BLAKE3/c/README.md

This is the C implementation of BLAKE3. The public API consists of one struct and five functions in blake3.h:

• typedef struct {...} blake3_hasher An incremental BLAKE3 hashing state, which can accept any number of updates.
• blake3_hasher_init(...) Initialize a blake3_hasher in the default hashing mode.
• blake3_hasher_init_keyed(...) Initialize a blake3_hasher in the keyed hashing mode, which accepts a 256-bit key.
• blake3_hasher_init_derive_key(...) Initialize a blake3_hasher in the key derivation mode, which accepts a context string of any length. In this mode, the key material is given as input after initialization. The context string should be hardcoded, globally unique, and application-specific. A good default format for such strings is "[application] [commit timestamp] [purpose]", e.g., "example.com 2019-12-25 16:18:03 session tokens v1".
• blake3_hasher_update(...) Add input to the hasher. This can be called any number of times.
• blake3_hasher_finalize(...) Finalize the hasher and emit an output of any length. This does not modify the hasher itself. It is possible to finalize again after adding more input.

Example

Here's an example program that hashes bytes from standard input and prints the result:

#include "blake3.h"
#include <stdio.h>

int main() {
  // Initialize the hasher.
  blake3_hasher hasher;
  blake3_hasher_init(&hasher);

  // Read input bytes from stdin.
  unsigned char buf[65536];
  size_t n;
  while ((n = fread(buf, 1, 65536, stdin)) > 0) {
    blake3_hasher_update(&hasher, buf, n);
  }

  // Finalize the hash. BLAKE3_OUT_LEN is the default output length, 32 bytes.
  uint8_t output[BLAKE3_OUT_LEN];
  blake3_hasher_finalize(&hasher, output, BLAKE3_OUT_LEN);

  // Print the hash as hexadecimal.
  for (size_t i = 0; i < BLAKE3_OUT_LEN; i++) {
    printf("%02x", output[i]);
  }
  printf("\n");
  return 0;
}

If you save the example code above as example.c, and you're on x86_64 with a Unix-like OS, you can compile a working binary like this:

gcc -O3 -o example example.c blake3.c blake3_dispatch.c blake3_portable.c \
    blake3_sse41_x86-64_unix.S blake3_avx2_x86-64_unix.S blake3_avx512_x86-64_unix.S

Building

The Makefile included in this implementation is for testing. It's expected that callers will have their own build systems. This section describes the compilation steps that build systems (or folks compiling by hand) should take. Note that these steps may change in future versions.

x86

Dynamic dispatch is enabled by default on x86. The implementation will query the CPU at runtime to detect SIMD support, and it will use the widest instruction set available. By default, blake3_dispatch.c expects to be linked with code for four different instruction sets: portable C, SSE4.1, AVX2, and AVX-512.

For each of the x86 SIMD instruction sets, two versions are available, one in assembly (with three flavors: Unix, Windows MSVC, and Windows GNU) and one using C intrinsics. The assembly versions are generally preferred: they perform better, they perform more consistently across different compilers, and they build more quickly. On the other hand, the assembly versions are x86_64-only, and you need to select the right flavor for your target platform.

Here's an example of building a shared library on x86_64 Linux using the assembly implementations:

gcc -shared -O3 -o libblake3.so blake3.c blake3_dispatch.c blake3_portable.c \
    blake3_sse41_x86-64_unix.S blake3_avx2_x86-64_unix.S blake3_avx512_x86-64_unix.S

When building the intrinsics-based implementations, you need to build each implementation separately, with the corresponding instruction set explicitly enabled in the compiler. Here's the same shared library using the intrinsics-based implementations:

gcc -c -fPIC -O3 -msse4.1 blake3_sse41.c -o blake3_sse41.o
gcc -c -fPIC -O3 -mavx2 blake3_avx2.c -o blake3_avx2.o
gcc -c -fPIC -O3 -mavx512f -mavx512vl blake3_avx512.c -o blake3_avx512.o
gcc -shared -O3 -o libblake3.so blake3.c blake3_dispatch.c blake3_portable.c \
    blake3_avx2.o blake3_avx512.o blake3_sse41.o

Note above that building blake3_avx512.c requires both -mavx512f and -mavx512vl under GCC and Clang, as shown above. Under MSVC, the single /arch:AVX512 flag is sufficient.

If you want to omit SIMD code on x86, you need to explicitly disable each instruction set. Here's an example of building a shared library on x86 with only portable code:

gcc -shared -O3 -o libblake3.so -DBLAKE3_NO_SSE41 -DBLAKE3_NO_AVX2 -DBLAKE3_NO_AVX512 \
    blake3.c blake3_dispatch.c blake3_portable.c

ARM NEON

The NEON implementation is not enabled by default on ARM, since not all ARM targets support it. To enable it, set BLAKE3_USE_NEON=1. Here's an example of building a shared library on ARM Linux with NEON support:

gcc -shared -O3 -o libblake3.so -DBLAKE3_USE_NEON blake3.c blake3_dispatch.c \
    blake3_portable.c blake3_neon.c

Note that on some targets (ARMv7 in particular), extra flags may be required to activate NEON support in the compiler. If you see an error like...

/usr/lib/gcc/armv7l-unknown-linux-gnueabihf/9.2.0/include/arm_neon.h:635:1: error: inlining failed
in call to always_inline ‘vaddq_u32’: target specific option mismatch

...then you may need to add something like -mfpu=neon-vfpv4 -mfloat-abi=hard.

Other Platforms

The portable implementation should work on most other architectures. For example:

gcc -shared -O3 -o libblake3.so blake3.c blake3_dispatch.c blake3_portable.c

Differences from the Rust Implementation

The single-threaded Rust and C implementations use the same algorithms, and their performance is the same if you use the assembly implementations or if you compile the intrinsics-based implementations with Clang. (Both Clang and rustc are LLVM-based.)

The C implementation does not currently support multi-threading. OpenMP support or similar might be added in the future.

Both the C and Rust implementations support output of any length, but only the Rust implementation provides an incremental (and seekable) output reader. This might also be added in the future.