Rust跨平台与条件编译总结

build.rs可实现正常项目编译前的额外操作，比如代码生成、编译所依赖的C/C++库等行为。为了让编程过程更可控，通常需要输出状态表示通过了某一阶段，或遇到什么错误，Cargo支持build.rs编译时输出不同类型的语句，比如info、warning、error等，以下为示例，参考自 rustdroid-native 。

println!("cargo:warning=Error executing process command <{:?}>: {}", cmd, e);
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调用C/C++编译器编译所依赖的第三方C/C++库

目前我看到比较完整的参考是官方的libstd/build.rs，编译我们业务所需的第三方库的命令几乎都可以从那找到“灵感”，下面贴出完整代码镇宅，关键操作是 build_libbacktrace() ，通过 cc::Build 实例把需要编译的C/C++代码声明起来， 理论上支持正则匹配 。

#![deny(warnings)]

extern crate build_helper;
extern crate cc;

use build_helper::native_lib_boilerplate;
use std::env;
use std::fs::File;

fn main() {
    let target = env::var("TARGET").expect("TARGET was not set");
    if cfg!(feature = "backtrace") &&
        !target.contains("cloudabi") &&
        !target.contains("emscripten") &&
        !target.contains("msvc") &&
        !target.contains("wasm32")
    {
        let _ = build_libbacktrace(&target);
    }

    if target.contains("linux") {
        if target.contains("android") {
            println!("cargo:rustc-link-lib=dl");
            println!("cargo:rustc-link-lib=log");
            println!("cargo:rustc-link-lib=gcc");
        } else if !target.contains("musl") {
            println!("cargo:rustc-link-lib=dl");
            println!("cargo:rustc-link-lib=rt");
            println!("cargo:rustc-link-lib=pthread");
        }
    } else if target.contains("freebsd") {
        println!("cargo:rustc-link-lib=execinfo");
        println!("cargo:rustc-link-lib=pthread");
    } else if target.contains("dragonfly") || target.contains("bitrig") ||
              target.contains("netbsd") || target.contains("openbsd") {
        println!("cargo:rustc-link-lib=pthread");
    } else if target.contains("solaris") {
        println!("cargo:rustc-link-lib=socket");
        println!("cargo:rustc-link-lib=posix4");
        println!("cargo:rustc-link-lib=pthread");
        println!("cargo:rustc-link-lib=resolv");
    } else if target.contains("apple-darwin") {
        println!("cargo:rustc-link-lib=System");

        // res_init and friends require -lresolv on macOS/iOS.
        // See #41582 and http://blog.achernya.com/2013/03/os-x-has-silly-libsystem.html
        println!("cargo:rustc-link-lib=resolv");
    } else if target.contains("apple-ios") {
        println!("cargo:rustc-link-lib=System");
        println!("cargo:rustc-link-lib=objc");
        println!("cargo:rustc-link-lib=framework=Security");
        println!("cargo:rustc-link-lib=framework=Foundation");
        println!("cargo:rustc-link-lib=resolv");
    } else if target.contains("windows") {
        println!("cargo:rustc-link-lib=advapi32");
        println!("cargo:rustc-link-lib=ws2_32");
        println!("cargo:rustc-link-lib=userenv");
        println!("cargo:rustc-link-lib=shell32");
    } else if target.contains("fuchsia") {
        println!("cargo:rustc-link-lib=zircon");
        println!("cargo:rustc-link-lib=fdio");
    } else if target.contains("cloudabi") {
        if cfg!(feature = "backtrace") {
            println!("cargo:rustc-link-lib=unwind");
        }
        println!("cargo:rustc-link-lib=c");
        println!("cargo:rustc-link-lib=compiler_rt");
    }
}

fn build_libbacktrace(target: &str) -> Result<(), ()> {
    let native = native_lib_boilerplate("libbacktrace", "libbacktrace", "backtrace", "")?;

    let mut build = cc::Build::new();
    build
        .flag("-fvisibility=hidden")
        .include("../libbacktrace")
        .include(&native.out_dir)
        .out_dir(&native.out_dir)
        .warnings(false)
        .file("../libbacktrace/alloc.c")
        .file("../libbacktrace/backtrace.c")
        .file("../libbacktrace/dwarf.c")
        .file("../libbacktrace/fileline.c")
        .file("../libbacktrace/posix.c")
        .file("../libbacktrace/read.c")
        .file("../libbacktrace/sort.c")
        .file("../libbacktrace/state.c");

    let any_debug = env::var("RUSTC_DEBUGINFO").unwrap_or_default() == "true" ||
        env::var("RUSTC_DEBUGINFO_LINES").unwrap_or_default() == "true";
    build.debug(any_debug);

    if target.contains("darwin") {
        build.file("../libbacktrace/macho.c");
    } else if target.contains("windows") {
        build.file("../libbacktrace/pecoff.c");
    } else {
        build.file("../libbacktrace/elf.c");

        let pointer_width = env::var("CARGO_CFG_TARGET_POINTER_WIDTH").unwrap();
        if pointer_width == "64" {
            build.define("BACKTRACE_ELF_SIZE", "64");
        } else {
            build.define("BACKTRACE_ELF_SIZE", "32");
        }
    }

    File::create(native.out_dir.join("backtrace-supported.h")).unwrap();
    build.define("BACKTRACE_SUPPORTED", "1");
    build.define("BACKTRACE_USES_MALLOC", "1");
    build.define("BACKTRACE_SUPPORTS_THREADS", "0");
    build.define("BACKTRACE_SUPPORTS_DATA", "0");

    File::create(native.out_dir.join("config.h")).unwrap();
    if !target.contains("apple-ios") &&
       !target.contains("solaris") &&
       !target.contains("redox") &&
       !target.contains("android") &&
       !target.contains("haiku") {
        build.define("HAVE_DL_ITERATE_PHDR", "1");
    }
    build.define("_GNU_SOURCE", "1");
    build.define("_LARGE_FILES", "1");

    build.compile("backtrace");
    Ok(())
}
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条件编译

所有的条件编译都由通过cfg配置实现，cfg支持any、all、not等逻辑谓词组合。

基本用法

在Cargo.toml中添加 [features] 段，然后列举需要组合的feature名，大体上相当于 gcc -条件1 -条件2 -条件3 ... 。

[features]
default = []
metal = ["gfx-backend-metal"]
vulkan = ["gfx-backend-vulkan"]
dx12 = ["gfx-backend-dx12"]
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mod级别条件编译

实现示例，参考 gl_generator.rs

#[cfg(feature = "unstable_generator_utils")]
pub mod generators;
#[cfg(not(feature = "unstable_generator_utils"))]
mod generators;
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编译特定CPU架构

指定target_arch + CPU架构名称字符串，如 #[cfg(target_arch= "x86")] ， #[cfg(any(target_arch = "arm", target_arch = "x86"))] 。

参考 libstd/os/android/raw.rs

#[cfg(any(target_arch = "arm", target_arch = "x86"))]
mod arch {
    use os::raw::{c_uint, c_uchar, c_ulonglong, c_longlong, c_ulong};
    use os::unix::raw::{uid_t, gid_t};

    #[stable(feature = "raw_ext", since = "1.1.0")]
    pub type dev_t = u64;
    #[stable(feature = "raw_ext", since = "1.1.0")]
    pub type mode_t = u32;

    #[stable(feature = "raw_ext", since = "1.1.0")]
    pub type blkcnt_t = u64;
    #[stable(feature = "raw_ext", since = "1.1.0")]
    pub type blksize_t = u64;
    #[stable(feature = "raw_ext", since = "1.1.0")]
    pub type ino_t = u64;
    #[stable(feature = "raw_ext", since = "1.1.0")]
    pub type nlink_t = u64;
    #[stable(feature = "raw_ext", since = "1.1.0")]
    pub type off_t = u64;
    #[stable(feature = "raw_ext", since = "1.1.0")]
    pub type time_t = i64;
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#[doc(include = "os/raw/char.md")]
#[cfg(any(all(target_os = "linux", any(target_arch = "aarch64",
                                       target_arch = "arm",
                                       target_arch = "powerpc",
                                       target_arch = "powerpc64",
                                       target_arch = "s390x")),
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[target.'cfg(any(target_os = "macos", all(target_os = "ios", target_arch = "aarch64")))'.dependencies.gfx-backend-metal]
git = "https://github.com/gfx-rs/gfx"
version = "0.1"
optional = true

[target.'cfg(target_os = "android")'.dependencies.gfx-backend-vulkan]
git = "https://github.com/gfx-rs/gfx"
version = "0.1"
optional = true

#[target.'cfg(windows)'.dependencies.gfx-backend-dx12]
#git = "https://github.com/gfx-rs/gfx"
#version = "0.1"
#optional = true
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编译成指定类型二进制包(.a/.so/.r)

目前还没找到支持编译出macOS/iOS支持的 .framework 办法。

在Cargo.toml中添加 [lib] 段，

• name 表示输出的库名， 最终输出文件名为lib+name.a或lib+name.so ， 比如libportability.so 。
• crate-type 表示输出的二进制包类型，比如
  • staticlib = .a iOS只认Rust输出.a，Android可以.a和.so，配置成 ["staticlib", "cdylib"] 在用cargo-lipo时会出警告不支持 cdylib ，忽略即可。
  • cdylib = .so
  • rlib = 给Rust用的静态库
  • dylib = 给Rust用的动态库
• path 表示库项目的入口文件，通常是src/lib.rs，如果改动了这一位置，可通过path = 新位置实现，比如：

[lib]
name = "portability"
crate-type = ["staticlib", "cdylib"]
path = "src/ios/lib.rs"
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SDK开发的“售后服务”

提供.a/.so给业务团队，这一过程可能会有人为失误导致大家对接失败，下面介绍些我们使用的小技巧。

读取.a静态库的iOS版本

在macOS terminal执行如下命令，用 / 查找 VERSION 。

otool -lv xyz.a | less
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参考： check-ios-deployment-target-of-a-static-library

nm查看导出符号

有时编码疏忽导致没给需要导出的C接口添加 #[no_mangle] 和 extern 等修饰，或者使用了不合理的优化attribute导致符号被优化掉，此时业务链接我们的库就会失败，因此，交付二进制包前用nm确认符号表是合格的工程师习惯。以下为示例代码。

nm -D ./target/release/libportability.so  | grep fun_call_exported_to_c
0000000000003190 T fun_call_exported_to_c
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