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expand the C README for public consumption
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c/README.md
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c/README.md
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This is the C implementation of BLAKE3. It's tested, and parts of it are
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linked into the Rust implementation for AVX-512 and NEON support.
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However, it doesn't yet have a friendly public interface.
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This is the C implementation of BLAKE3. The public API consists of one
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struct and five functions in [`blake3.h`](blake3.h):
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This implementation is simpler than the Rust implementation. It doesn't
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support multithreading, and it doesn't parallelize parent hashes, so
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throughput is lower.
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- `typedef struct {...} blake3_hasher`: An incremental BLAKE3 hashing
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state, which can accept any number of updates.
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- `blake3_hasher_init(...)`: Initialize a `blake3_hasher` in the default
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hashing mode.
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- `blake3_hasher_init_keyed(...)`: Initialize a `blake3_hasher` in the
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keyed hashing mode, which accepts a 256-bit key.
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- `blake3_hasher_init_derive_key(...)`: Initialize a `blake3_hasher` in
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the key derivation mode, which accepts a context string of any length.
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In this mode, the key material is given as input after initialization.
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The context string should be hardcoded, globally unique, and
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application-specific. A good default format for such strings is
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`"[application] [commit timestamp] [purpose]"`, e.g., `"example.com
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2019-12-25 16:18:03 session tokens v1"`.
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- `blake3_hasher_update(...)`: Add input to the hasher. This can be
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called any number of times.
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- `blake3_hasher_finalize(...)`: Finalize the hasher and emit an output
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of any length. This does not modify the hasher itself. It is possible
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to finalize again after adding more input.
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## Example
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```c
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#include <stdio.h>
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#include "blake3.h"
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int main() {
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// Initialize the hasher.
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blake3_hasher hasher;
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blake3_hasher_init(&hasher);
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// Write some input bytes.
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blake3_hasher_update(&hasher, "foo", 3);
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blake3_hasher_update(&hasher, "bar", 3);
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blake3_hasher_update(&hasher, "baz", 3);
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// Finalize the hash. BLAKE3_OUT_LEN is 32 bytes, but extended outputs are
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// also supported.
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uint8_t output[BLAKE3_OUT_LEN];
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blake3_hasher_finalize(&hasher, output, BLAKE3_OUT_LEN);
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// Print the hash as hexadecimal.
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for (size_t i = 0; i < BLAKE3_OUT_LEN; i++) {
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printf("%02x", output[i]);
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}
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printf("\n");
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return 0;
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}
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```
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## Building
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The Makefile included in this implementation is for testing. It's
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expected that callers will have their own build systems. This section
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describes the compilation steps that build systems (or folks compiling
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by hand) should take. Note that these steps may change in future
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versions of this repo.
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### x86
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Dynamic dispatch is enabled by default on x86. The implementation will
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query the CPU at runtime to detect SIMD support, and it will use the
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widest SIMD vectors the CPU supports. Each of the
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instruction-set-specific implementation files needs to be compiled with
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the corresponding instruction set explicitly enabled. Here's an example
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of building a shared library on Linux:
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```bash
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gcc -c -fPIC -O3 -msse4.1 blake3_sse41.c -o blake3_sse41.o
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gcc -c -fPIC -O3 -mavx2 blake3_avx2.c -o blake3_avx2.o
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gcc -c -fPIC -O3 -mavx512f -mavx512vl blake3_avx512.c -o blake3_avx512.o
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gcc -shared -O3 blake3.c blake3_dispatch.c blake3_portable.c \
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blake3_avx2.o blake3_avx512.o blake3_sse41.o -o libblake3.so
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```
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If you want to omit SIMD code on x86, you need to explicitly disable
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each instruction set. Here's an example of building a shared library on
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Linux with no SIMD support:
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```bash
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gcc -shared -O3 -DBLAKE3_NO_SSE41 -DBLAKE3_NO_AVX2 -DBLAKE3_NO_AVX512 \
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blake3.c blake3_dispatch.c blake3_portable.c -o libblake3.so
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```
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### ARM
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TODO: add NEON support to `blake3_dispatch.c`.
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### Other Platforms
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The portable implementation should work on most other architectures. For
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example:
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```bash
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gcc -shared -O3 blake3.c blake3_dispatch.c blake3_portable.c -o libblake3.so
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```
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Most multi-platform builds should build `blake3_sse41.c`,
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`blake3_avx2.c`, and `blake3_avx512.c` when targetting x86, and skip
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them for all other platforms. It could be possible to `#ifdef` out the
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contents of those files on non-x86 platforms, but flags like `-msse4.1`
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generally cause errors anyway when they're not supported, so skipping
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these build steps entirely is usually necessary.
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## Differences from the Rust Implementation
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The single-threaded Rust and C implementations use the same algorithms
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and have essentially the same performance if you compile with Clang.
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(Both Clang and rustc are LLVM-based.) Note that performance is
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currently better with Clang than with GCC.
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The C implementation does not currently support multi-threading. OpenMP
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support or similar might be added in the future.
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Both the C and Rust implementations support output of any lenght, but
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only the Rust implementation provides an incremental (and seekable)
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output reader. This might also be added in the future.
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@ -15,6 +15,8 @@ extern "C" {
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#define BLAKE3_MAX_DEPTH 54
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#define BLAKE3_MAX_SIMD_DEGREE 16
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// This struct is a private implementation detail. It has to be here because
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// it's part of blake3_hasher below.
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typedef struct {
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uint32_t cv[8];
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uint64_t chunk_counter;
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