## When Should I Use Syscall.RawSyscall() vs. Syscall.Syscall() in Go?

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

Background

The Go language utilizes system calls to interact with the underlying operating system. The syscall package provides the necessary functionality for performing system calls. Two significant functions within this package are Syscall.RawSyscall() and Syscall.Syscall().

Syscall.RawSyscall()

Purpose:
Provides a way to perform system calls directly, allowing for fine-grained control.

Parameters:

• trap: The system call number.
• a1, a2, a3: Arguments to pass to the system call.

Return Values:

• r1, r2: Return values from the system call.
• err: An error code indicating any issues encountered.

Syscall.Syscall()

Purpose:
Offers a higher-level interface for making system calls, handling some of the lower-level details on behalf of the caller.

Parameters:
Similar to Syscall.RawSyscall(), but also accepts a system call name as the first parameter.

Return Values:

• r1, r2: Return values from the system call.
• err: An error code indicating any issues encountered.

Differences between Syscall and RawSyscall

The primary difference between the two functions lies in their approach to handling system call execution. Syscall.Syscall() utilizes a helper function in the runtime library to notify the scheduler that it's entering a blocking operation, allowing the scheduler to yield control to another goroutine or thread. In contrast, Syscall.RawSyscall() does not perform this notification, meaning that the current goroutine will block until the system call completes.

Assembly Considerations

The implementation of Syscall.RawSyscall() in assembly for Darwin/amd64 provides insights into the underlying assembly instructions used to perform system calls.

61  TEXT ·RawSyscall(SB),7,
62      MOVQ    16(SP), DI
63      MOVQ    24(SP), SI
...
70      SYSCALL
...
76  ok1:
...
80      RET

• Lines 61-68: Load arguments (trap number, arguments) from stack into registers.
• Line 70: Execute the system call via the SYSCALL instruction.
• Line 76 (ok1): Conditional jump label used to handle successful system call execution.
• Lines 77-80: Return results (AX) to the caller if the system call succeeded.

zsyscall and Custom Syscall Functions

zsyscall refers to a subpackage within syscall that provides wrappers around system calls specific to the z/OS operating system.

As for creating your own syscall functions, you would typically use Syscall.Syscall() and specify the system call name and arguments as parameters. However, if you need more precise control or want to avoid runtime overhead, you can opt for Syscall.RawSyscall().

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