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The memchr crate provides heavily optimized routines for searching bytes.

Build status

Dual-licensed under MIT or the UNLICENSE.



The memchr function is traditionally provided by libc, but its performance can vary significantly depending on the specific implementation of libc that is used. They can range from manually tuned Assembly implementations (like that found in GNU's libc) all the way to non-vectorized C implementations (like that found in MUSL).

To smooth out the differences between implementations of libc, at least on x86_64 for Rust 1.27+, this crate provides its own implementation of memchr that should perform competitively with the one found in GNU's libc. The implementation is in pure Rust and has no dependency on a C compiler or an Assembler.

Additionally, GNU libc also provides an extension, memrchr. This crate provides its own implementation of memrchr as well, on top of memchr2, memchr3, memrchr2 and memrchr3. The difference between memchr and memchr2 is that memchr2 permits finding all occurrences of two bytes instead of one. Similarly for memchr3.

Compiling without the standard library

memchr links to the standard library by default, but you can disable the std feature if you want to use it in a #![no_std] crate:

memchr = { version = "2", default-features = false }

On x86 platforms, when the std feature is disabled, the SSE2 implementation of memchr will be used in compilers that support it. When std is enabled, the AVX implementation of memchr will be used if the CPU is determined to support it at runtime.

Using libc

memchr is a routine that is part of libc, although this crate does not use libc by default. Instead, it uses its own routines, which are either vectorized or generic fallback routines. In general, these should be competitive with what‘s in libc, although this has not been tested for all architectures. If using memchr from libc is desirable and a vectorized routine is not otherwise available in this crate, then enabling the libc feature will use libc’s version of memchr.

The rest of the functions in this crate, e.g., memchr2 or memrchr3, are not a standard part of libc, so they will always use the implementations in this crate. One exception to this is memrchr, which is an extension commonly found on Linux. On Linux, memrchr is used in precisely the same scenario as memchr, as described above.

Minimum Rust version policy

This crate's minimum supported rustc version is 1.28.0.

The current policy is that the minimum Rust version required to use this crate can be increased in minor version updates. For example, if crate 1.0 requires Rust 1.20.0, then crate 1.0.z for all values of z will also require Rust 1.20.0 or newer. However, crate 1.y for y > 0 may require a newer minimum version of Rust.

In general, this crate will be conservative with respect to the minimum supported version of Rust.