Adapt API style of lock_memory to match the one of the other functions (backport #209)

include/realtime_tools/realtime_helpers.hpp

@@ -63,10 +63,24 @@ bool configure_sched_fifo(int priority);
  * will not swap out the pages to disk i.e., the pages are guaranteed to stay in
  * RAM until later unlocked - which is important for realtime applications.
  * \param[out] message a message describing the result of the operation
- * \returns true if memory locking succeeded, false otherwise
+ * \returns true if memory locking succeeded, false otherwise.
 */
+
+[[deprecated("Use std::pair<bool, std::string> lock_memory() instead.")]]
 bool lock_memory(std::string & message);
 
+/**
+ * Locks the memory pages of the calling thread to prevent page faults.
+ * By calling this method, the programs locks all pages mapped into the address
+ * space of the calling process and future mappings. This means that the kernel
+ * will not swap out the pages to disk i.e., the pages are guaranteed to stay in
+ * RAM until later unlocked - which is important for realtime applications.
+ * \param[out] message a message describing the result of the operation
+ * \returns  a pair of a boolean indicating whether the operation succeeded or not
+ * and a message describing the result of the operation
+*/
+std::pair<bool, std::string> lock_memory();
+
 /**
  * Configure the caller thread affinity - Tell the scheduler to prefer a certain
  * set of cores for the given thread handle.

src/realtime_helpers.cpp

@@ -65,10 +65,16 @@ bool configure_sched_fifo(int priority)
 }
 
 bool lock_memory(std::string & message)
+{
+  const auto lock_result = lock_memory();
+  message = lock_result.second;
+  return lock_result.first;
+}
+
+std::pair<bool, std::string> lock_memory()
 {
 #ifdef _WIN32
-  message = "Memory locking is not supported on Windows.";
-  return false;
+  return {false, "Memory locking is not supported on Windows."};
 #else
   auto is_capable = [](cap_value_t v) -> bool {
     bool rc = false;
@@ -86,6 +92,7 @@ bool lock_memory(std::string & message)
     return rc;
   };
 
+  std::string message;
   if (mlockall(MCL_CURRENT | MCL_FUTURE) == -1) {
     if (!is_capable(CAP_IPC_LOCK)) {
       message = "No proper privileges to lock the memory!";
@@ -105,10 +112,10 @@ bool lock_memory(std::string & message)
     } else {
       message = "Unknown error occurred!";
     }
-    return false;
+    return {false, message};
   } else {
     message = "Memory locked successfully!";
-    return true;
+    return {true, message};
   }
 #endif
 }