CS50 Week 4, Lab 4 - Volume. fputc vs fwrite - c

Question

I'm trying to copy wav samples using two slightly different ways here. First way gives me correct output file sound and the second one - distorted sound:
fwrite case:
int16_t samp;
while (fread(&samp, sizeof(int16_t), 1, input))
{
samp *= factor;
fwrite(&samp, sizeof(int16_t), 1, output);
}
fputc case:
int16_t samp;
while ((samp = fgetc(input)) != EOF)
{
samp *= factor;
fputc(samp, output);
}
I already know that it happens exactly because of samp *= factor; in second case. Volume properly changes when I use numbers without a decimal point, and sound gets distorted when I use floating-point numbers.
So my question is: why it works in the fwrite case, and doesn't work in fputc case? Both of them are int being multiplied by a float. How does it even work in a first case?
Here below is a whole program for reference:
// Modifies the volume of an audio file
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
// Number of bytes in .wav header
const int HEADER_SIZE = 44;
int main(int argc, char *argv[])
{
// Check command-line arguments
if (argc != 4)
{
printf("Usage: ./volume input.wav output.wav factor\n");
return 1;
}
// Open files and determine scaling factor
FILE *input = fopen(argv[1], "r");
if (input == NULL)
{
printf("Could not open file.\n");
return 1;
}
FILE *output = fopen(argv[2], "w");
if (output == NULL)
{
printf("Could not open file.\n");
return 1;
}
float factor = atof(argv[3]);
// Copy header from input file to output file
uint8_t *hder = malloc(sizeof(uint8_t) * HEADER_SIZE);
fread(hder, sizeof(uint8_t), HEADER_SIZE, input);
fwrite(hder, sizeof(uint8_t), HEADER_SIZE, output);
free(hder);
// Read samples from input file and write updated data to output file
int16_t samp;
while (fread(&samp, sizeof(int16_t), 1, input))
{
samp *= factor;
fwrite(&samp, sizeof(int16_t), 1, output);
}
// while ((samp = fgetc(input)) != EOF)
// {
// samp *= factor;
// fputc(samp, output);
// }
// Close files
fclose(input);
fclose(output);
}

Answer 1

From the fputc docs on cppreference.com:
Internally, the character is converted to unsigned char just before being written.
When you call fputc your 16-bit samp is being converted to an 8-bit unsigned char. With fwrite, both bytes are being written to the output file.