How do `RawSyscall()` and `Syscall()` in Go differ in terms of runtime interaction and usage scenarios?

Understanding Syscall.RawSyscall() and Syscall.Syscall() in Go

Introduction

Syscall package in Go provides low-level interaction with the operating system. Two notable functions in this package are RawSyscall() and Syscall(), which offer different ways to execute system calls.

RawSyscall

The RawSyscall() function performs a raw system call by taking the following parameters:

• trap: The system call number to be executed.
• a1, a2, a3: Arguments for the system call.

It returns the following:

• r1, r2: Results from the system call.
• err: An error value indicating any errors encountered.

Assembly Code

For Darwin/amd64 systems, the assembly code for RawSyscall() can be found at http://golang.org/src/pkg/syscall/asm_darwin_amd64.s. Lines 61-80 implement the system call:

• Line 71 jumps to label ok1 if the system call was successful.
• Line 76 is the ok1 label, where the return values (AX and DX) are stored in the return address.

Zsyscall

Zsyscall is a submodule of Syscall that provides high-performance implementations of certain system calls. The functions in Zsyscall are named in a such a way that they start with the letter a, such as Zsyscall.AioRead(). They follow the same interface as normal Syscall functions.

Differences Between Syscall and RawSyscall

While both Syscall and RawSyscall can execute system calls, there are subtle differences:

• Syscall: Notifies the runtime when it starts and ends a system call.
• RawSyscall: Does not notify the runtime, allowing it to block indefinitely while executing the system call.

Usage Scenarios

• Syscall: Use for run-of-the-mill system calls where the runtime should be aware of the call.
• RawSyscall: Use for situations where the runtime should not be notified and the system call is known to be blocking.

Implementing Custom System Call Functions

To implement custom system call functions:

1. Identify the system call number for the desired operation.
2. Write assembly code that follows the system call interface (similar to RawSyscall).
3. Define a Go function that wraps the assembly code and provides an easy-to-use interface.

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