rpoll(3) BEGEMOT Library rpoll(3)


rpoll - callback functions for file descriptors and timers


# include <rpoll.h>

typedef void (*poll_f)(int fd , int mask , void *arg );


typedef void (*timer_f)(int tid , void *arg );

int poll_register(int fd , poll_f func , void *arg , int mask );

void poll_unregister(int handle );

int poll_start_timer(u_int msecs , int repeat , timer_f func , void *arg );

void poll_stop_timer(int handle );

int poll_start_utimer(unsigned long long usecs , int repeat , timer_f func , void *arg );

void poll_dispatch(int wait );


Many programs need to read from several file descriptors at the same time. Typically in these programs one of select(3c) or poll(2) is used. These calls are however clumsy to use and the usage of one of these calls is probably not portable to other systems - not all systems support both calls.

The rpoll(l) family of functions is designed to overcome these restrictions. They support the well known and understood technique of event driven programing and, in addition to select(3c) and poll(2) also support timers.

Each event on a file descriptor or each timer event is translated into a call to a user defined callback function. These functions need to be registered. A file descriptor is registered with poll_register. fd is the file descriptor to watch, mask is an event mask. It may be any combination of POLL_IN to get informed when input on the file descriptor is possible, POLL_OUT to get informed when output is possible or POLL_EXCEPT to get informed when an exceptional condition occures. An example of an exceptional condition is the arrival of urgent data. (Note, that an end of file condition is signaled via POLL_IN). func is the user function to be called and arg is a user supplied argument for this function. The callback functions is called with the file descriptor, a mask describing the actual events (from the set supplied in the registration) and the user argument. poll_register returns a handle, which may be used later to de-register the file descriptor. A file descriptor may be registered more than once, if the function, the user arguments or both differ in the call to poll_register. If func and arg are the same, then no new registration is done, instead the event mask of the registration is changed to reflect the new mask.

A registered file descriptor may be de-registered by calling poll_unregister with the handle returned by poll_register.

A timer is created with poll_start_timer or poll_start_utimer. msecs is the number of milliseconds in poll_start_timer while usecs is the number of microseconds in poll_start_utimer, after which the timer event will be generated. If the functions use the poll(2) system call, then usecs is rounded to milliseconds and poll_start_timer is called. repeat selects one-short behavior (if 0) or a repeatable timer (if not 0). A one-short timer will automatically unregistered after expiry. func is the user function which will be called with a timer id and the user supplied arg. poll_start_timer and poll_start_utimer return a timer id, which may be used to cancel the timer with poll_stop_timer. A one-short timer should be canceled only if it has not yet fired.

poll_dispatch must be called to actually dispatch events. wait is a flag, which should be 0, if only a poll should be done. In this case, the function returns, after polling the registered file descriptors and timers. If wait is not 0, poll_dispatch waits until an event occures. All events are dispatch (i.e. callback functions called) and poll_dispatch returns.

Typical use is:



poll(2), select(3C)


poll_register , poll_start_timer and poll_start_utimer return a handle which may be used to unregister the file descriptor or cancel the timer.

Both functions and poll_dispatch call xrealloc(l) and can end in panic(l).


System call or memory allocation errors are fatal and are handle by calling panic(l). The one exception is a return of EINTR from select(3c) or poll(2) in poll_dispatch. In this case poll_dispatch simply returns.


Obscure sequences of poll_start_timer and poll_stop_timer in callback functions may probably break the code.

The semantics of POLL_EXCEPT are not clear.


Hartmut Brandt, harti@freebsd.org
8 Dec 2006 BEGEMOT