volume.c

// 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 header[44];
    fread(&header, sizeof(uint8_t), HEADER_SIZE, input);
    fwrite(&header, sizeof(uint8_t), HEADER_SIZE, output);

    // Read samples from input file and write updated data to output file
    // Data is stored in blocks of 2 * 8 bytes, assigning int16_t will cover its length
    int16_t buffer;
    int check = 1;
    while (check)
    {
        // fread() returns number of bytes. Stop if EOF reached
        int successful = fread(&buffer, sizeof(int16_t), 1, input);
        if (!successful)
        {
            check = 0;
            break;
        }
        
        // Multiply times factor and write to filea
        buffer *= factor; 
        fwrite(&buffer, sizeof(int16_t), 1, output);
    }

    // Close files
    fclose(input);
    fclose(output);
}