## What are the differences between Syscall and RawSyscall in Go, and when should each be used?

Details of Syscall.RawSyscall() and Syscall.Syscall() in Go

Syscall.RawSyscall() and Syscall.Syscall() are functions in the Go syscall package that provide direct access to the operating system's system calls. They allow developers to interact with the operating system at a low level, enabling the creation of custom system calls or interfacing with operating system-specific features.

Parameters and Return Values

Syscall.RawSyscall() takes the following parameters:

• trap: The system call number.
• a1, a2, a3: Additional arguments passed to the system call.
• r1, r2: Return values from the system call.

Syscall.Syscall() takes similar parameters but also includes two additional parameters:

• err: An error code returned by the system call.

Assembly Code in Syscall.RawSyscall()

The assembly code for Syscall.RawSyscall() on Darwin/amd64 (located in syscall/asm_darwin_amd64.s) can be summarized as follows:

• It creates a system call frame by pushing arguments onto the stack (lines 61-65).
• It adds a constant to the AX register, which is the system call number (line 69).
• It performs a system call (line 70).
• Based on the result of the system call, the program jumps to either line 71 or 76 (label ok1).

Meaning of `ok1:

Label ok1 (line 76) is used to handle successful system call execution. If the system call succeeds, the assembly code jumps to this label and returns the results in registers AX (r1) and DX (r2). Otherwise, it returns -1 in register AX (r1) and 0 in register DX (r2) (lines 72-74).

Zsyscalls

The package syscall/zsyscall_darwin_amd64.go contains "zipped" system calls that do not perform arguments validation. They are typically used in netpoll functions to improve performance.

Syscall vs. RawSyscall

The main difference between Syscall and RawSyscall is that Syscall notifies the Go runtime system that a blocking system call is about to be executed (lines 14, 28, and 34 of syscall.go). This allows the runtime to yield the CPU to other goroutines while the system call is executing. In contrast, RawSyscall does not notify the runtime, meaning that the program will block until the system call completes.

Usage

Syscall and RawSyscall can be used to implement custom system calls or interact with operating system-specific features that are not supported by the standard Go syscall package. For example, you might use Syscall to implement a new file system or access a hardware device directly.

To use Syscall or RawSyscall, you need to first determine the system call number and arguments. You can find this information in the documentation for the specific operating system and architecture you are targeting. Once you have this information, you can use the syscall package to create a system call.

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