diff --git a/cpp2rust/compat/platform_flags.h b/cpp2rust/compat/platform_flags.h
index 09ceb590..0f563870 100644
--- a/cpp2rust/compat/platform_flags.h
+++ b/cpp2rust/compat/platform_flags.h
@@ -9,7 +9,7 @@
 static inline std::vector<std::string> getPlatformClangBeginFlags() {
   std::vector<std::string> flags = {
       "-resource-dir=" CLANG_RESOURCE_DIR,
-      "-I" COMPAT_INCLUDE_DIR,
+      "-isystem" COMPAT_INCLUDE_DIR,
       "-D_FORTIFY_SOURCE=0",
   };
 #ifdef MACOS_SDK_PATH
diff --git a/cpp2rust/converter/converter.cpp b/cpp2rust/converter/converter.cpp
index 703343d6..cab732b0 100644
--- a/cpp2rust/converter/converter.cpp
+++ b/cpp2rust/converter/converter.cpp
@@ -276,7 +276,7 @@ bool Converter::Convert(clang::Decl *decl) { return TraverseDecl(decl); }
 
 bool Converter::VisitTranslationUnitDecl(clang::TranslationUnitDecl *decl) {
   for (auto *child : decl->decls()) {
-    if (IsConvertibleDecl(child) &&
+    if (IsUserDefinedDecl(child) &&
         (IsInMainFile(child) || !decl_ids_.contains(GetID(child)))) {
       Convert(child);
     }
diff --git a/cpp2rust/converter/converter_lib.cpp b/cpp2rust/converter/converter_lib.cpp
index 4f60cf64..051aed2d 100644
--- a/cpp2rust/converter/converter_lib.cpp
+++ b/cpp2rust/converter/converter_lib.cpp
@@ -129,7 +129,7 @@ bool IsInMainFile(const clang::Decl *decl) {
   return src_mgr.isInMainFile(src_mgr.getExpansionLoc(loc));
 }
 
-bool IsConvertibleDecl(const clang::Decl *decl) {
+bool IsUserDefinedDecl(const clang::Decl *decl) {
   const auto &ctx = decl->getASTContext();
   const auto &src_mgr = ctx.getSourceManager();
   const auto src_loc = decl->getLocation();
@@ -138,6 +138,21 @@ bool IsConvertibleDecl(const clang::Decl *decl) {
          !src_mgr.isInSystemMacro(src_loc);
 }
 
+bool RefersToUserDefinedDecl(const clang::Expr *expr) {
+  expr = expr->IgnoreParenImpCasts();
+  const clang::Decl *decl = nullptr;
+  if (const auto *call = llvm::dyn_cast<clang::CallExpr>(expr)) {
+    decl = call->getDirectCallee();
+  } else if (const auto *ref = llvm::dyn_cast<clang::DeclRefExpr>(expr)) {
+    decl = ref->getDecl();
+  } else if (const auto *member = llvm::dyn_cast<clang::MemberExpr>(expr)) {
+    decl = member->getMemberDecl();
+  } else if (const auto *ctor = llvm::dyn_cast<clang::CXXConstructExpr>(expr)) {
+    decl = ctor->getConstructor();
+  }
+  return decl && IsUserDefinedDecl(decl);
+}
+
 bool IsUnsignedArithOp(const clang::BinaryOperator *expr) {
   clang::QualType lhs_type;
   clang::QualType rhs_type;
diff --git a/cpp2rust/converter/converter_lib.h b/cpp2rust/converter/converter_lib.h
index aa5d0c2b..c208ea1c 100644
--- a/cpp2rust/converter/converter_lib.h
+++ b/cpp2rust/converter/converter_lib.h
@@ -39,7 +39,9 @@ bool IsComparisonWithNullOp(const clang::BinaryOperator *expr);
 
 bool IsInMainFile(const clang::Decl *decl);
 
-bool IsConvertibleDecl(const clang::Decl *decl);
+bool IsUserDefinedDecl(const clang::Decl *decl);
+
+bool RefersToUserDefinedDecl(const clang::Expr *expr);
 
 bool IsUnsignedArithOp(const clang::BinaryOperator *expr);
 
diff --git a/cpp2rust/converter/mapper.cpp b/cpp2rust/converter/mapper.cpp
index e2b444f4..c86f903a 100644
--- a/cpp2rust/converter/mapper.cpp
+++ b/cpp2rust/converter/mapper.cpp
@@ -385,6 +385,9 @@ search(std::unordered_multimap<std::string, T> &map, const std::string &txt,
 }
 
 TranslationRule::ExprRule *search(const clang::Expr *expr) {
+  if (RefersToUserDefinedDecl(expr)) {
+    return nullptr;
+  }
   auto qualified_name = ToString(expr);
   auto [rule, subs] =
       search(exprs_, qualified_name, GetExprMapKey(qualified_name));
@@ -598,7 +601,8 @@ const TranslationRule::ExprRule *GetExprRule(const clang::Expr *expr) {
 
 std::string MapFunctionName(const clang::FunctionDecl *decl) {
   assert(decl);
-  if (exprs_.contains(GetExprMapKey(ToString(decl)))) {
+  if (!IsUserDefinedDecl(decl) &&
+      exprs_.contains(GetExprMapKey(ToString(decl)))) {
     return std::format("libcc2rs::{}_{}", decl->getNameAsString(),
                        model_ == Model::kRefCount ? "refcount" : "unsafe");
   }
diff --git a/tests/unit/out/refcount/user_defined_same_as_libc.rs b/tests/unit/out/refcount/user_defined_same_as_libc.rs
new file mode 100644
index 00000000..599f7bb3
--- /dev/null
+++ b/tests/unit/out/refcount/user_defined_same_as_libc.rs
@@ -0,0 +1,29 @@
+extern crate libcc2rs;
+use libcc2rs::*;
+use std::cell::RefCell;
+use std::collections::BTreeMap;
+use std::io::prelude::*;
+use std::io::{Read, Seek, Write};
+use std::os::fd::AsFd;
+use std::rc::{Rc, Weak};
+pub fn fopen_0(path: Ptr<u8>, mode: Ptr<u8>) -> Ptr<::std::fs::File> {
+    let path: Value<Ptr<u8>> = Rc::new(RefCell::new(path));
+    let mode: Value<Ptr<u8>> = Rc::new(RefCell::new(mode));
+    (*path.borrow()).clone();
+    (*mode.borrow()).clone();
+    return Ptr::null();
+}
+pub fn main() {
+    std::process::exit(main_0());
+}
+fn main_0() -> i32 {
+    let fp: Value<Ptr<::std::fs::File>> = Rc::new(RefCell::new(
+        ({
+            let _path: Ptr<u8> = Ptr::from_string_literal("/tmp/irrelevant-file");
+            let _mode: Ptr<u8> = Ptr::from_string_literal("r");
+            fopen_0(_path, _mode)
+        }),
+    ));
+    assert!(((((*fp.borrow()).is_null()) as i32) != 0));
+    return 0;
+}
diff --git a/tests/unit/out/unsafe/user_defined_same_as_libc.rs b/tests/unit/out/unsafe/user_defined_same_as_libc.rs
new file mode 100644
index 00000000..2b284523
--- /dev/null
+++ b/tests/unit/out/unsafe/user_defined_same_as_libc.rs
@@ -0,0 +1,27 @@
+extern crate libc;
+use libc::*;
+extern crate libcc2rs;
+use libcc2rs::*;
+use std::collections::BTreeMap;
+use std::io::{Read, Seek, Write};
+use std::os::fd::{AsFd, FromRawFd, IntoRawFd};
+use std::rc::Rc;
+pub unsafe fn fopen_0(mut path: *const u8, mut mode: *const u8) -> *mut ::std::fs::File {
+    &(path);
+    &(mode);
+    return std::ptr::null_mut();
+}
+pub fn main() {
+    unsafe {
+        std::process::exit(main_0() as i32);
+    }
+}
+unsafe fn main_0() -> i32 {
+    let mut fp: *mut ::std::fs::File = (unsafe {
+        let _path: *const u8 = (b"/tmp/irrelevant-file\0".as_ptr().cast_mut()).cast_const();
+        let _mode: *const u8 = (b"r\0".as_ptr().cast_mut()).cast_const();
+        fopen_0(_path, _mode)
+    });
+    assert!(((((fp).is_null()) as i32) != 0));
+    return 0;
+}
diff --git a/tests/unit/user_defined_same_as_libc.c b/tests/unit/user_defined_same_as_libc.c
new file mode 100644
index 00000000..57863b8a
--- /dev/null
+++ b/tests/unit/user_defined_same_as_libc.c
@@ -0,0 +1,14 @@
+#include <assert.h>
+#include <stdio.h>
+
+FILE *fopen(const char *path, const char *mode) {
+  (void)path;
+  (void)mode;
+  return NULL;
+}
+
+int main() {
+  FILE *fp = fopen("/tmp/irrelevant-file", "r");
+  assert(fp == NULL);
+  return 0;
+}