What is the correct way to connect and invoke assembly code using extern functions in Rust?

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The correct way to connect and invoke assembly code using extern functions in Rust involves the following steps:

1. Write the assembly code function in a separate file with extension ".S" using the appropriate syntax for the target architecture.

2. Create a header file that declares the input and output parameters of the assembly function.

3. In the Rust code, use the "extern" keyword to import the assembly function as an external function.

4. Use the unsafe keyword to indicate that the function is not guaranteed to be safe.

5. Call the assembly function using the extern function name and passing the appropriate arguments.

Here is a sample code demonstrating how to connect and invoke an assembly code function named "add_asm" in Rust:

Assembly code (add_asm.S):

.globl add_asm
add_asm:
mov r0, r0, lsl #1
bx lr

Header file (add_asm.h):

#ifndef ADD_ASM_H
#define ADD_ASM_H
int add_asm(int number);
#endif

Rust code:

extern {
    fn add_asm(number: i32) -> i32;
}

unsafe {
    let result = add_asm(5);
    println!("Result: {}", result);
}

In this example, we use the "extern" keyword to import the "addasm" function as an external function, and define its input parameter and output types. We also use the "unsafe" keyword to indicate that the function may not be safe. Finally, we call the "addasm" function passing the integer value 5 as the argument, and print the result.