Syscall Wrapper Refactoring#

Purpose#

LLVM-libc is transitioning to a centralized system for Linux syscalls. The goal is to move all direct syscall_impl calls into a dedicated directory: src/__support/OSUtil/linux/syscall_wrappers/.

This refactor provides several benefits:

  • Reusability: Allows multiple entrypoints to share a single syscall implementation without public APIs depending on other public APIs.

  • Type Safety: Using ErrorOr<T> ensures that error conditions are handled explicitly.

  • Consistency: Standardizes the conversion of syscall return values into errno-compatible objects.

  • Maintainability: Centralizes platform-specific syscall logic, making it easier to audit and update.

The Pattern#

Each syscall should have its own header-only library in the syscall_wrappers directory. The wrapper function should return an ErrorOr<T>. Wrappers live in the linux_syscalls namespace to make call sites self-documenting and to clearly identify any leakage into OS-generic code.

Example Wrapper (src/__support/OSUtil/linux/syscall_wrappers/read.h):#

#include "hdr/types/ssize_t.h"
#include "src/__support/OSUtil/linux/syscall.h" // For syscall_impl
#include "src/__support/common.h"
#include "src/__support/error_or.h"
#include "src/__support/macros/config.h"
#include <sys/syscall.h> // For syscall numbers

namespace LIBC_NAMESPACE_DECL {
namespace linux_syscalls {

LIBC_INLINE ErrorOr<ssize_t> read(int fd, void *buf, size_t count) {
  ssize_t ret = syscall_impl<ssize_t>(SYS_read, fd, buf, count);
  if (ret < 0) {
    return Error(-static_cast<int>(ret));
  }
  return ret;
}

} // namespace linux_syscalls
} // namespace LIBC_NAMESPACE_DECL

How to Migrate#

  1. Cleanup Existing Implementation: If the syscall was previously implemented in OSUtil/linux/fcntl.cpp (or similar), remove the old implementation to replace it with the new wrapper.

  2. Create the Wrapper: Add a new header file in src/__support/OSUtil/linux/syscall_wrappers/.

  3. Update CMake: Add a add_header_library target for the new wrapper in src/__support/OSUtil/linux/syscall_wrappers/CMakeLists.txt.

  4. Refactor Entrypoints:

    • Include the new wrapper header (e.g., read.h).

    • Replace direct syscall_impl calls with linux_syscalls::<function_name>.

    • Update the entrypoint’s DEPENDS in CMakeLists.txt to include the new wrapper target.