le_signals.h

    1 /**
    2  * @page c_signals Signals API
    3  *
    4  * @ref le_signals.h "API Reference"
    5  *
    6  * <HR>
    7  *
    8  * Signals are software interrupts that can be sent to a running process or thread to
    9  * indicate exceptional situations. The action taken when an event is received depends on the
   10  * current settings for the signal and may be set to either:
   11  *
   12  *  - Operating systems default action.
   13  *  - Ignore the signal.
   14  *  - A custom handler.
   15  *
   16  * When a signal is received, unless it is ignored or blocked the action for the signal will preempt
   17  * any code that is currently executing.  Also, signals are asynchronous and may arrive at any time.
   18  * See http://man7.org/linux/man-pages/man7/signal.7.html for more details.
   19  *
   20  * The asynchronous and preemptive nature of signals can be difficult to deal with and is often a
   21  * source of race conditions.  Moreover asynchronous and preemptive signal handling is often
   22  * unnecessary so code often looks something like this:
   23  *
   24  * @code
   25  * // A global volatile atomic flag.
   26  * static volatile int GotSignal;
   27  *
   28  * void sigHandler(int sigNum)
   29  * {
   30  *      // Must only use asynch-signal-safe functions in this handler, see signals(7) man page for
   31  *      // more details.
   32  *
   33  *      // Set the flag.
   34  *      GotSignal = 1;
   35  * }
   36  *
   37  * int main (void)
   38  * {
   39  *      while(1)
   40  *      {
   41  *          // Do something.
   42  *          ...
   43  *
   44  *          if (GotSignal = 1)
   45  *          {
   46  *              // Clear the flag.
   47  *              GotSignal = 0;
   48  *
   49  *              // Process the signal.
   50  *              ...
   51  *          }
   52  *      }
   53  * }
   54  * @endcode
   55  *
   56  * In this code sample, the signal handler is only used to set a flag, while the main loop handles
   57  * the actual signal processing.  But handling signals this way requires the main loop to run
   58  * continuously.  This code is also prone to errors. For example, if the clearing of the flags was
   59  * done after processing of the signal, any signals received during processing of the signals will
   60  * be lost.
   61  *
   62  * @section c_signals_eventHandlers Signal Event Handlers
   63  *
   64  * The Legato signals API provides a simpler alternative, called signal events.  Signal events can
   65  * be used to receive and handle signals synchronously without the need for a sit-and-wait loop or
   66  * even a block-and-wait call.
   67  *
   68  * To use signal events, the desired signals must first be blocked using le_sig_Block() (see
   69  * @ref c_signals_blocking).  Then set a signal event handler for the desired signal using
   70  * @c le_sig_SetEventHandler().  Once a signal to the thread is received, the signal event handler
   71  * is called by the thread's Legato event loop (see @ref c_eventLoop for more details).  The handler
   72  * is called synchronously in the context of the thread that set the handler.  Be aware that if the
   73  * thread's event loop is not called or is blocked by some other code, the signal event handler will
   74  * also be blocked.
   75  *
   76  * Here is an example using signal events to handle the SIGCHLD signals:
   77  *
   78  * @code
   79  * // SIGCHILD event handler that will be called as a synchronous event.
   80  * static void SigChildEventHandler(int sigNum)
   81  * {
   82  *      // Handle SIGCHLD event.
   83  *      ...
   84  *
   85  *      // There is no need to limit ourselves to async-signal-safe functions because we are now in
   86  *      // a synchronous event handler.
   87  * }
   88  *
   89  *
   90  * COMPONENT_INIT
   91  * {
   92  *      // Block Signals that we are going to set event handlers for.
   93  *      le_sig_Block(SIGCHLD);
   94  *
   95  *      // Setup the signal event handler.
   96  *      le_sig_SetEventHandler(SIGCHILD, SigChildEventHandler);
   97  * }
   98  * @endcode
   99  *
  100  *
  101  * @section c_signals_mixedHandlers Mixing Asynchronous Signal Handlers with Synchronous Signal Event Handlers
  102  *
  103  * Signal events work well when dealing with signals synchronously, but when signals must be dealt
  104  * with in an asynchronously, traditional signal handlers are still preferred.  In fact, signal
  105  * event handlers are not allowed for certain signals like program error signals (ie. SIGFPE,
  106  * etc.) because they indicate a serious error in the program and all code outside of signal
  107  * handlers are considered unreliable.  This means that asynchronous signal handlers are the only
  108  * option when dealing with program error signals.
  109  *
  110  * Signal event handlers can be used in conjunction with asynchronous signal handlers but
  111  * only if they do not deal with the same signals.  In fact all signals that use signal events must
  112  * be blocked for every thread in the process.  The Legato framework takes care of this for you when
  113  * you set the signals you want to use in the Legato build system.
  114  *
  115  * If your code explicitly unblocks a signal where you currently have signal event handlers, the
  116  * signal event handlers will no longer be called until the signal is blocked again.
  117  *
  118  * @section c_signals_multiThread Multi-Threading Support
  119  *
  120  * In a multi-threaded system, signals can be sent to either the process or a specific thread.
  121  * Signals directed at a specific thread will be received by that thread; signals directed at
  122  * the process are received by one of the threads in the process that has a handler for the signal.
  123  *
  124  * It is unspecified which thread will actually receive the signal so it's recommended
  125  * to only have one signal event handler per signal.
  126  *
  127  * @section c_signals_limitations Limitations and Warnings
  128  *
  129  * A limitation of signals in general (not just with signal events) is called signal merging.
  130  * Signals that are received but not yet handled are said to be pending.  If another signal of the
  131  * same type is received while the first signal is pending, then the two signals will merge into a
  132  * single signal and there will be only one handler function call.  Consequently, it is not possible
  133  * to reliably know how many signals arrived.
  134  *
  135  *
  136  * @warning Signals are difficult to deal with in general because of their asynchronous nature and
  137  * although, Legato has simplified the situation with signal events certain limitations still exist.
  138  * If possible, avoid using them.
  139  *
  140  * @section c_signals_blocking Blocking signals
  141  *
  142  * Signals that are to be used with a signal event handlers must be blocked for the entire process.
  143  * To ensure this use le_sig_Block() to block signals in the process' first thread.  All other
  144  * threads will inherit the signal mask from the first thread.
  145  *
  146  * The example below shows how to use a signal event in a separate thread.
  147  *
  148  * @code
  149  * // SIGCHILD event handler that will be called as a synchronous event in the context of the
  150  * // workThread.
  151  * static void SigChildEventHandler(int sigNum)
  152  * {
  153  *      // Handle SIGCHILD event.
  154  *      ...
  155  *
  156  *      // There is no need to limit ourselves to async-signal-safe functions because we are now in
  157  *      // a synchronous event handler.
  158  * }
  159  *
  160  *
  161  * // Work thread's main function.
  162  * static void* WorkThreadMain(void* context)
  163  * {
  164  *      // Setup the signal event handler.
  165  *      le_sig_SetEventHandler(SIGCHILD, SigChildEventHandler);
  166  *
  167  *      // Start this thread's event loop.
  168  *      le_event_RunLoop();
  169  *
  170  *      return NULL;
  171  * }
  172  *
  173  *
  174  * // Main thread code.
  175  * COMPONENT_INIT
  176  * {
  177  *      // Block Signals that we are going to set event handlers for in the main thread so that all
  178  *      // subsequent threads will inherit the same signal mask.
  179  *      le_sig_Block(SIGCHLD);
  180  *
  181  *      // Create and start a work thread that will actually handle the signal.
  182  *      le_thread_Ref_t workThread = le_thread_Create("workThread", WorkThread, NULL);
  183  *
  184  *      le_thread_Start(workThread);
  185  * }
  186  * @endcode
  187  *
  188  * <HR>
  189  *
  190  * Copyright (C) Sierra Wireless Inc. Use of this work is subject to license.
  191  */
  192 
  193 //--------------------------------------------------------------------------------------------------
  194 /** @file le_signals.h
  195  *
  196  * Legato @ref c_signals include file.
  197  *
  198  * Copyright (C) Sierra Wireless Inc. Use of this work is subject to license.
  199  */
  200 
  201 #ifndef LEGATO_SIGNALS_INCLUDE_GUARD
  202 #define LEGATO_SIGNALS_INCLUDE_GUARD
  203 
  204 
  205 //--------------------------------------------------------------------------------------------------
  206 /**
  207  *  Prototype for the signal event handler functions.
  208  */
  209 //--------------------------------------------------------------------------------------------------
  210 typedef void (*le_sig_EventHandlerFunc_t)
  211 (
  212     int sigNum      ///< [IN] The signal that was received.
  213 );
  214 
  215 
  216 //--------------------------------------------------------------------------------------------------
  217 /**
  218  * Blocks a signal in the calling thread.
  219  *
  220  * Signals that an event handler will be set for must be blocked for all threads in the process.  To
  221  * ensure that the signals are blocked in all threads call this function in the process' first
  222  * thread, all subsequent threads will inherit the signal mask.
  223  *
  224  * @note Does not return on failure.
  225  */
  226 //--------------------------------------------------------------------------------------------------
  227 void le_sig_Block
  228 (
  229     int sigNum              ///< [IN] Signal to block.
  230 );
  231 
  232 
  233 //--------------------------------------------------------------------------------------------------
  234 /**
  235  * Sets a signal event handler for the calling thread.  Each signal can only have a single event
  236  * handler per thread.  The most recent event handler set will be called when the signal is
  237  * received.  sigEventHandler can be set to NULL to remove a previously set handler.
  238  *
  239  * @note sigNum Cannot be SIGKILL or SIGSTOP or any program error signals: SIGFPE, SIGILL,
  240  * SIGSEGV, SIGBUS, SIGABRT, SIGIOT, SIGTRAP, SIGEMT, SIGSYS.
  241  *
  242  * @note Does not return on failure.
  243  */
  244 //--------------------------------------------------------------------------------------------------
  245 void le_sig_SetEventHandler
  246 (
  247     int sigNum,                                 ///< [IN] Signal to set the event handler for.
  248                                                 ///  See parameter documentation in comments above.
  249     le_sig_EventHandlerFunc_t sigEventHandler   ///< [IN] Event handler to call when a signal is
  250                                                 ///  received.
  251 );
  252 
  253 
  254 //--------------------------------------------------------------------------------------------------
  255 /**
  256  * Removes all signal event handlers for the calling thread and cleans up any resources used for
  257  * signal events.  This should be called before the thread exits.
  258  */
  259 //--------------------------------------------------------------------------------------------------
  260 void le_sig_DeleteAll
  261 (
  262     void
  263 );
  264 
  265 
  266 #endif // LEGATO_SIGNALS_INCLUDE_GUARD
le_sig_DeleteAll
void le_sig_DeleteAll(void)
le_sig_Block
void le_sig_Block(int sigNum)
le_sig_EventHandlerFunc_t
void(* le_sig_EventHandlerFunc_t)(int sigNum)
Definition: le_signals.h:211
le_sig_SetEventHandler
void le_sig_SetEventHandler(int sigNum, le_sig_EventHandlerFunc_t sigEventHandler)