From 3810389c9452c983815251c7af9121d663c82460 Mon Sep 17 00:00:00 2001
From: Maz <mazenshaar@hotmail.com>
Date: Tue, 20 Oct 2026 16:36:32 +0400
Subject: [PATCH] fix(libc): preserve zero in alternate wide octal formatting

---
 core/libs/prx/libc/src/FormattingWide.cpp |  2 ++
 core/libs/tests/GuestWideFormatting.cpp   | 26 +++++++++++++++++++++++
 2 files changed, 28 insertions(+)

diff --git a/core/libs/prx/libc/src/FormattingWide.cpp b/core/libs/prx/libc/src/FormattingWide.cpp
index 1526f39fefe..594cf50f956 100644
--- a/core/libs/prx/libc/src/FormattingWide.cpp
+++ b/core/libs/prx/libc/src/FormattingWide.cpp
@@ -167,6 +167,8 @@ std::u16string FormatWide(const char16_t* format, VaList* source, bool secure =
                 if (length == "h") value = static_cast<unsigned short>(value);
                 if (length == "hh") value = static_cast<unsigned char>(value);
             } else value = args.Next<unsigned long long>();
+            if (conversion == u'o' && value == 0 && precision == 0 && spec.find('#') != std::string::npos)
+                spec = spec.substr(0, spec.find('.')) + ".1";
             AppendNumber(out, spec + "ll" + static_cast<char>(conversion), value);
         } else if (In(conversion, "aAeEfFgG")) {
             if (length == "L") AppendNumber(out, spec + "L" + static_cast<char>(conversion), args.Next<long double>());
diff --git a/core/libs/tests/GuestWideFormatting.cpp b/core/libs/tests/GuestWideFormatting.cpp
index 818f401ffd1..8ebfa2753ee 100644
--- a/core/libs/tests/GuestWideFormatting.cpp
+++ b/core/libs/tests/GuestWideFormatting.cpp
@@ -276,7 +276,33 @@ static void CheckWcsftime() {
     Require(WcsftimeThrows(buffer, u"%Y", nullptr), "wcsftime null time");
 }
 
+static void CheckAlternateOctalZero() {
+    struct Case { const char16_t* format; const char16_t* expected; unsigned value; };
+    for (const auto& test : {
+        Case{u"%#.0o", u"0", 0}, Case{u"%#.o", u"0", 0}, Case{u"%#.00o", u"0", 0},
+        Case{u"[%#5.0o]", u"[    0]", 0}, Case{u"[%-#5.0o]", u"[0    ]", 0},
+        Case{u"[%#05.0o]", u"[    0]", 0}, Case{u"%.0o", u"", 0}, Case{u"%#.0x", u"", 0},
+        Case{u"%#.0o", u"01", 1}, Case{u"%#.1o", u"0", 0}, Case{u"%#.3o", u"000", 0},
+        Case{u"%#.0hho", u"0", 256}, Case{u"%#.0ho", u"0", 65536}
+    }) {
+        char16_t buffer[32];
+        const auto expected = std::u16string(test.expected);
+        Require(swprintf_nid_postfix(buffer, 32, test.format, test.value) == static_cast<int>(expected.size()) && buffer == expected, "swprintf alternate octal zero");
+        Require(Format(buffer, 32, test.format, test.value) == static_cast<int>(expected.size()) && buffer == expected, "vswprintf alternate octal zero");
+        Require(snwprintf_s_nid_postfix(buffer, 32, test.format, test.value) == static_cast<int>(expected.size()) && buffer == expected, "snwprintf_s alternate octal zero");
+    }
+    char16_t buffer[16];
+    Require(swprintf_nid_postfix(buffer, 16, u"%#.*o", 0, 0u) == 1 && buffer == std::u16string(u"0"), "octal zero dynamic precision");
+    Require(swprintf_nid_postfix(buffer, 16, u"%#.*o", -1, 0u) == 1 && buffer == std::u16string(u"0"), "octal zero omitted precision");
+    Require(swprintf_nid_postfix(buffer, 16, u"%2$#.*1$o", 0, 0u) == 1 && buffer == std::u16string(u"0"), "octal zero positional precision");
+    Require(swprintf_nid_postfix(buffer, 16, u"%#.0llo", 0ull) == 1 && buffer == std::u16string(u"0"), "octal zero long long");
+    buffer[0] = u'x';
+    buffer[1] = u'y';
+    Require(snwprintf_s_nid_postfix(buffer, 1, u"%#.0o", 0u) == 1 && buffer[0] == 0 && buffer[1] == u'y', "octal zero bounded output");
+}
+
 int main() {
+    CheckAlternateOctalZero();
     CheckBounded();
     CheckPositional();
     CheckPointer();