Most efficient way to split an array to write to different files

I’m reading double data in from a DAQ that has 2 channels. Data is stored in read_buf, the first 1000 samples are channel 0 and the second 1000 are channel 1. I have no control over this concatenation of channel data.

I have set up 2 files like so,

FILE *fptr_0;
if ((fptr_0 = fopen("channel_0.bin", "wb")) == NULL)
{
    printf("Error opening file.\n");
    exit(1);
}

FILE *fptr_1;
if ((fptr_1 = fopen("channel_1.bin", "wb")) == NULL)
{
    printf("Error opening file.\n");
    exit(1);
}

I would then like to split read_buf and send the first half to fptr_0 and the second to fptr_1. I can read out the first half but am flummoxed on how to read out the second half. How do I point fptr_1 to just the second half of read_buf?

Do I have to copy each half into a new array?

What I have so far which works for the first half followed by what I’m not getting,

double read_buf[2000];
result = DAQ_func(device, &status, read_buf);
fwrite(&read_buf, sizeof(double), (sizeof(read_buf) / sizeof(read_buf[0])) / 2, fptr_0);
fwrite( ??? , sizeof(double), (sizeof(read_buf) / sizeof(read_buf[0])) / 2, fptr_1);

>Solution :

??? is read_buf + 1000 (no &).

fwrite(read_buf + 1000, sizeof (double), (sizeof read_buf / sizeof read_buf[0]) / 2, fptr_1);

Or if you prefer &read_buf[1000] which is the same.