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Rust FFI programming - libc crate

2022-06-28 06:40:00 【51CTO】

Previous warning ： If the Unix Environment system programming without basic knowledge , This article will be in the clouds .

We're doing it Rust When developing and compiling , Often in dependency list , notice libc This crate The figure of . We don't usually rely directly on this crate, But dependence （ The dependence of ……） You might use this crate. in general , It is Rust One of the most basic and bottom layers in ecology crate 了 .

libc What is it?

libc It is the original system library of each platform FFI binding . Its code address is at ：https://github.com/rust-lang/libc. You can see , This is a Rust A library officially maintained .

libc Provided with Rust The most basic system on each supported platform C All necessary facilities for dealing with the warehouse . It exports the type of underlying platform 、 Functions and constants .

All content is placed directly in libc Under this namespace , No more modules . therefore , You can use ​​libc::foo​​ Access any exported content in this library in this form .

It can work with std In combination with , It can also be in no_std Environmental use .

libc Import of

In the project ​​Cargo.toml​​ Add the following configuration , You can import libc 了 .

      
      

       
       [dependencies]
       
       
libc = "0.2"
      
      

      
      
    
       
       
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libc Content classification of

libc The underlying layer will be exported C These things from the library ：

  • C type , such as typedefs, Primitive types , enumeration , Structure, etc
  • C Constant , For example, use ​​#define​​ Those constants defined by the instruction
  • C Static variables
  • C function （ Export by function signature defined in their header file ）
  • C macro , stay Rust Will be implemented as ​​#[inline]​​ function

in addition ,libc All exported in C struct It's all done ​​Copy​​ and ​​Clone​​ trait.

ok , be familiar with C Classmate , It should have been known ,C The interface of , It's just that . Now? libc It's all led out .

What is the result of the export ？ Open it directly ​ ​https://docs.rs/libc/0.2.69/libc/index.html ​​ see , There will be a long web page in front of you . Yes

  • Structs   Corresponding C Symbols in
  • Enums   Corresponding C Enumeration in
  • Constants   Corresponding C Constant in
  • Functions   Corresponding C Function interface in
  • Type Definitions   Corresponding C Medium typedef Defined symbols

These symbols , Probably 99% No one can guarantee that they used it 20% above , Even many students who focus on upper level development have never seen these names .

This is a great set of things , It is the essence of the system that has been accumulated over the years in the history of computer engineering. . The essence of this system is called. Unix Environment programming . This system 《UNIX Environment advanced programming （ The first 3 edition ）》 There is an authoritative explanation in this book .

The essence of this set of things lies in its essence. , It's not just a simple list of symbols , It contains a set of exquisite mechanism to drive . Yes , It's a set of mechanisms . This mechanism is composed of several different parts , There is a very clear distinction between these parts （Unix Of KISS principle ）, But in terms of design concept , It's the same smell again . therefore , This set of things , We call it engineering 、 technology 、 philosophy 、 Even art .

This set of things is modern IT Industry , The cornerstone of the Internet .

libc The boundaries of

be familiar with linux The students of system development all know ,linux The system itself has a libc library , It is the basic library of almost all applications . Basically linux Next C All code written in language must be linked to this library to run .

and Rust Of libc crate, Not exactly equivalent to C Of libc Encapsulation of Libraries . The differences are as follows ：

  • Linux （ And others unix-like platform ） Next , Exported is libc, libm, librt, libdl, libutil and libpthread Symbols of these libraries .
  • OSX Next , Exported is libsystem_c, libsystem_m, libsystem_pthread, libsystem_malloc and libdyld Symbols of these libraries .
  • Windows Next , Exported is VS CRT（VS C RunTime VS C Runtime library ） Symbols in . But these symbols , Symbols compared to the first two platforms , Much less in quantity . therefore , You can say that directly ,Rust libc crate stay Windows Limited functionality on the platform . stay Windows On the platform , It is recommended to use ​​winapi​​ This crate Development .


give an example ： Use libc Create child process

That's amazing , Let's meet libc What does Lushan look like . below , Let's use an example —— Create a subprocess —— To show libc Usage of , As well as Rust Different thread operations in standard library .

Rust There is no facility to create child processes in the standard library , But you can create a child thread . As a comparative demonstration , Let's create a new thread ：


      
      

       
       use std::thread;
       
       

       
       

       
       
fn main() {
       
       
    let child = thread::spawn(move || {
       
       
        println!("hello, I am a new rust thread!");
       
       
    });
       
       

       
       

       
       
    let res = child.join();
       
       
    println!("{:?}", res);
       
       

       
       

       
       
    println!("Hello, I am main thread!");
       
       
}
      
      

      
      
    
       
       
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The above code will be output ：

      
      

       
       hello, I am a new rust thread!
       
       
Ok(())
       
       
Hello, I am main thread!
      
      

      
      
    
       
       
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Let's see libc How to create a subprocess ：

      
      

       
       fn main() {
       
       
    unsafe {
       
       
        let pid = libc::fork();
       
       
        if pid > 0 {
       
       
            println!("Hello, I am parent thread: {}", libc::getpid());
       
       
        }
       
       
        else if pid == 0 {
       
       
            println!("Hello, I am child thread: {}", libc::getpid());
       
       
            println!("My parent thread: {}", libc::getppid());
       
       
        }
       
       
        else {
       
       
            println!("Fork creation failed!");
       
       
        }
       
       
    }
       
       
}
      
      

      
      
    
       
       
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This code will have output similar to the following ：


      
      

       
       Hello, I am parent thread: 5722
       
       
Hello, I am child thread: 5724
       
       
My parent thread: 5722
      
      

      
      
    
       
       
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Specific process id Numbers , Every run can change .

A simple comparison of two programs , You can find ：

  1. libc All function calls for , all must In the ​​unsafe​​ In block . Because all its calls are unsafe Of ;
  2. std Thread operation encapsulation of , To use , image .libc Process operations for , And C Language system programming is the same , It's just another set of ideas and programming style ;
  3. std The thread operation of is simple , But there is also a lack of finer particle size control . and libc Operations that can be performed on a process （ And subsequent function expansion of subprocesses , Signal management in parent process, etc ）, Complete control , More flexible , Powerful ;
  4. std Process cannot be implemented by itself fork The function of .

Example address of the above code ：​ ​https://github.com/daogangtang/learn-rust/tree/master/07libctest​​


Which things are Rust std Can't do it libc What can be done ？

Almost everything about underlying programming （ Of course, this sentence is not rigorous ）.

Just a few examples ：​​dup2​​ Do you have a standard library ？​​openpty​​ Do you have a standard library ？​​ioctl​​ Do you have a standard library ？

ioctl No, , That's with the bottom floor say byebye La （ And then isolated from serious embedded development ）. Of course , You can say , Then I'll take it Rust Write your own operating system . Right? , Do you use Rust Write operating system , It doesn't work std ah .

It should be said , Use libc, class Unix All system programming on the platform , Previously, only by C Work done , It can be used now Rust Here we go . On this level ,C What can be done ,Rust Can do it .

adopt libc This floor ,Rust Breaking into the field of system programming .

Probably , Some students have to explain again , It's just a library , It's no big deal .Python There are also packages for operating system base Libraries ,Python The same can be done for system development . This is not enough to prove Rust Is a system programming language ,Rust There is no difference at this point .

In fact, it only needs one sentence to answer this question ： Because of me Rust The package of is zero cost （ Zero cost ） Of .

Yes, It's that simple . Zero cost abstraction gives Rust Ability of system programming .


libc And std::os::*::raw The relationship between ？

Careful students will find out , In the standard library os Under module , There's something about libc Duplication .

page ​ ​https://doc.rust-lang.org/std/os/raw/index.html​​ Contains ​​c_char, c_double, c_float, c_int, c_long, c_longlong, c_schar, c_short, c_uchar, c_uint, c_ulong, c_ulonglong, c_ushort​​.

and libc in , For these , And redefined a （ such as ：https://docs.rs/libc/0.2.69/libc/type.c_char.html）. Why? ？

std::os::raw These definitions in , Can be used with some simple C Code to interact , For example, there is no system call C Code . This is the time , No need to introduce it again libc The library .

Once a system call is generated or Unix Environment programming , Then we have to introduce libc Library to operate .

std Here are some more  std::os::*::raw Module , These modules are now Deprecated 了 （ such as ：https://doc.rust-lang.org/std/os/unix/raw/index.html）. Clearly noted in the document ：


      
      

       
       Deprecated since 1.8.0:
       
       

       
       

       
       
these type aliases are no longer supported by the standard library, the libc crate on crates.io should be used instead for the correct definitions
      
      

      
      
    
       
       
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in other words , These things , Go to libc Middle search , use libc To implement these functions .


summary

We should be glad ,Rust Humanized and convenient programming mode provided by standard library .

meanwhile , We should be glad again ,Rust And C Close consanguinity of , Let us Rustaceans Easy to use with little performance loss C The way and thinking of the lowest level system programming .

This little fortune （ possibility ）, Not everyone can have it .

I want to master Rust And feel happy .


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