What\'s the Difference Between `Syscall.RawSyscall()` and `Syscall.Syscall()` in Go?

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

For those new to Go's syscall package, the Syscall.RawSyscall() and Syscall.Syscall() functions can be daunting. Here's a detailed overview to clarify their nuances:

Syscall.RawSyscall()

• Parameters:

  • trap: Numeric code of the system call to execute
  • a1, a2, a3: First three arguments passed to the system call

• Return Values:

  • r1, r2: Results of the system call
  • err: Error code if the system call failed

Assembly Code Implementation:

For Darwin/amd64 systems, the assembly code for Syscall.RawSyscall() can be found here: https://golang.org/src/pkg/syscall/asm_darwin_amd64.s?h=RawSyscall

• Lines 61-80: This section sets up the arguments for the system call, executes it, and handles error handling.
• Line 76: The ok1 label indicates a successful system call, where the results are returned.

Syscall.Syscall()

Syscall.Syscall() differs from Syscall.RawSyscall() only by calling runtime.entersyscall() and runtime.exitsyscall() functions before and after executing the system call. This allows the Go runtime to track and control the goroutine's use of system calls.

Usage:

Syscall.Syscall() should typically be used instead of Syscall.RawSyscall() for most purposes. Syscall.Syscall() handles Goroutine context switching, enabling preemption and multitasking. Only use Syscall.RawSyscall() if you have a specific need for bypassing the runtime context switching mechanism.

Example:

To write your own syscall function, one approach is:

1. Understand the details of the system call you want to use from the operating system documentation.
2. Write an assembly function that follows the calling conventions for the specific operating system and architecture.
3. Convert the system call number to a uintptr and call Syscall.RawSyscall() with appropriate parameters.

This example, however, requires intimate knowledge of assembly programming and system-level details, so it's recommended to use Syscall.Syscall() whenever possible.

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