From 3d57d3e176cce2445ef945e7e318bcdd1eac19db Mon Sep 17 00:00:00 2001
From: tamnd <1218621+tamnd@users.noreply.github.com>
Date: Wed, 9 Sep 2026 14:15:49 +0700
Subject: [PATCH] Define the stdio family for JIT'd code on Windows (#326)

A Mojo program that formats a float failed with "Symbols not found: [ snprintf ]". There is no snprintf on a Windows machine to find. The UCRT headers declare it inline over __stdio_common_vsprintf, so every program gets its own copy while it is being compiled and ucrtbase.dll exports nothing by that name. JIT'd code never had that moment, so it emits a plain call and there is nothing to bind it to. Nine names in the printf and scanf family are like this.

Six more resolve, but through ntdll, which carries its own copies from the old NT C runtime. Those are not the UCRT functions, they differ in floating point conversion, and swprintf does not even take a buffer size there. That half was the worse one, because it went unnoticed.

This defines all fifteen as absolute symbols in the platform stdlib, pointing at the copies already linked into mojo.exe, which links the UCRT statically. Defining them settles both halves at once, since a generator is only ever asked about a name the dylib does not already have.
---
 .../lib/ExecutionEngine/ExecutionEngine.cpp   | 55 +++++++++++++++++++
 1 file changed, 55 insertions(+)

diff --git a/overlay/Mojo/lib/ExecutionEngine/ExecutionEngine.cpp b/overlay/Mojo/lib/ExecutionEngine/ExecutionEngine.cpp
index 329d480..96878d1 100644
--- a/overlay/Mojo/lib/ExecutionEngine/ExecutionEngine.cpp
+++ b/overlay/Mojo/lib/ExecutionEngine/ExecutionEngine.cpp
@@ -34,6 +34,9 @@
 #include "llvm/Target/TargetMachine.h"
 #include "llvm/TargetParser/Host.h"
 
+#include <stdio.h>
+#include <wchar.h>
+
 using namespace M;
 using namespace KGEN;
 
@@ -92,6 +95,58 @@ static ErrorOrSuccess setupPlatform(llvm::orc::JITDylib &platformStdlib,
                      library);
       platformStdlib.addGenerator(std::move(*systemGenerator));
     }
+
+    // Half of the printf and scanf family is not a function anywhere on a
+    // Windows machine. The UCRT headers declare those names inline over
+    // `__stdio_common_*`, so an ordinary program gets its own copy while it is
+    // being compiled and ucrtbase exports nothing to match. JIT'd code has no
+    // such moment and emits a plain call, which then has nothing to bind to.
+    //
+    // The other half does resolve, through ntdll, which carries its own copies
+    // from the old NT C runtime. Those are not the UCRT functions and they
+    // differ where it is easiest to notice, in floating point conversions, so
+    // that half is the worse one. Defining these names outright settles both
+    // cases, because a generator is only ever asked about a name the dylib does
+    // not already have.
+    //
+    // A Windows build of mojo links the UCRT statically, so the addresses below
+    // are our own copies, which are the ones that behave correctly. They are
+    // also the reason for the preprocessor check: these are names out of a
+    // Windows C library, and some of them are only spelled this way there.
+#ifdef _WIN32
+    llvm::orc::SymbolMap stdio;
+    auto defineStdio = [&](StringRef name, auto *function) {
+      stdio[session.intern(name)] = {
+          llvm::orc::ExecutorAddr::fromPtr(function),
+          llvm::JITSymbolFlags::Exported | llvm::JITSymbolFlags::Callable};
+    };
+    defineStdio("printf", &printf);
+    defineStdio("vprintf", &vprintf);
+    defineStdio("fprintf", &fprintf);
+    defineStdio("vfprintf", &vfprintf);
+    defineStdio("sprintf", &sprintf);
+    defineStdio("vsprintf", &vsprintf);
+    defineStdio("snprintf", &snprintf);
+    defineStdio("vsnprintf", &vsnprintf);
+    defineStdio("scanf", &scanf);
+    defineStdio("fscanf", &fscanf);
+    defineStdio("sscanf", &sscanf);
+    defineStdio("_snprintf", &_snprintf);
+    defineStdio("_vsnprintf", &_vsnprintf);
+    // The two wide ones need spelling out. In C++ the UCRT headers give them a
+    // template overload that deduces the size of an array argument, so the bare
+    // name is an overload set rather than one address.
+    defineStdio("swprintf",
+                static_cast<int (*)(wchar_t *, size_t, const wchar_t *, ...)>(
+                    &swprintf));
+    defineStdio("vswprintf",
+                static_cast<int (*)(wchar_t *, size_t, const wchar_t *,
+                                    va_list)>(&vswprintf));
+    if (auto errOr = toModularErrorOr(platformStdlib.define(
+            llvm::orc::absoluteSymbols(std::move(stdio))));
+        failed(errOr))
+      return errOr.takeError();
+#endif
   }
 
   return success();