From 1337bd7e45f51cbf2bdd556d9056cb7efb2d4a6b Mon Sep 17 00:00:00 2001
From: milescoviello <kitslayer29@gmail.com>
Date: Mon, 12 Oct 2026 08:00:21 +0000
Subject: [PATCH] fix(libc): leave a stream that is reading untouched in fflush
 on Linux

fflush() passed every stream to glibc's fflush(). For a stream that is
reading, glibc seeks the descriptor back and drops the read buffer, and
with a pushed-back character it gets the position wrong: after fgetc()
returned 'a' from "abcdef\n" and ungetc('Z'), fflush() returned 0 and
the next fread() returned "bcdef\n\n". The pushed-back 'Z' was lost and
the last byte came back twice.

FreeBSD's fflush() writes out only a stream that is writing and leaves
every other stream as it is, lib/libc/stdio/fflush.c:

	if ((fp->_flags & (__SWR | __SRW)) == 0)
		retval = 0;
	else
		retval = __sflush(fp);

and __sflush():

	t = fp->_flags;
	if ((t & __SWR) == 0)
		return (0);

fflush() on Linux now returns 0 without calling the host for a stream
whose last operation was not a write (__fwriting() is 0). fflush(NULL)
and streams that are writing are unchanged.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
---
 core/libs/prx/libc/src/FileAndHeap.cpp   | 10 +++++++++-
 core/libs/tests/GuestStandardStreams.cpp | 24 ++++++++++++++++++++++++
 2 files changed, 33 insertions(+), 1 deletion(-)

diff --git a/core/libs/prx/libc/src/FileAndHeap.cpp b/core/libs/prx/libc/src/FileAndHeap.cpp
index 87c352e2b..45212dc36 100644
--- a/core/libs/prx/libc/src/FileAndHeap.cpp
+++ b/core/libs/prx/libc/src/FileAndHeap.cpp
@@ -18,6 +18,10 @@
 #include "prx/libc/include/FilesystemError.hpp"
 #include "prx/libc/include/GuestArena.hpp"
 
+#ifdef __linux__
+#include <stdio_ext.h>
+#endif
+
 struct GuestFileMode {
     std::string native;
     bool writes = false;
@@ -272,7 +276,11 @@ int APS5_VABI fputs_nid_postfix(const char* str, FileStream* stream) {
 }
 
 int APS5_VABI fflush_nid_postfix(FileStream* stream) {
-    if (std::fflush(stream ? GetNativeStream(stream) : nullptr) != 0) throw std::runtime_error("fflush: flush failed");
+    std::FILE* native = stream ? GetNativeStream(stream) : nullptr;
+#ifdef __linux__
+    if (native && !__fwriting(native)) return 0;
+#endif
+    if (std::fflush(native) != 0) throw std::runtime_error("fflush: flush failed");
     return 0;
 }
 
diff --git a/core/libs/tests/GuestStandardStreams.cpp b/core/libs/tests/GuestStandardStreams.cpp
index 3173ea8d9..953f513e8 100644
--- a/core/libs/tests/GuestStandardStreams.cpp
+++ b/core/libs/tests/GuestStandardStreams.cpp
@@ -53,6 +53,7 @@ int APS5_VABI setvbuf_nid_postfix(FileStream*, char*, int, std::size_t);
 void APS5_VABI setbuf_nid_postfix(FileStream*, char*);
 FileStream* APS5_VABI fdopen_nid_postfix(int, const char*);
 int APS5_VABI fclose_nid_postfix(FileStream*);
+int APS5_VABI fflush_nid_postfix(FileStream*);
 int APS5_VABI _Getmbcurmax_nid_postfix();
 int APS5_VABI ___mb_cur_max_nid_postfix();
 void APS5_VABI flockfile_nid_postfix(FileStream*);
@@ -280,6 +281,28 @@ static void CheckRewind() {
 #endif
 }
 
+static void CheckFlushWhileReading() {
+    const auto filename = "anyps5-flush-read-" + std::to_string(std::chrono::steady_clock::now().time_since_epoch().count());
+    { std::ofstream seed(filename, std::ios::binary); seed << "abcdef\n"; Require(seed.good()); }
+    for (const char* mode : {"r", "r+"}) {
+        FileStream* stream = fopen_nid_postfix(filename.c_str(), mode);
+        Require(stream != nullptr);
+        Require(fgetc_nid_postfix(stream) == 'a' && ungetc_nid_postfix('Z', stream) == 'Z');
+        Require(fflush_nid_postfix(stream) == 0);
+        char rest[16]{};
+        const auto count = fread_nid_postfix(rest, 1, sizeof(rest) - 1, stream);
+        if (count != 7 || std::strcmp(rest, "Zbcdef\n") != 0) {
+            std::string shown;
+            for (std::size_t index = 0; index < count; ++index) shown += rest[index] == '\n' ? std::string("\\n") : std::string(1, rest[index]);
+            std::fprintf(stderr, "fflush of a \"%s\" stream that is reading changed the data read next: \"%s\" (%zu bytes), expected \"Zbcdef\\n\" (7 bytes)\n",
+                mode, shown.c_str(), count);
+            std::abort();
+        }
+        Require(fclose_nid_postfix(stream) == 0);
+    }
+    std::filesystem::remove(filename);
+}
+
 static bool CheckBinaryModes() {
     const auto directory = "anyps5-byte-stream-" + std::to_string(std::chrono::steady_clock::now().time_since_epoch().count());
     Require(std::filesystem::create_directory(directory));
@@ -336,6 +359,7 @@ static bool CheckBinaryModes() {
 
 int main() {
     Require(_Getmbcurmax_nid_postfix() == 1 && _Getmbcurmax_nid_postfix() == ___mb_cur_max_nid_postfix());
+    CheckFlushWhileReading();
     Require(fdopen_nid_postfix(-1, "rb") == nullptr && *__error_nid_postfix() == 9);
     Require(fdopen_nid_postfix(0, nullptr) == nullptr && *__error_nid_postfix() == 22);
     Require(fdopen_nid_postfix(0, "invalid") == nullptr && *__error_nid_postfix() == 22);