From 26265e29f4599660a4bfcbc196fc98110bdac6d0 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Alex=20R=C3=B8nne=20Petersen?= <alex@alexrp.com>
Date: Tue, 13 Oct 2026 20:06:22 +0200
Subject: [PATCH] std.c, libzigc: fix ffs() family signatures and
 implementation bugs

---
 lib/c/strings.zig  | 8 ++++----
 lib/std/c.zig      | 4 ++--
 test/c/strings.zig | 8 ++------
 3 files changed, 8 insertions(+), 12 deletions(-)

diff --git a/lib/c/strings.zig b/lib/c/strings.zig
index 111da9ad0f04eff48c890c6b33b5481c733a8aae..11643e066a12da636297f4a1e63c8f7fd48c3365 100644
--- a/lib/c/strings.zig
+++ b/lib/c/strings.zig
@@ -44,19 +44,19 @@ fn rindex(str: [*:0]const c_char, value: c_int) callconv(.c) ?[*:0]c_char {
     return @constCast(str[std.mem.findScalarLast(u8, std.mem.span(@as([*:0]const u8, @ptrCast(str))), @truncate(@as(c_uint, @bitCast(value)))) orelse return null ..]);
 }
 
-fn firstBitSet(comptime T: type, value: T) T {
-    return @bitSizeOf(T) - @clz(value);
+fn firstBitSet(comptime T: type, value: T) c_int {
+    return if (value == 0) 0 else @as(c_int, @ctz(value)) + 1;
 }
 
 fn ffs(i: c_int) callconv(.c) c_int {
     return firstBitSet(c_int, i);
 }
 
-fn ffsl(i: c_long) callconv(.c) c_long {
+fn ffsl(i: c_long) callconv(.c) c_int {
     return firstBitSet(c_long, i);
 }
 
-fn ffsll(i: c_longlong) callconv(.c) c_longlong {
+fn ffsll(i: c_longlong) callconv(.c) c_int {
     return firstBitSet(c_longlong, i);
 }
 
diff --git a/lib/std/c.zig b/lib/std/c.zig
index 3bffe16ea78947721e7c4d9d39280d2b0d6ce37e..69c29af84368b22df157932076f46198545e88f9 100644
--- a/lib/std/c.zig
+++ b/lib/std/c.zig
@@ -11579,8 +11579,8 @@ pub const wcsdup = switch (builtin.abi) {
 };
 
 pub extern "c" fn ffs(i: c_int) c_int;
-pub extern "c" fn ffsl(i: c_long) c_long;
-pub extern "c" fn ffsll(i: c_longlong) c_longlong;
+pub extern "c" fn ffsl(i: c_long) c_int;
+pub extern "c" fn ffsll(i: c_longlong) c_int;
 
 pub extern "c" fn erand48(xsubi: *[3]c_ushort) f64;
 pub extern "c" fn jrand48(xsubi: *[3]c_ushort) c_long;
diff --git a/test/c/strings.zig b/test/c/strings.zig
index baea1a8fe1f658dbd4c6a9491b4c14d57d22b842..c7f180d0540ac7960e156bb047ac05e7ab34a30d 100644
--- a/test/c/strings.zig
+++ b/test/c/strings.zig
@@ -26,9 +26,8 @@ fn testFfs(comptime T: type) !void {
     try testing.expectEqual(0, ffs(0));
 
     for (0..@bitSizeOf(T)) |i| {
-        const bit = @as(T, 1) << @intCast(i);
-
-        try testing.expectEqual(@as(T, @intCast(i + 1)), ffs(bit));
+        try testing.expectEqual(@as(c_int, @intCast(i + 1)), ffs(@as(T, 1) << @intCast(i)));
+        try testing.expectEqual(@as(c_int, @intCast(i + 1)), ffs(@as(T, -1) << @intCast(i)));
     }
 }
 
@@ -41,9 +40,6 @@ test "ffs" {
     if (builtin.target.os.tag == .netbsd) return; // no ffsl/ffsll until 11
 
     try testFfs(c_long);
-
-    if (@sizeOf(usize) == 4) return error.SkipZigTest; // TODO
-
     try testFfs(c_longlong);
 }
 
-- 
2.54.0