Alt Package <alt.h>

The alt.h functions allow a program to detect which of a group of input channel becomes ready first.

There are two sets of functions: the nowait functions return a status value if none of the specified channels is ready to communicate; the others wait until a channel becomes ready.

There are two ways to specify which channels are to be tested: the vec functions use an array of pointers to the channels; the others use a variable-length

argument list of pointers to channels.

The alt.h functions may be used only on virtual or internal channels. They cannot be used directly on physical channels that run over a hardware link.

If you need to use the alt.h functions on a physical channel, consider adding an intermediate guard thread to echo messages from the physical channel to an internal channel, as shown below.

#include <chan.h>

#define SIZE 64

struct map {

   CHAN *phys_chan;

   CHAN *internal_chan;

};

CHAN internal0, internal1;

void guard(void *arg) {

   struct map *s = (struct map *)arg;

   char buf[SIZE];

   for (;;) {

      chan_in_message(SIZE, buf, s->phys_chan);

      chan_out_message(SIZE, buf, s->internal_chan);

   }

}

main(int argc, char *argv[], char *envp[],

     CHAN *in[], int ins, CHAN *out[], int outs) {

   struct map s0, s1;

   s0.phys_chan = in[0]; s0.internal_chan = &internal0;

   s1.phys_chan = in[1]; s1.internal_chan = &internal1;

   chan_init(&internal0); thread_new(guard, 1024, &s0);

   chan_init(&internal1); thread_new(guard, 1024, &s1);

   for (;;) {

      int i = alt_wait(2, &internal0, &internal1);

      ...

   }

}

Here, the alt_wait function is applied to two internal channels, internal0 and internal1, but the effect is to alt on the underlying ports, in[0] and in[1], which may be physical channels. Clearly, to echo them properly to the internal channels, guard threads must know the format of the expected messages.