From a2ea36a51767659edf7feeee87eb237263e0dc4a Mon Sep 17 00:00:00 2001
From: Andrew Kelley <andrew@ziglang.org>
Date: Wed, 21 Jan 2026 18:11:16 -0800
Subject: [PATCH] zig libc: modify errno helper to eliminate `@intCast`

The vast majority of libc functions return `c_int` for the return value,
when setting errno. This utility function is for those cases.

Other cases can hand-roll the logic, or additional helpers can be added.
---
 lib/c/common.zig      | 24 ++++++++++++++----------
 lib/c/sys/utsname.zig |  2 +-
 2 files changed, 15 insertions(+), 11 deletions(-)

diff --git a/lib/c/common.zig b/lib/c/common.zig
index 09dc5a8c0cbf2fe18160a2310ab4d33ffbfa7545..8d2a79db54b33515ef75c98bf5eb1defe23cb19a 100644
--- a/lib/c/common.zig
+++ b/lib/c/common.zig
@@ -14,16 +14,20 @@ pub const visibility: std.builtin.SymbolVisibility = if (linkage != .internal)
 else
     .default;
 
-/// Checks whether the syscall has had an error, storing it in `std.c.errno` and returning -1.
-/// Otherwise returns the result.
-pub fn linuxErrno(r: usize) isize {
-    const linux = std.os.linux;
-
-    return switch (linux.errno(r)) {
-        .SUCCESS => @bitCast(r),
-        else => |err| blk: {
-            std.c._errno().* = @intFromEnum(err);
-            break :blk -1;
+/// Given a low-level syscall return value, sets errno and returns `-1`, or on
+/// success returns the result.
+pub fn errno(syscall_return_value: usize) c_int {
+    return switch (builtin.os.tag) {
+        .linux => {
+            const signed: isize = @bitCast(syscall_return_value);
+            const casted: c_int = @intCast(signed);
+            if (casted < 0) {
+                @branchHint(.unlikely);
+                std.c._errno().* = -casted;
+                return -1;
+            }
+            return casted;
         },
+        else => comptime unreachable,
     };
 }
diff --git a/lib/c/sys/utsname.zig b/lib/c/sys/utsname.zig
index 06f849ba6df4e8aa642e1361e6f9e5d57fff7367..6082c1b6cecf6c15c8454cf362fef9e49cc1e3e8 100644
--- a/lib/c/sys/utsname.zig
+++ b/lib/c/sys/utsname.zig
@@ -13,7 +13,7 @@ comptime {
 }
 
 fn unameLinux(uts: *std.os.linux.utsname) callconv(.c) c_int {
-    return @intCast(common.linuxErrno(std.os.linux.uname(uts)));
+    return common.errno(std.os.linux.uname(uts));
 }
 
 fn unameWasi(uts: *std.c.utsname) callconv(.c) c_int {
-- 
2.54.0