le_fdMonitor.h

    1 /**
    2  * @page c_fdMonitor File Descriptor Monitor API
    3  *
    4  * @ref le_fdMonitor.h "API Reference"
    5  *
    6  * <HR>
    7  *
    8  * In a POSIX environment, like Linux, file descriptors (fds) are used for most process I/O.
    9  * Many components need to be notified when one or more fds are
   10  * ready to read from or write to, or if there's an error or hang-up.
   11  *
   12  *Although it's common to block a thread on a call to @c read(), @c write(),
   13  * @c accept(), @c select(), @c poll() (or some variantion of these), if that's done in a thread
   14  * shared with other components, the other components won't run when needed. To avoid this, Legato
   15  * has methods to monitor fds reporting related events so they won't interfere with other software
   16  * sharing the same thread.
   17  *
   18  * @section c_fdMonitorStartStop Start/Stop Monitoring
   19  *
   20  * le_fdMonitor_Create() creates a <b> File Descriptor Monitor </b> and starts monitoring an fd.
   21  * A handler function and set of events is also provided to le_fdMonitor_Create().
   22  *
   23  * @code
   24     // Monitor for data available to read.
   25     le_fdMonitor_Ref_t fdMonitor = le_fdMonitor_Create("Serial Port",      // Name for diagnostics
   26                                                        fd,                 // fd to monitor
   27                                                        SerialPortHandler,  // Handler function
   28                                                        POLLIN);            // Monitor readability
   29  * @endcode
   30  *
   31  * When an fd no longer needs to be monitored, the File Descriptor Monitor object
   32  * is deleted by calling le_fdMonitor_Delete().
   33  *
   34  * @code
   35  *
   36     le_fdMonitor_Delete(fdMonitor);
   37 
   38  * @endcode
   39  *
   40  * @warning Always delete the Monitor object for an fd <b> before closing the fd </b>. After an
   41  *          fd is closed, it could get reused for something completely different. If monitoring of
   42  *          the new fd incarnation is started before the old Monitor object is deleted, deleting
   43  *          the old Monitor will cause monitoring of the new incarnation to fail.
   44  *
   45  *
   46  * @section c_fdMonitorEvents Event Types
   47  *
   48  * Events that can be handled:
   49  *
   50  * - @c POLLIN = Data available to read.
   51  * - @c POLLPRI = Urgent data available to read (e.g., out-of-band data on a socket).
   52  * - @c POLLOUT = Writing to the fd should accept some data now.
   53  * - @c POLLRDHUP = Other end of stream socket closed or shutdown.
   54  * - @c POLLERR = Error occurred.
   55  * - @c POLLHUP = Hang up.
   56  *
   57  * These are bitmask values and can be combined using the bit-wise OR operator ('|') and tested
   58  * for using the bit-wise @e and ('&') operator.
   59  *
   60  * @note @c POLLRDHUP, @c POLLERR and @c POLLHUP can't be disabled.
   61  * Monitoring these events is always enabled as soon as the File Descriptor
   62  * Monitor is created regardless of the set of events given to le_fdMonitor_Create().
   63  *
   64  * @section c_fdTypes FD Types
   65  *
   66  * The fd type affects how events are monitored:
   67  *
   68  * - @ref c_fdTypes_files
   69  * - @ref c_fdTypes_pipes
   70  * - @ref c_fdTypes_sockets
   71  * - @ref c_fdTypes_terminals
   72  *
   73  * @subsection c_fdTypes_files Files
   74  *
   75  * - POLLIN and POLLOUT are always SET
   76  * - NONE of the other EVENTS are ever set
   77  *
   78  * @subsection c_fdTypes_pipes Pipes
   79  *
   80  * Pipe fd events indicate two conditions for reading from a pipe and two conditions for writing to
   81  * a pipe.
   82  *
   83  * |                      |  Event            |   Condition                                     |
   84  * | ---------------------| ----------------- | ----------------------------------------------- |
   85  * | READING from a pipe  | POLLHUP           | NO DATA in the pipe and the WRITE END is closed |
   86  * |                      | POLLIN            | DATA in the pipe and the WRITE END is open      |
   87  * |                      | POLLIN + POLLHUP  | DATA in the pipe BUT the WRITE END is closed    |
   88  * | WRITING to the pipe  | POLLERR           | NO SPACE in the pipe and the READ END is closed |
   89  * |                      | POLLOUT           | SPACE in the pipe and the READ END is open      |
   90  * |                      | POLLOUT + POLLERR | SPACE in the pipe BUT the READ END is closed    |
   91  *
   92  * @subsection c_fdTypes_sockets Sockets
   93  *
   94  * Socket activity (establishing/closing) is monitored for connection-orientated sockets including
   95  * SOCK_STREAM and SOCK_SEQPACKET. Input and output data availability for all socket types is
   96  * monitored.
   97  * | Event                        | Condition                                                    |
   98  * | ---------------------------- | ------------------------------------------------------------ |
   99  * | POLLIN                       | Input is available from the socket                           |
  100  * | POLLOUT                      | Possible to send data on the socket                          |
  101  * | POLLIN                       | Incoming connection being established on the listen port     |
  102  * | POLLPRI                      | Out of band data received only on TCP                        |
  103  * | POLLIN + POLLOUT + POLLRDHUP | Peer closed the connection in a connection-orientated socket |
  104  *
  105  *
  106  * @subsection c_fdTypes_terminals Terminals and Pseudo-Terminals
  107  *
  108  * Terminals and pseudo-terminals operate in pairs. When one terminal pair closes, an event is
  109  * generated to indicate the closure. POLLIN, POLLOUT and POLLPRI are the event indicators related
  110  * to terminal status.
  111  *
  112  * | Event   | Condition             |
  113  * | ------- | --------------------- |
  114  * | POLLIN  | Ready to receive data |
  115  * | POLLOUT | Ready to send data    |
  116  * | POLLPRI | Master/pseudo terminal detects slave state has changed (in packet mode only). |
  117  * | POLLHUP | Either half of the terminal pair has closed. |
  118  *
  119  *
  120  * @section c_fdMonitorHandlers Handler Functions
  121  *
  122  * Parameters to the fd event handler functions are the fd and the events active for the
  123  * fd. The events are passed as a bit mask; the bit-wise AND operator ('&') must be used to
  124  * check for specific events.
  125  *
  126  * @code
  127 
  128 COMPONENT_INIT
  129 {
  130     // Open the serial port.
  131     int fd = open("/dev/ttyS0", O_RDWR|O_NONBLOCK);
  132     LE_FATAL_IF(fd == -1, "open failed with errno %d (%m)", errno);
  133 
  134     // Create a File Descriptor Monitor object for the serial port's file descriptor.
  135     // Monitor for data available to read.
  136     le_fdMonitor_Ref_t fdMonitor = le_fdMonitor_Create("Serial Port",      // Name for diagnostics
  137                                                        fd,                 // fd to monitor
  138                                                        SerialPortHandler,  // Handler function
  139                                                        POLLIN);            // Monitor readability
  140 }
  141 
  142 static void SerialPortHandler(int fd, short events)
  143 {
  144     if (events & POLLIN)    // Data available to read?
  145     {
  146         char buff[MY_BUFF_SIZE];
  147 
  148         ssize_t bytesRead = read(fd, buff, sizeof(buff));
  149 
  150         ...
  151     }
  152 
  153     if ((events & POLLERR) || (events & POLLHUP) || (events & POLLRDHUP))   // Error or hang-up?
  154     {
  155         ...
  156     }
  157 }
  158 
  159  * @endcode
  160  *
  161  * @section c_fdMonitorEnableDisable Enable/Disable Event Monitoring
  162  *
  163  * The set of fd events being monitored can be adjusted using le_fdMonitor_Enable()
  164  * and le_fdMonitor_Disable().  However, @c POLLRDHUP, @c POLLERR and @c POLLHUP can't be disabled.
  165  *
  166  * CPU cycles (and power) can be saved by disabling monitoring when not needed.
  167  * For example, @c POLLOUT monitoring should be disabled while nothing needs to be written
  168  * to the fd, so that the event handler doesn't keep getting called with a @c POLLOUT event
  169  * because the fd is writeable.
  170  *
  171  * @code
  172 static void StartWriting()
  173 {
  174     // Enable monitoring for POLLOUT.  When connection is ready, handler will be called.
  175     le_fdMonitor_Enable(FdMonitorRef, POLLOUT);
  176 }
  177 
  178 static void ConnectionEventHandler(int fd, int event)
  179 {
  180     if (event & POLLOUT)
  181     {
  182         // Connection is ready for us to send some data.
  183         le_result_t result = SendWaitingData();
  184         if (result == LE_NOT_FOUND)
  185         {
  186             // Buffer empty, stop monitoring POLLOUT so handler doesn't keep getting called.
  187             le_fdMonitor_Disable(le_fdMonitor_GetMonitor(), POLLOUT);
  188         }
  189         ...
  190     }
  191     ...
  192 }
  193  * @endcode
  194  *
  195  * If an event occurs on an fd while monitoring of that event is disabled,
  196  * the event will be ignored. If that event is later enabled, and that event's
  197  * trigger condition is still true (e.g., the fd still has data available to be read), then the
  198  * event will be reported to the handler at that time. If the event trigger condition is gone
  199  * (e.g., the fd no longer has data available to read), then the event will not be reported
  200  * until its trigger condition becomes true again.
  201  *
  202  * If events occur on different fds at the same time, the order in which the handlers
  203  * are called is implementation-dependent.
  204  *
  205  *
  206  * @section c_fdMonitorHandlerContext Handler Function Context
  207  *
  208  * Calling le_fdMonitor_GetMonitor() inside the handler function fetches a reference to the
  209  * File Descriptor Monitor object for the event being handled.  This is handy to enable and
  210  * disable event monitoring from inside the handler.
  211  *
  212  * If additional data needs to be passed to the handler function, the context pointer can be set
  213  * to use le_fdMonitor_SetContextPtr() and retrieved inside the handler function with
  214  * le_fdMonitor_GetContextPtr(). le_event_GetContextPtr() can also be used, but
  215  * le_fdMonitor_GetContextPtr() is preferred as it double checks it's being called inside
  216  * a File Descriptor Monitor's handler function.
  217  *
  218  * @code
  219  *
  220  * static void SerialPortHandler(int fd, short events)
  221  * {
  222  *     MyContext_t* contextPtr = le_fdMonitor_GetContextPtr();
  223  *
  224  *     // Process the fd event(s).
  225  *     ...
  226  * }
  227  *
  228  * static void StartDataTransmission(const char* port, uint8_t* txBuffPtr, size_t txBytes)
  229  * {
  230  *     // Open the serial port.
  231  *     int fd = open(port, O_RDWR|O_NONBLOCK);
  232  *     LE_FATAL_IF(fd == -1, "open failed with errno %d (%m)", errno);
  233  *
  234  *     // Create a File Descriptor Monitor object for the serial port's file descriptor.
  235  *     // Monitor for write buffer space availability.
  236  *     le_fdMonitor_Ref_t fdMonitor = le_fdMonitor_Create("Port", fd, SerialPortHandler, POLLOUT);
  237  *
  238  *     // Allocate a data block and populate with stuff we need in SerialPortHandler().
  239  *     MyContext_t* contextPtr = le_mem_ForceAlloc(ContextMemPool);
  240  *     contextPtr->txBuffPtr = txBuffPtr;
  241  *     contextPtr->bytesRemaining = txBytes;
  242  *
  243  *     // Make this available to SerialPortHandler() via le_fdMonitor_GetContextPtr().
  244  *     le_fdMonitor_SetContextPtr(fdMonitor, contextPtr);
  245  * }
  246  *
  247  * @endcode
  248  *
  249  *
  250  * @section c_fdMonitorPowerManagement Power Management
  251  *
  252  * If your process has the privilege of being able to block the system from going to sleep,
  253  * whenever the fd that is being monitored has a pending event, the system will be kept
  254  * awake.  To allow the system to go to sleep while this fd has a pending event, you can call
  255  * le_fdMonitor_SetDeferrable() with @c isDeferrable flag set to 'true'.
  256  *
  257  *
  258  * @section c_fdMonitorThreading Threading
  259  *
  260  * fd monitoring is performed by the Event Loop of the thread that
  261  * created the Monitor object for that fd. If that the is blocked, events
  262  * won't be detected for that fd until the thread is unblocked and returns to its
  263  * Event Loop. Similarly, if the thread that creates a File Descriptor Monitor object doesn't
  264  * run an Event Loop at all, no events will be detected for that fd.
  265  *
  266  * It's not recommended to monitor the same fd in two threads at the same time, because the threads
  267  * will race to handle any events on that fd.
  268  *
  269  *
  270  * @section c_fdMonitorTroubleshooting Troubleshooting
  271  *
  272  * The "fdMonitor" logging keyword can be enabled to view fd monitoring activity.
  273  *
  274  * <hr>
  275  *
  276  * Copyright (C) Sierra Wireless Inc. Use of this work is subject to license.
  277  */
  278 
  279 //--------------------------------------------------------------------------------------------------
  280 /** @file le_fdMonitor.h
  281  *
  282  * Legato @ref c_fdMonitor include file.
  283  *
  284  * Copyright (C) Sierra Wireless Inc. Use of this work is subject to license.
  285  */
  286 
  287 #ifndef LEGATO_FDMONITOR_INCLUDE_GUARD
  288 #define LEGATO_FDMONITOR_INCLUDE_GUARD
  289 
  290 
  291 //--------------------------------------------------------------------------------------------------
  292 /**
  293  * File Descriptor Monitor reference.
  294  *
  295  * Used to refer to File Descriptor Monitor objects.
  296  */
  297 //--------------------------------------------------------------------------------------------------
  298 typedef struct le_fdMonitor* le_fdMonitor_Ref_t;
  299 
  300 
  301 //--------------------------------------------------------------------------------------------------
  302 /**
  303  * Prototype for file descriptor event handler functions.
  304  *
  305  * Events that can be received:
  306  *
  307  * - @c POLLIN = Data available to read.
  308  * - @c POLLPRI = Urgent data available to read (e.g., out-of-band data on a socket).
  309  * - @c POLLOUT = Writing to the fd should accept some data now.
  310  * - @c POLLRDHUP = Other end of stream socket closed or shutdown.
  311  * - @c POLLERR = Error occurred.
  312  * - @c POLLHUP = Hang up.
  313  *
  314  * These are bitmask values that may appear in the @c events parameter.  Use the bit-wise AND
  315  * operator ('&') to test for specific events.
  316  *
  317  * @code
  318  *
  319  * if (events & POLLIN)
  320  * {
  321  *     // Data available to read.
  322  *     ...
  323  * }
  324  *
  325  * if (events & POLLERR)
  326  * {
  327  *     // An error occured.
  328  *     ...
  329  * }
  330  *
  331  * ...
  332  *
  333  * @endcode
  334  *
  335  * @param fd        [in] File descriptor.
  336  *
  337  * @param events    [in] Bit map of events that occurred.  Use bitwise AND ('&') to test for events.
  338  */
  339 //--------------------------------------------------------------------------------------------------
  340 typedef void (*le_fdMonitor_HandlerFunc_t)
  341 (
  342     int   fd,
  343     short events
  344 );
  345 
  346 
  347 //--------------------------------------------------------------------------------------------------
  348 /**
  349  * Creates a File Descriptor Monitor.
  350  *
  351  * Creates an object that will monitor a given file descriptor for events.
  352  *
  353  * The monitoring will be performed by the event loop of the thread that created the Monitor object.
  354  * If that thread is blocked, no events will be detected for that file descriptor until that
  355  * thread is unblocked and returns to its event loop.
  356  *
  357  * Events that can be enabled for monitoring:
  358  *
  359  * - @c POLLIN = Data available to read.
  360  * - @c POLLPRI = Urgent data available to read (e.g., out-of-band data on a socket).
  361  * - @c POLLOUT = Writing to the fd should accept some data now.
  362  *
  363  * These are bitmask values and can be combined using the bit-wise OR operator ('|').
  364  *
  365  * The following events are always monitored, even if not requested:
  366  *
  367  * - @c POLLRDHUP = Other end of stream socket closed or shutdown.
  368  * - @c POLLERR = Error occurred.
  369  * - @c POLLHUP = Hang up.
  370  *
  371  * @return
  372  *      Reference to the object, which is needed for later deletion.
  373  *
  374  * @note Doesn't return on failure, there's no need to check the return value for errors.
  375  */
  376 //--------------------------------------------------------------------------------------------------
  377 le_fdMonitor_Ref_t le_fdMonitor_Create
  378 (
  379     const char*             name,       ///< [in] Name of the object (for diagnostics).
  380     int                     fd,         ///< [in] File descriptor to be monitored for events.
  381     le_fdMonitor_HandlerFunc_t handlerFunc, ///< [in] Handler function.
  382     short                   events      ///< [in] Initial set of events to be monitored.
  383 );
  384 
  385 
  386 //--------------------------------------------------------------------------------------------------
  387 /**
  388  * Enables monitoring for events on a file descriptor.
  389  *
  390  * Events that can be enabled for monitoring:
  391  *
  392  * - @c POLLIN = Data available to read.
  393  * - @c POLLPRI = Urgent data available to read (e.g., out-of-band data on a socket).
  394  * - @c POLLOUT = Writing to the fd should accept some data now.
  395  *
  396  * These are bitmask values and can be combined using the bit-wise OR operator ('|').
  397  */
  398 //--------------------------------------------------------------------------------------------------
  399 void le_fdMonitor_Enable
  400 (
  401     le_fdMonitor_Ref_t  monitorRef, ///< [in] Reference to the File Descriptor Monitor object.
  402     short               events      ///< [in] Bit map of events.
  403 );
  404 
  405 
  406 //--------------------------------------------------------------------------------------------------
  407 /**
  408  * Disables monitoring for events on a file descriptor.
  409  *
  410  * Events that can be disabled for monitoring:
  411  *
  412  * - @c POLLIN = Data available to read.
  413  * - @c POLLPRI = Urgent data available to read (e.g., out-of-band data on a socket).
  414  * - @c POLLOUT = Writing to the fd should accept some data now.
  415  *
  416  * These are bitmask values and can be combined using the bit-wise OR operator ('|').
  417  */
  418 //--------------------------------------------------------------------------------------------------
  419 void le_fdMonitor_Disable
  420 (
  421     le_fdMonitor_Ref_t  monitorRef, ///< [in] Reference to the File Descriptor Monitor object.
  422     short               events      ///< [in] Bit map of events.
  423 );
  424 
  425 
  426 //--------------------------------------------------------------------------------------------------
  427 /**
  428  * Sets if processing of events on a given fd is deferrable (the system is allowed to go to
  429  * sleep while there are monitored events pending for this fd) or urgent (the system will be kept
  430  * awake until there are no monitored events waiting to be handled for this fd).
  431  *
  432  * If the process has @c CAP_EPOLLWAKEUP (or @c CAP_BLOCK_SUSPEND) capability, then fd events are
  433  * considered urgent by default.
  434  *
  435  * If the process doesn't have @c CAP_EPOLLWAKEUP (or @c CAP_BLOCK_SUSPEND) capability, then fd
  436  * events are always deferrable, and calls to this function have no effect.
  437  */
  438 //--------------------------------------------------------------------------------------------------
  439 void le_fdMonitor_SetDeferrable
  440 (
  441     le_fdMonitor_Ref_t monitorRef,  ///< [in] Reference to the File Descriptor Monitor object.
  442     bool               isDeferrable ///< [in] true (deferrable) or false (urgent).
  443 );
  444 
  445 
  446 //--------------------------------------------------------------------------------------------------
  447 /**
  448  * Sets the Context Pointer for File Descriptor Monitor's handler function.  This can be retrieved
  449  * by the handler using le_fdMonitor_GetContextPtr() (or le_event_GetContextPtr()) when the handler
  450  * function is running.
  451  */
  452 //--------------------------------------------------------------------------------------------------
  453 void le_fdMonitor_SetContextPtr
  454 (
  455     le_fdMonitor_Ref_t  monitorRef, ///< [in] Reference to the File Descriptor Monitor.
  456     void*               contextPtr  ///< [in] Opaque context pointer value.
  457 );
  458 
  459 
  460 //--------------------------------------------------------------------------------------------------
  461 /**
  462  * Gets the Context Pointer for File Descriptor Monitor's handler function.
  463  *
  464  * @return  The context pointer set using le_fdMonitor_SetContextPtr(), or NULL if it hasn't been
  465  *          set.
  466  *
  467  * @note    This only works inside the handler function.  The difference between this function and
  468  *          le_event_GetContextPtr() is that le_fdMonitor_GetContextPtr() will double check that
  469  *          it's being called inside of a File Descriptor Monitor's handler function.
  470  */
  471 //--------------------------------------------------------------------------------------------------
  472 void* le_fdMonitor_GetContextPtr
  473 (
  474     void
  475 );
  476 
  477 
  478 //--------------------------------------------------------------------------------------------------
  479 /**
  480  * Gets the file descriptor that an FD Monitor object is monitoring.
  481  *
  482  * @return  The fd.
  483  */
  484 //--------------------------------------------------------------------------------------------------
  485 int le_fdMonitor_GetFd
  486 (
  487     le_fdMonitor_Ref_t  monitorRef  ///< [in] Reference to the File Descriptor Monitor.
  488 );
  489 
  490 
  491 //--------------------------------------------------------------------------------------------------
  492 /**
  493  * Gets a reference to the File Descriptor Monitor whose handler function is currently running.
  494  *
  495  * @return  File Descriptor Monitor reference.
  496  *
  497  * @note    This only works inside the handler function.
  498  **/
  499 //--------------------------------------------------------------------------------------------------
  500 le_fdMonitor_Ref_t le_fdMonitor_GetMonitor
  501 (
  502     void
  503 );
  504 
  505 
  506 //--------------------------------------------------------------------------------------------------
  507 /**
  508  * Deletes a file descriptor monitor object.
  509  */
  510 //--------------------------------------------------------------------------------------------------
  511 void le_fdMonitor_Delete
  512 (
  513     le_fdMonitor_Ref_t monitorRef  ///< [in] Reference to the File Descriptor Monitor object.
  514 );
  515 
  516 
  517 #endif // LEGATO_FDMONITOR_INCLUDE_GUARD
le_fdMonitor_GetContextPtr
void * le_fdMonitor_GetContextPtr(void)
le_fdMonitor_HandlerFunc_t
void(* le_fdMonitor_HandlerFunc_t)(int fd, short events)
Definition: le_fdMonitor.h:341
le_fdMonitor_SetContextPtr
void le_fdMonitor_SetContextPtr(le_fdMonitor_Ref_t monitorRef, void *contextPtr)
le_fdMonitor_Delete
void le_fdMonitor_Delete(le_fdMonitor_Ref_t monitorRef)
le_fdMonitor_GetMonitor
le_fdMonitor_Ref_t le_fdMonitor_GetMonitor(void)
le_fdMonitor_GetFd
int le_fdMonitor_GetFd(le_fdMonitor_Ref_t monitorRef)
le_fdMonitor_Disable
void le_fdMonitor_Disable(le_fdMonitor_Ref_t monitorRef, short events)
le_fdMonitor_Enable
void le_fdMonitor_Enable(le_fdMonitor_Ref_t monitorRef, short events)
le_fdMonitor_SetDeferrable
void le_fdMonitor_SetDeferrable(le_fdMonitor_Ref_t monitorRef, bool isDeferrable)
le_fdMonitor_Ref_t
struct le_fdMonitor * le_fdMonitor_Ref_t
Definition: le_fdMonitor.h:298
le_fdMonitor_Create
le_fdMonitor_Ref_t le_fdMonitor_Create(const char *name, int fd, le_fdMonitor_HandlerFunc_t handlerFunc, short events)