mixer.c

在linux下多媒体开发实例linux下多媒体开发

/*
 * mixer.c
 * Example of a simple mixer program.
 */

#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>
#include <sys/ioctl.h>
#include <fcntl.h>
#include <linux/soundcard.h>

/* names of available mixer devices */
const char *sound_device_names[] = SOUND_DEVICE_NAMES;

int fd;                  /* file descriptor for mixer device */
int devmask, stereodevs; /* bit masks of mixer information */
char *name;              /* program name */

/* display command usage and exit with error status */
void usage()
{
  int i;

  fprintf(stderr, "usage: %s <device> <left-gain%%> <right-gain%%>\n"
	  "       %s <device> <gain%%>\n\n"
	  "Where <device> is one of:\n", name, name);
  for (i = 0 ; i < SOUND_MIXER_NRDEVICES ; i++)
    if ((1 << i) & devmask) /* display only valid devices */
      fprintf(stderr, "%s ", sound_device_names[i]);
  fprintf(stderr, "\n");
  exit(1);
}

int main(int argc, char *argv[])
{
  int left, right, level;  /* gain settings */
  int status;              /* return value from system calls */
  int device;              /* which mixer device to set */
  int i;                   /* general purpose loop counter */
  char *dev;               /* mixer device name */

  /* save program name */
  name = argv[0];

  /* open mixer, read only */
  fd = open("/dev/mixer", O_RDONLY);
  if (fd == -1) {
    perror("unable to open /dev/mixer");
    exit(1);
  }
  
  /* get needed information about the mixer */
  status = ioctl(fd, SOUND_MIXER_READ_DEVMASK, &devmask);
  if (status == -1)
    perror("SOUND_MIXER_READ_DEVMASK ioctl failed");
  status = ioctl(fd, SOUND_MIXER_READ_STEREODEVS, &stereodevs);
  if (status == -1)
    perror("SOUND_MIXER_READ_STEREODEVS ioctl failed");

  /* check that user passed two or three arguments on command line */
  if (argc != 3 && argc != 4)
    usage();

  /* save mixer device name */
  dev = argv[1];

  /* figure out which device to use */
  for (i = 0 ; i < SOUND_MIXER_NRDEVICES ; i++)
    if (((1 << i) & devmask) && !strcmp(dev, sound_device_names[i]))
      break;
  if (i == SOUND_MIXER_NRDEVICES) { /* didn't find a match */
    fprintf(stderr, "%s is not a valid mixer device\n", dev);
    usage();
  }

  /* we have a valid mixer device */
  device = i;

  /* get gain values */
  if (argc == 4) {
    /* both left and right values given */
    left  = atoi(argv[2]);
    right = atoi(argv[3]);
  } else {
    /* left and right are the same */
    left  = atoi(argv[2]);
    right = atoi(argv[2]);
  }
  
  /* display warning if left and right gains given for non-stereo device */
  if ((left != right) && !((1 << i) & stereodevs)) {
    fprintf(stderr, "warning: %s is not a stereo device\n", dev);
  }
  
  /* encode both channels into one value */
  level = (right << 8) + left;
  
  /* set gain */
  status = ioctl(fd, MIXER_WRITE(device), &level);
  if (status == -1) {
    perror("MIXER_WRITE ioctl failed");
    exit(1);
  }

  /* unpack left and right levels returned by sound driver */
  left  = level & 0xff;
  right = (level & 0xff00) >> 8;

  /* display actual gain setting */
  fprintf(stderr, "%s gain set to %d%% / %d%%\n", dev, left, right);

  /* close mixer device and exit */
  close(fd);
  return 0;
}