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This is not production ready code. There’s a lot of error checking that’s not being done. But this should at least give you an idea of how to begin using the PulseAudio asynchronous API.

To compile this program, you will need to link against the pulse library. For example, if you save this program as "pulsedevicelist.c", you should run: gcc -Wall -o pulsedevicelist pulsedevicelist.c -lpulse

#include <stdio.h>
#include <string.h>
#include <pulse/pulseaudio.h>

// Field list is here: http://0pointer.de/lennart/projects/pulseaudio/doxygen/structpa__sink__info.html
typedef struct pa_devicelist {
        uint8_t initialized;
        char name[512];
        uint32_t index;
        char description[256];
} pa_devicelist_t;

void pa_state_cb(pa_context *c, void *userdata);
void pa_sinklist_cb(pa_context *c, const pa_sink_info *l, int eol, void *userdata);
void pa_sourcelist_cb(pa_context *c, const pa_source_info *l, int eol, void *userdata);
int pa_get_devicelist(pa_devicelist_t *input, pa_devicelist_t *output);


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

    // This is where we'll store the input device list
    pa_devicelist_t pa_input_devicelist[16];

    // This is where we'll store the output device list
    pa_devicelist_t pa_output_devicelist[16];

    if (pa_get_devicelist(pa_input_devicelist, pa_output_devicelist) < 0) {
        fprintf(stderr, "failed to get device list\n");
        return 1;
    }

    for (ctr = 0; ctr < 16; ctr++) {
        if (! pa_output_devicelist[ctr].initialized) {
            break;
        }
        printf("=======[ Output Device #%d ]=======\n", ctr+1);
        printf("Description: %s\n", pa_output_devicelist[ctr].description);
        printf("Name: %s\n", pa_output_devicelist[ctr].name);
        printf("Index: %d\n", pa_output_devicelist[ctr].index);
        printf("\n");
    }

    for (ctr = 0; ctr < 16; ctr++) {
        if (! pa_input_devicelist[ctr].initialized) {
            break;
        }
        printf("=======[ Input Device #%d ]=======\n", ctr+1);
        printf("Description: %s\n", pa_input_devicelist[ctr].description);
        printf("Name: %s\n", pa_input_devicelist[ctr].name);
        printf("Index: %d\n", pa_input_devicelist[ctr].index);
        printf("\n");
    }
    return 0;
}

int pa_get_devicelist(pa_devicelist_t *input, pa_devicelist_t *output) {
    // Define our pulse audio loop and connection variables
    pa_mainloop *pa_ml;
    pa_mainloop_api *pa_mlapi;
    pa_operation *pa_op;
    pa_context *pa_ctx;

    // We'll need these state variables to keep track of our requests
    int state = 0;
    int pa_ready = 0;

    // Initialize our device lists
    memset(input, 0, sizeof(pa_devicelist_t) * 16);
    memset(output, 0, sizeof(pa_devicelist_t) * 16);

    // Create a mainloop API and connection to the default server
    pa_ml = pa_mainloop_new();
    pa_mlapi = pa_mainloop_get_api(pa_ml);
    pa_ctx = pa_context_new(pa_mlapi, "test");

    // This function connects to the pulse server
    pa_context_connect(pa_ctx, NULL, 0, NULL);

    // This function defines a callback so the server will tell us it's state.
    // Our callback will wait for the state to be ready.  The callback will
    // modify the variable to 1 so we know when we have a connection and it's
    // ready.
    // If there's an error, the callback will set pa_ready to 2
    pa_context_set_state_callback(pa_ctx, pa_state_cb, &pa_ready);

    // Now we'll enter into an infinite loop until we get the data we receive
    // or if there's an error
    for (;;) {
        // We can't do anything until PA is ready, so just iterate the mainloop
        // and continue
        if (pa_ready == 0) {
            pa_mainloop_iterate(pa_ml, 1, NULL);
            continue;
        }
        // We couldn't get a connection to the server, so exit out
        if (pa_ready == 2) {
            pa_context_disconnect(pa_ctx);
            pa_context_unref(pa_ctx);
            pa_mainloop_free(pa_ml);
            return -1;
        }
        // At this point, we're connected to the server and ready to make
        // requests
        switch (state) {
            // State 0: we haven't done anything yet
            case 0:
                // This sends an operation to the server.  pa_sinklist_info is
                // our callback function and a pointer to our devicelist will
                // be passed to the callback The operation ID is stored in the
                // pa_op variable
                pa_op = pa_context_get_sink_info_list(pa_ctx,
                        pa_sinklist_cb,
                        output
                        );

                // Update state for next iteration through the loop
                state++;
                break;
            case 1:
                // Now we wait for our operation to complete.  When it's
                // complete our pa_output_devicelist is filled out, and we move
                // along to the next state
                if (pa_operation_get_state(pa_op) == PA_OPERATION_DONE) {
                    pa_operation_unref(pa_op);

                    // Now we perform another operation to get the source
                    // (input device) list just like before.  This time we pass
                    // a pointer to our input structure
                    pa_op = pa_context_get_source_info_list(pa_ctx,
                            pa_sourcelist_cb,
                            input
                            );
                    // Update the state so we know what to do next
                    state++;
                }
                break;
            case 2:
                if (pa_operation_get_state(pa_op) == PA_OPERATION_DONE) {
                    // Now we're done, clean up and disconnect and return
                    pa_operation_unref(pa_op);
                    pa_context_disconnect(pa_ctx);
                    pa_context_unref(pa_ctx);
                    pa_mainloop_free(pa_ml);
                    return 0;
                }
                break;
            default:
                // We should never see this state
                fprintf(stderr, "in state %d\n", state);
                return -1;
        }
        // Iterate the main loop and go again.  The second argument is whether
        // or not the iteration should block until something is ready to be
        // done.  Set it to zero for non-blocking.
        pa_mainloop_iterate(pa_ml, 1, NULL);
    }
}

// This callback gets called when our context changes state.  We really only
// care about when it's ready or if it has failed
void pa_state_cb(pa_context *c, void *userdata) {
        pa_context_state_t state;
        int *pa_ready = userdata;

        state = pa_context_get_state(c);
        switch  (state) {
                // There are just here for reference
                case PA_CONTEXT_UNCONNECTED:
                case PA_CONTEXT_CONNECTING:
                case PA_CONTEXT_AUTHORIZING:
                case PA_CONTEXT_SETTING_NAME:
                default:
                        break;
                case PA_CONTEXT_FAILED:
                case PA_CONTEXT_TERMINATED:
                        *pa_ready = 2;
                        break;
                case PA_CONTEXT_READY:
                        *pa_ready = 1;
                        break;
        }
}

// pa_mainloop will call this function when it's ready to tell us about a sink.
// Since we're not threading, there's no need for mutexes on the devicelist
// structure
void pa_sinklist_cb(pa_context *c, const pa_sink_info *l, int eol, void *userdata) {
    pa_devicelist_t *pa_devicelist = userdata;
    int ctr = 0;

    // If eol is set to a positive number, you're at the end of the list
    if (eol > 0) {
        return;
    }

    // We know we've allocated 16 slots to hold devices.  Loop through our
    // structure and find the first one that's "uninitialized."  Copy the
    // contents into it and we're done.  If we receive more than 16 devices,
    // they're going to get dropped.  You could make this dynamically allocate
    // space for the device list, but this is a simple example.
    for (ctr = 0; ctr < 16; ctr++) {
        if (! pa_devicelist[ctr].initialized) {
            strncpy(pa_devicelist[ctr].name, l->name, 511);
            strncpy(pa_devicelist[ctr].description, l->description, 255);
            pa_devicelist[ctr].index = l->index;
            pa_devicelist[ctr].initialized = 1;
            break;
        }
    }
}

// See above.  This callback is pretty much identical to the previous
void pa_sourcelist_cb(pa_context *c, const pa_source_info *l, int eol, void *userdata) {
    pa_devicelist_t *pa_devicelist = userdata;
    int ctr = 0;

    if (eol > 0) {
        return;
    }

    for (ctr = 0; ctr < 16; ctr++) {
        if (! pa_devicelist[ctr].initialized) {
            strncpy(pa_devicelist[ctr].name, l->name, 511);
            strncpy(pa_devicelist[ctr].description, l->description, 255);
            pa_devicelist[ctr].index = l->index;
            pa_devicelist[ctr].initialized = 1;
            break;
        }
    }
}