DESCRIPTION

After setting up all the contexts, resources, endpoints sessions etc., the underlying CoAP and (D)TLS need to send (and possibly re-send) created packets as well as receive packets for processing.

The coap_io_process() function is the primary function applications should use. There are internal functions that coap_io_process() calls which are available to use if absolutely necessary. These internal functions and how to use them is different depending on whether libcoap has been compiled to use epoll (Linux systems only) or not.

For epoll libcoap, coap_io_process() in simple terms calls coap_io_prepare_epoll(), does an epoll_wait() and then calls coap_io_do_epoll() if needed to make sure that all event based i/o has been completed.

For non-epoll libcoap, coap_io_process() in simple terms calls coap_io_prepare_io() to set up sockets[], sets up all of the select() parameters based on the COAP_SOCKET_WANT* values in the sockets[], does a select(), updates the sockets[] with COAP_SOCKET_CAN_* as appropriate and then calls coap_io_do_io() to make sure that all current i/o has been completed.

FUNCTIONS

Function: coap_io_process()

The coap_io_process() function will process any outstanding packets to send for the specified context, process any available input packets and then wait for processing any new input packets, or for when to re-transmit a packet, for up to timeout_ms milli-seconds before returning. There are 2 special case timeout_ms values.

#define COAP_IO_WAIT    0
#define COAP_IO_NO_WAIT ((uint32_t)-1)

If timeout_ms is set to COAP_IO_WAIT, then coap_io_process() will block until the next internal action (e.g. packet retransmit) if any, or block until the next packet is received whichever is the sooner and do the necessary processing. If timeout_ms is set to COAP_IO_NO_WAIT, then coap_io_process() will return immediately after processing without waiting for any new input packets to arrive.

NOTE: coap_io_process() should not be called from within a callback handler as defined using the coap_register_*_handler() as coap_io_process() will likely recursively call the same handler.

There are two methods of how to call coap_io_process().

:NOTE:* With multi-threading protection enabled, it is possible to call coap_io_process() from multiple threads to do some load balancing.

See EXAMPLES below.

Function: coap_io_prepare_epoll()

The coap_io_prepare_epoll() function for the specified context will iterate through the endpoints and sessions to transmit any triggered observer responses as well as handling any timed out packet re-transmissions. Returned, based on now, is the number of milli-secs needed to delay until the next time that coap_io_prepare_epoll() needs to get called. After this call an epoll_wait() should done.

Function: coap_io_do_epoll()

The coap_io_do_epoll() function for the specified context will iterate through the nevents of events returned by epoll_wait() and execute the appropriate low level i/o function to send / receive / process the packets. Where appropriate, structure information (endpoints, sessions etc.) is updated with the value of now in the lower level functions.

Function: coap_io_prepare_io()

The coap_io_prepare_io() function for the specified context will iterate through the endpoints and sessions to add all of sockets waiting for network traffic (COAP_SOCKET_WANT_* is set) found to sockets (limited by max_sockets) and updates num_sockets with the number of sockets found. Furthermore, any triggered observer responses are transmitted as well as handling any timed out packet re-transmissions. Returned, based on now, is the number of milli-secs needed to delay until the next time that coap_io_prepare_io() needs to get called. After this call a select() should done on all the file descriptors (COAP_WANT_READ for readfds etc.), and any that are returned active should set the appropriate COAP_SOCKET_CAN_* in the sockets.

Function: coap_io_do_io()

The coap_io_do_io() function for the specified context will iterate through the endpoints and sessions to find all of sockets that have COAP_SOCKET_CAN_* set and then execute the appropriate low level i/o function to send / receive / process the packets. Where appropriate, structure information (endpoints, sessions etc.) is updated with the value of now in the lower level functions.

Function: coap_io_process_with_fds()

The coap_io_process_with_fds() function is the same as coap_process_io() but supports additional select() style parameters nfds, readfds, writefds and exceptfds. This provides the ability to add in additional non libcoap FDs to test for in the internal select() call which can then tested after the return from coap_io_process_with_fds(). readfds, writefds and exceptfds can either point to a defined and pre-filled fd_set structure or NULL if not required. nfds needs to be set to the maximum FD to test for in readfds, writefds or exceptfds if any of them are set plus 1. If none of them are set, then nfds should be set to 0.

NOTE: The additional parameters for coap_io_process_with_fds() are only used if there is no epoll support in libcoap. If there is epoll support, then coap_context_get_coap_fd() should be used and this returned FD along with other non libcoap FDs can separately be monitored using method 2 above.

Function: coap_context_get_coap_fd()

The coap_context_get_coap_fd() function obtains from the specified context a single file descriptor that can be monitored by a select() or as an event returned from a epoll_wait() call. This file descriptor will get updated with information (read, write etc. available) whenever any of the internal to libcoap file descriptors (sockets) change state.

Function: coap_io_pending()

The coap_io_pending() function checks to see if there are any outstanding i/o requests / responses associated with context as well as if Observe has been set up (client only) and large transfers are in process.

Function: coap_io_get_fds()

The coap_io_get_fds() function is used to get all of the libcoap internally used file descriptors associated with context in a read or write pending state.

read_fds[] is a defined array to hold all the file descriptors that are in a read pending state with a size of max_read_fds. have_read_fds is returned with the number of file descriptors in read_fds[].

write_fds[] is a defined array to hold all the file descriptors that are in a write pending state with a size of max_write_fds. have_write_fds is returned with the number of file descriptors in write_fds[].

rem_timeout_ms is updated with the remaining milli-seconds before coap_io_process() needs to be called again to handle any internal timeouts. If rem_timeout_ms is 0, then there is no timeout and the next event will be a change in state of one of the file descriptors.

Function: coap_can_exit()

The coap_can_exit() function checks to see if there are any outstanding PDUs to transmit associated with context and returns 1 if there is nothing outstanding else 0. This function does not check that all requests transmitted have been responded to.

Function: coap_socket_get_fd()

The coap_socket_get_fd() function obtains the file descriptor from the given socket. The file descriptor can be used to integrate libcoap in an external event loop instead of using one of its builtin event loops.

Function: coap_socket_get_flags()

The coap_socket_get_flags() function obtains the event flags from the given socket.

Function: coap_socket_set_flags()

The coap_socket_set_flags() function sets the event flags for the given socket.

RETURN VALUES

coap_io_process() and coap_io_process_with_fds() return the time, in milli-seconds, that was spent in the function. If -1 is returned, there was an unexpected error.

coap_context_get_coap_fd() returns a non-negative number as the file descriptor to monitor, or -1 if epoll is not configured in libcoap.

coap_io_prepare_io() and coap_io_prepare_epoll() return the number of milli-seconds that need to be waited before the function should next be called.

coap_io_pending() returns 1 if there is outstanding i/o else returns 0.

coap_io_get_fds() returns 1 if file descriptors returned, else returns 0.

coap_can_exit() returns 1 if there is nothing outstanding to transmit else returns 0.

coap_socket_get_fd() returns the file descriptor or COAP_INVALID_SOCKET if the platform does not use file descriptors.

coap_socket_get_flags() returns the OR-ed COAP_SOCKET* flags for this socket.

EXAMPLES

Method One - use coap_io_process()

#include <coap3/coap.h>

int
main(int argc, char *argv[]) {

  coap_context_t *ctx = NULL;
  unsigned wait_ms;
  /* Remove (void) definition if variable is used */
  (void)argc;
  (void)argv;

  /* Initialize libcoap library */
  coap_startup();

  /* Create the libcoap context */
  ctx = coap_new_context(NULL);
  if (!ctx) {
    exit(1);
  }
  /* See coap_block(3) */
  coap_context_set_block_mode(ctx,
                              COAP_BLOCK_USE_LIBCOAP | COAP_BLOCK_SINGLE_BODY);


  /* Other Set up Code */

  wait_ms = COAP_RESOURCE_CHECK_TIME * 1000;

  while (1) {
    int result = coap_io_process(ctx, wait_ms);
    if (result < 0) {
      /* There is an internal issue */
      break;
    }
    /* Do any other housekeeping */
  }
  coap_free_context(ctx);
  coap_cleanup();

  /* Do any other cleanup */

  exit(0);

}

Method One - coap_io_process_with_fds

#include <coap3/coap.h>

int
main(int argc, char *argv[]) {

  coap_context_t *ctx = NULL;
  unsigned wait_ms;
  fd_set readfds;
  int nfds = 0;
  /* Remove (void) definition if variable is used */
  (void)argc;
  (void)argv;

  /* Initialize libcoap library */
  coap_startup();

  /* Create the libcoap context */
  ctx = coap_new_context(NULL);
  if (!ctx) {
    exit(1);
  }
  /* See coap_block(3) */
  coap_context_set_block_mode(ctx,
                              COAP_BLOCK_USE_LIBCOAP | COAP_BLOCK_SINGLE_BODY);


  FD_ZERO(&readfds);
  /* Set up readfds and nfds to handle other non libcoap FDs */

  /* Other Set up Code */

  wait_ms = COAP_RESOURCE_CHECK_TIME * 1000;

  while (1) {
    int result = coap_io_process_with_fds(ctx, wait_ms, nfds, &readfds, NULL, NULL);
    if (result < 0) {
      /* There is an internal issue */
      break;
    }
    /* Check if set non libcoap FDs and process accordingly */

    /* Do any other housekeeping */
  }
  coap_free_context(ctx);
  coap_cleanup();

  /* Do any other cleanup */

  exit(0);

}

Method Two - select() based on monitorable file descriptor

#include <coap3/coap.h>

#include <errno.h>

int
main(int argc, char *argv[]) {

  coap_context_t *ctx = NULL;
  int coap_fd;
  fd_set m_readfds;
  int nfds;
  /* Remove (void) definition if variable is used */
  (void)argc;
  (void)argv;

  /* Initialize libcoap library */
  coap_startup();

  /* Create the libcoap context */
  ctx = coap_new_context(NULL);
  if (!ctx) {
    exit(1);
  }
  /* See coap_block(3) */
  coap_context_set_block_mode(ctx,
                              COAP_BLOCK_USE_LIBCOAP | COAP_BLOCK_SINGLE_BODY);

  coap_fd = coap_context_get_coap_fd(ctx);
  if (coap_fd == -1) {
    /* epoll is not supported */
    exit(1);
  }
  FD_ZERO(&m_readfds);
  FD_SET(coap_fd, &m_readfds);
  nfds = coap_fd + 1;

  /* Other Set up Code */

  while (1) {
    fd_set readfds = m_readfds;
    int result;
    /* Wait until any i/o takes place */
    result = select(nfds, &readfds, NULL, NULL, NULL);
    if (result == -1) {
      if (errno != EAGAIN) {
        coap_log_debug("select: %s (%d)\n", coap_socket_strerror(), errno);
        break;
      }
    }
    if (result > 0) {
      if (FD_ISSET(coap_fd, &readfds)) {
        result = coap_io_process(ctx, COAP_IO_NO_WAIT);
        if (result < 0) {
          /* There is an internal issue */
          break;
        }
      }
    }
    /* Do any other housekeeping */
  }
  coap_free_context(ctx);
  coap_cleanup();

  /* Do any other cleanup */

  exit(0);

}

Method Two - epoll_wait() based on monitorable file descriptor

#include <coap3/coap.h>

#include <sys/epoll.h>

#include <errno.h>

#define MAX_EVENTS 10

int
main(int argc, char *argv[]) {

  coap_context_t *ctx = NULL;
  int coap_fd;
  int epoll_fd;
  struct epoll_event ev;
  struct epoll_event events[MAX_EVENTS];
  int nevents;
  int i;
  /* Remove (void) definition if variable is used */
  (void)argc;
  (void)argv;

  /* Initialize libcoap library */
  coap_startup();

  /* Create the libcoap context */
  ctx = coap_new_context(NULL);
  if (!ctx) {
    exit(1);
  }
  /* See coap_block(3) */
  coap_context_set_block_mode(ctx,
                              COAP_BLOCK_USE_LIBCOAP | COAP_BLOCK_SINGLE_BODY);

  coap_fd = coap_context_get_coap_fd(ctx);
  if (coap_fd == -1) {
    exit(1);
  }
  epoll_fd = epoll_create1(0);
  if (epoll_fd == -1) {
    exit(2);
  }
  ev.events = EPOLLIN;
  ev.data.fd = coap_fd;
  if (epoll_ctl(epoll_fd, EPOLL_CTL_ADD, coap_fd, &ev) == -1) {
    exit(3);
  }

  /* Other Set up Code */

  while (1) {
    int result;
    /* Wait until any i/o takes place */
    nevents = epoll_wait(epoll_fd, events, MAX_EVENTS, -1);
    if (nevents == -1) {
      if (errno != EAGAIN) {
        coap_log_debug("epoll_wait: %s (%d)\n", coap_socket_strerror(), errno);
        break;
      }
    }
    for (i = 0; i < nevents; i++) {
      if (events[i].data.fd == coap_fd) {
        result = coap_io_process(ctx, COAP_IO_NO_WAIT);
        if (result < 0) {
          /* There is an internal issue */
          break;
        }
      } else {
        /* Process other events */
      }
    }
    /* Do any other housekeeping */
  }

  if (epoll_ctl(epoll_fd, EPOLL_CTL_DEL, coap_fd, &ev) == -1) {
    coap_log_debug("epoll_ctl: %s (%d)\n", coap_socket_strerror(), errno);
  }
  coap_free_context(ctx);
  coap_cleanup();

  /* Do any other cleanup */

  exit(0);

}

SEE ALSO

coap_block(3), coap_context(3), coap_init(3) and coap_supported(3)

FURTHER INFORMATION

See

"RFC7252: The Constrained Application Protocol (CoAP)"

for further information.

BUGS

Please raise an issue on GitHub at https://github.com/obgm/libcoap/issues to report any bugs.

Please raise a Pull Request at https://github.com/obgm/libcoap/pulls for any fixes.

AUTHORS

The libcoap project <libcoap-developers@lists.sourceforge.net>