Note that all of the code in preExecFunction must be async-signal-safe

std/process.d

@@ -2159,6 +2159,8 @@ struct Config
         /**
         A function that is called before `exec` in $(LREF spawnProcess).
         It returns `true` if succeeded and otherwise returns `false`.
+        Please note that the code in this function must only use
+        async-signal-safe functions.
         On Windows, this member is not available.
         */
         bool function() nothrow @nogc @safe preExecFunction;