Q:

About fwrite() errors in C

Reads the file and multiplies the data by 2. After that, I wrote a
program that writes to another file. This file is a 16-bit file. By
the way, only a certain number is written to the created file. I do
not know why this is happening. Please help me.
C (visual studio 2017) 
#define _CRT_SECURE_NO_WARNINGS
#define SIZE 16000
typedef short data_type;

#include<stdio.h>
#include<stdlib.h>
#include<string.h>

int main() {

    FILE *fd, *fd2;
    data_type *data;
    int n;

    data = (data_type*)malloc(sizeof(data_type) * SIZE);
    for (int i = 0; i < SIZE; i++)
        data[i] = 0;

    if ((fd = fopen("C:\\Users\\SeoHyeon\\Documents\\test16kSam16bMono.pcm", "r")) == NULL)
        printf("Error opening file1");
    if ((fd2 = fopen("C:\\Users\\SeoHyeon\\Documents\\test16kSam16bMono2.pcm", "w")) == NULL)
        printf("Error opening file2");

    n = fread(data, sizeof(data_type), SIZE, fd);

    for (int i = 0; i < SIZE; i++)
        data[i] = data[i] * 2.0;

    if (fwrite(data, sizeof(data_type), SIZE, fd2) != SIZE) {
        printf("Error writing to file.\n");
        exit(1);
    }

    fclose(fd);
    fclose(fd2);

    free(data);

    return 0;
}

A:

It could be that the program is not opening both the files in binary mode.
For example:
FILE *fin = fopen("input_file.bin", "rb");  // <-- Note the "rb"

If your file is opened in text mode, which is the default, if there's an EOF character in the data, the file-input could close prematurely.
EDIT: Also, you should handle the error when your file-handles fd and fd2 are NULL.  There's also a couple of other error conditions you aren't checking for, but I leave these as an exercise for the reader.

