ftp.nice.ch/pub/next/unix/audio/sndutil.1.3.s.tar.gz#/sndutil-1.3/sndtools/sndsum.c

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/*
 * sndsum.c
 *	add one sound to another
 */
#import <stdio.h>
#import <sound/sound.h>
#import <architecture/byte_order.h>
#import <c.h>

static void fails(char *s, char *s2)
{
    printf("sndsum: ");	/* error message  */
    printf(s,s2);
    printf("\n\n");
    exit(1);			/* error exit */
}

void main (int argc, char *argv[])
{
    int err, nchans, sampleCount;
    SNDSoundStruct *sndin1;
    SNDSoundStruct *sndin2;
    FILE *outstream;
    short *data1;
    short *data2;

    if (argc != 4) {
	fprintf(stderr,"%s - add one sound to another\n",argv[0]);
	fprintf(stderr, "Usage: sndsum in1.snd in2.snd out.snd\n");
	fprintf(stderr, "       (out = in1 + in2)\n");
	exit(1);
    }

/*
 * Input sound file
 */
    err = SNDReadSoundfile(argv[1], &sndin1);
    if (err) {
	fprintf(stderr, "Cannot read soundfile: %s\n", argv[1]);
	exit(1);
    }

    err = SNDReadSoundfile(argv[2], &sndin2);
    if (err) {
	fprintf(stderr, "Cannot read soundfile: %s\n", argv[2]);
	exit(1);
    }

    nchans = sndin1->channelCount;
    if (sndin1->dataFormat != SND_FORMAT_LINEAR_16
	|| sndin1->dataFormat != sndin2->dataFormat) {
	fprintf(stderr, "Both soundfiles must be 16 bit linear\n");
	exit(1);
    }
    sampleCount = sndin1->dataSize / sizeof(short); /* channel independent */
    sampleCount = MIN(sampleCount,sndin2->dataSize / sizeof(short));
    data1 = (short *) ((char *)sndin1 + sndin1->dataLocation);
    data2 = (short *) ((char *)sndin2 + sndin2->dataLocation);
    {
      register short *sp1 = data1;
      register short *sp2 = data2;
      register int i;
      for (i = 0; i < sampleCount; i++) {
	*sp1 = (signed short) NXSwapHostShortToBig((signed short)
       NXSwapBigShortToHost(*sp1) + (signed short) NXSwapBigShortToHost(*sp2));
	sp1++;
	sp2++;
      }
    }	  

/*
 * Output sound file
 */
    if (NULL == (outstream = fopen(argv[3],"w")))
      fails("could not open output file '%s'", argv[3]);
    if (SNDWriteHeader(fileno(outstream), sndin1))
      fails("could not write output file '%s'", argv[3]);
    fwrite(data1, sizeof(short), sampleCount, outstream);
}

These are the contents of the former NiCE NeXT User Group NeXTSTEP/OpenStep software archive, currently hosted by Netfuture.ch.