bench runner can run external command

bench/bench.cpp

@@ -57,6 +57,8 @@ std::stringstream strout;
 parse_options popts;
 bool with_file_io = false;
 
+char const* external_command = nullptr;
+
 #if defined(__clang__)
 string_view toolset = "clang";
 #elif defined(__GNUC__)
@@ -173,6 +175,22 @@ print_prefix(
         arch << "," << impl.name();
 }
 
+void
+run_external(char const* op, file_item const& f)
+{
+    if( !external_command )
+        return;
+
+    std::string command = external_command;
+    command += ' ';
+    command += op;
+    command += ' ';
+    command += toolset;
+    command += ' ';
+    command += f.name;
+    (void)std::system( command.c_str() );
+}
+
 void
 bench(
     string_view verb,
@@ -203,7 +221,10 @@ bench(
             repeat = 1000;
             for(unsigned k = 0; k < Trials; ++k)
             {
+                run_external("start", vf[i]);
                 auto result = run_for(std::chrono::seconds(5), f);
+                run_external("finish", vf[i]);
+
                 result.calls *= repeat;
                 result.mbs = megabytes_per_second(
                     vf[i], result.calls, result.millis);
@@ -1159,6 +1180,8 @@ main(
         return 4;
     }
 
+    external_command = std::getenv("BOOST_JSON_BENCH_EXTERNAL_COMMAND");
+
     file_list vf;
 
     for( int i = 1; i < argc; ++i )