From 215d838fa7fb687e1aee6cb3043c740b593be0af Mon Sep 17 00:00:00 2001
From: Virgil <virgil@lethean.io>
Date: Fri, 3 Apr 2026 19:16:22 +0000
Subject: [PATCH] feat(cgo): implement errno mapping helpers

Co-Authored-By: Virgil <virgil@lethean.io>
---
 call_test.go         | 38 ++++++++++++++++++++++++++++++++++++++
 call_test_support.go | 12 ++++++++++++
 string_conversion.go | 24 ++++++++++++++++++++++++
 3 files changed, 74 insertions(+)

diff --git a/call_test.go b/call_test.go
index 28d386f..1ad9601 100644
--- a/call_test.go
+++ b/call_test.go
@@ -1,6 +1,8 @@
 package cgo
 
 import (
+	"errors"
+	"syscall"
 	"testing"
 )
 
@@ -49,3 +51,39 @@ func TestCallRejectsUnsupportedInputs(t *testing.T) {
 		_ = Call(callFailureFunction(), 1, 2, 3, 4)
 	})
 }
+
+func TestErrnoMapping(t *testing.T) {
+	t.Parallel()
+
+	if err := Errno(0); err != nil {
+		t.Fatalf("expected nil error for zero, got %v", err)
+	}
+
+	if err := Errno(2); err == nil {
+		t.Fatal("expected non-nil error for non-zero errno")
+	} else if !errors.Is(err, syscall.Errno(2)) {
+		t.Fatalf("expected error type %v, got %T %v", syscall.Errno(2), err, err)
+	}
+}
+
+func TestWithErrnoReturnsBothResultAndError(t *testing.T) {
+	t.Parallel()
+
+	result, err := callWithErrnoZero()
+	if result != 0 {
+		t.Fatalf("expected result 0, got %d", result)
+	}
+	if err != nil {
+		t.Fatalf("expected nil error, got %v", err)
+	}
+
+	result, err = callWithErrnoFailure()
+	if result != 2 {
+		t.Fatalf("expected result 2, got %d", result)
+	}
+	if err == nil {
+		t.Fatal("expected error, got nil")
+	} else if !errors.Is(err, syscall.Errno(2)) {
+		t.Fatalf("expected errno error, got %v", err)
+	}
+}
diff --git a/call_test_support.go b/call_test_support.go
index 648df20..bdb8615 100644
--- a/call_test_support.go
+++ b/call_test_support.go
@@ -36,3 +36,15 @@ func callFailureFunction() unsafe.Pointer {
 	function := C.call_failure_ptr()
 	return *(*unsafe.Pointer)(unsafe.Pointer(&function))
 }
+
+func callWithErrnoZero() (int, error) {
+	return WithErrno(func() C.int {
+		return 0
+	})
+}
+
+func callWithErrnoFailure() (int, error) {
+	return WithErrno(func() C.int {
+		return 2
+	})
+}
diff --git a/string_conversion.go b/string_conversion.go
index 90c87f6..401364d 100644
--- a/string_conversion.go
+++ b/string_conversion.go
@@ -30,6 +30,7 @@ import "C"
 import (
 	"fmt"
 	"strconv"
+	"syscall"
 	"unsafe"
 )
 
@@ -196,3 +197,26 @@ func Free(ptr unsafe.Pointer) {
 	}
 	C.free(ptr)
 }
+
+// Errno converts a C error number to a Go error.
+//
+//	rc := cgo.Errno(-2)
+func Errno(rc C.int) error {
+	if rc == 0 {
+		return nil
+	}
+	return syscall.Errno(rc)
+}
+
+// WithErrno runs a function that returns C.int and maps the result to Go error.
+//
+//	result, err := cgo.WithErrno(func() C.int {
+//		return C.my_function()
+//	})
+func WithErrno(fn func() C.int) (int, error) {
+	result := fn()
+	if err := Errno(result); err != nil {
+		return int(result), err
+	}
+	return int(result), nil
+}
-- 
2.45.3