Fix $fflush and autoflush with --threads (#7782).

Fixes #7782.

Changes

@@ -161,7 +161,7 @@ Verilator 5.049 devel
 * Fix FSM detect unchecked casts and variable redeclaration (#7758). [Adam Kostrzewski, Antmicro Ltd.]
 * Fix no-scope internal error on virtual interface method calls (#7759). [Yilou Wang]
 * Fix 'case (_) inside' with x wildcards (#7766). [Geza Lore, Testorrent USA, Inc.]
-
+* Fix $fflush and autoflush with --threads (#7782).
 
 
 Verilator 5.048 2026-04-26

include/verilated.cpp

@@ -318,6 +318,13 @@ void VL_PRINTF_MT(const char* formatp, ...) VL_MT_SAFE {
     }});
 }
 
+void VL_FFLUSH_MT() VL_MT_SAFE {
+    va_list ap;
+    VerilatedThreadMsgQueue::post(VerilatedMsg{[=]() {  //
+        Verilated::runFlushCallbacks();
+    }});
+}
+
 template <typename... snprintf_args_ts>
 static size_t _vl_snprintf_string(std::string& str, const char* format,
                                   snprintf_args_ts... args) VL_MT_SAFE {

include/verilated_funcs.h

@@ -74,10 +74,8 @@ extern void VL_FATAL_MT(const char* filename, int linenum, const char* hier,
 extern void VL_WARN_MT(const char* filename, int linenum, const char* hier,
                        const char* msg) VL_MT_SAFE;
 
-// clang-format off
 /// Print a string, multithread safe. Eventually VL_PRINTF will get called.
 extern void VL_PRINTF_MT(const char* formatp, ...) VL_ATTR_PRINTF(1) VL_MT_SAFE;
-// clang-format on
 
 /// Print a debug message from internals with standard prefix, with printf style format
 extern void VL_DBG_MSGF(const char* formatp, ...) VL_ATTR_PRINTF(1) VL_MT_SAFE;
@@ -85,6 +83,9 @@ extern void VL_DBG_MSGF(const char* formatp, ...) VL_ATTR_PRINTF(1) VL_MT_SAFE;
 /// Print a debug message from string via VL_DBG_MSGF
 inline void VL_DBG_MSGS(const std::string& str) VL_MT_SAFE { VL_DBG_MSGF("%s", str.c_str()); }
 
+/// Flush stdout
+extern void VL_FFLUSH_MT() VL_MT_SAFE;
+
 // EMIT_RULE: VL_RANDOM:  oclean=dirty
 inline IData VL_RANDOM_I() VL_MT_SAFE { return vl_rand64(); }
 inline QData VL_RANDOM_Q() VL_MT_SAFE { return vl_rand64(); }

src/V3EmitCFunc.h

@@ -1134,7 +1134,7 @@ public:
     }
     void visit(AstFFlush* nodep) override {
         if (!nodep->filep()) {
-            putns(nodep, "Verilated::runFlushCallbacks();\n");
+            putns(nodep, "VL_FFLUSH_MT();");
         } else {
             putns(nodep, "VL_FFLUSH_I(");
             iterateAndNextConstNull(nodep->filep());