for dynamic linking but it does not allow static linking of proprietary software without providing source code or linkable object files. The permissive BSD license is a non-copyleft license that is compatible in both directions. A BSD-licensed glibc substitute could act as an isolation layer between the copyleft core (kernel) and the non-copyleft applications, and was therefore chosen by Google for its Bionic as a glibc substitute. Small size: Bionic was much smaller than the GNU C Library; more importantly its memory requirements were (and remain) much lower. Speed: Bionic was designed for CPUs at relatively low clock frequencies.
Supported architectures
Bionic only supports Linux kernels, but currently supports the arm, arm64, riscv64, x86, and x86-64 architectures. The platform itself required armv7 with Neon since Marshmallow, though the Android Native Development Kit (NDK) continued to support armv5 (which it called armeabi) up until NDK r16. The NDK still supports armv7 both with and without Neon. Historically there was partial SH-4 support in the platform, but no devices ever shipped and support has since been removed. The NDK never supported SH-4, and MIPS and MIPS64 support were removed from the NDK in r17.
Components
Some parts of the libc source, such as stdio, are from the BSDs (mainly OpenBSD), whereas others, such as the pthread implementation, were written from scratch.
The dynamic memory allocator implementation has changed over time. Before Lollipop there was a single native memory allocator, Doug Lea's dlmalloc. For Lollipop and Marshmallow there were two implementations: dlmalloc and jemalloc. jemalloc gives much higher performance than dlmalloc, but at the cost of extra memory required for bookkeeping. Most devices used jemalloc but low-memory devices still used dlmalloc. For Nougat through Android 10, all devices use jemalloc. Low-memory devices use a "svelte" configuration of jemalloc that disables the tcache to nearly match the lower memory overhead of dlmalloc while keeping most of the speed of jemalloc. In Android 11, the memory allocator for most devices was switched to Scudo, which sacrifices some of jemalloc's high performance for additional security hardening features. Low-memory devices, however, are still permitted to use jemalloc.
Some 64-bit devices, like the Nexus 9, are effectively low-memory devices because of the extra space requirements of 64-bit pointers and hosting of two zygotes. (Zygote is an Android system service that is the parent of all Android application processes.)
The libm source is largely FreeBSD's, but with optimized assembler contributed by the various SoC vendors.
The dynamic linker (and libdl) were written from scratch.
Bionic doesn't include libthread_db (used by gdbserver), but the NDK does. The Android platform includes a statically linked gdbserver, so that developers can use the latest gdb even on old devices.