Mix with libsndfile and libao

/*
** gcc -o mix mix.c -lao -lsndfile -lm
**
*/

#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <string.h>
#include <math.h>
#include <ao/ao.h>
#include <sndfile.h>

#define BUFFSIZE 512

int playfiles (int, FILE *[], int[]);

int main(int argc, char *argv[])
{
    int filesc = (argc - 1) / 2;
    FILE *fp[filesc];
    int starttime[filesc];

    if (argc < 2 || (argc - 1) % 2 != 0) {
        printf("usage: %s file1 startsec1 file2 startsec2 fileN startsecN...\n", argv[0]);
        exit(1);
    }
    for(int i=0; i < filesc; i++) {
        fp[i] = fopen(argv[i * 2 + 1], "rb");
        starttime[i] = atoi(argv[i * 2 + 2]);
        if (fp[i] == NULL)  {
            printf("Cannot open %s.\n", argv[i+1]);
            exit(-1);
        }
    }

    ao_initialize ();
    playfiles (filesc, fp, starttime);
    ao_shutdown ();

    return 0;
}

int playfiles (int filesc, FILE *fp[], int starttime[])
{
    int default_driver;
    ao_device *device;
    ao_sample_format format;
    SNDFILE *sndfile[filesc];
    SF_INFO sf_info[filesc];
    int startframe[filesc];
    float gain = 1.0f / filesc;
    float *filebuffer;
    float *mixbuffer;
    short *outbuffer;

    int buflen;
    int toread = 0;

    for(int i=0; i < filesc; i++) {
        sndfile[i] = sf_open_fd(fileno(fp[i]), SFM_READ, &sf_info[i], 1);
        toread += sf_info[i].frames * sf_info[i].channels;
        // convert starttime to startframe
        startframe[i] = (int) (((float) (sf_info[0].samplerate * sf_info[0].channels) / (float) BUFFSIZE) * starttime[i]);
        printf ("File %d start time %d sec, %d frame to skip\n", i, starttime[i], startframe[i]);
    }

    // all files MUST be in the same format / channels
    buflen = BUFFSIZE * sf_info[0].channels;
    filebuffer = malloc(buflen * sizeof(float));
    mixbuffer= malloc(buflen * sizeof(float));
    outbuffer= malloc(buflen * sizeof(short));

    default_driver = ao_default_driver_id();
    memset(&format, 0, sizeof(ao_sample_format));

    format.byte_format = AO_FMT_NATIVE;
    format.bits = 16;
    format.channels = sf_info[0].channels;
    format.rate = sf_info[0].samplerate;

    device = ao_open_live(default_driver, &format, NULL);
    if (device == NULL) {
        printf("Error opening sound device.\n");
        exit(-2);
    }

    while (toread > 0) {
        for(int item = 0; item < buflen; item++) {
            mixbuffer[item] = 0;
        }

        for(int i=0; i < filesc; i++) {
            if (startframe[i] == 0) {
                sf_count_t item_read = sf_read_float (sndfile[i], filebuffer, BUFFSIZE); // WHY BUFFSIZE? Shouldn't be BUFFSIZE * channels?

                if (item_read > 0) {
                    toread -= item_read;

                    // mix samples
                    for(int item = 0; item < item_read; item++) {
                        mixbuffer[item] += (filebuffer[item] * gain);
                    }
                }
            } else {
                startframe[i]--;
            }
        }

        // convert from float to short
        for(int item = 0; item < buflen; item++) {
            int value = mixbuffer[item] * 32768;

            // limit to valid short values
            if (value > 32767)
                value = 32767;
            else if (value < -32768)
                value = -32768;

            outbuffer[item] =  (short) value;
        }

        ao_play(device, (char *)outbuffer, buflen);
    }

    ao_close(device);
    for(int i=0; i < filesc; i++) {
        sf_close(sndfile[i]);
    }

    free(filebuffer);
    free(mixbuffer);
    free(outbuffer);
}