CS50 Lab4: Volume--about malloc( ) - c

Question

I am working on the CS50 Intro to CS course--Lab 4: Volume. And I think I have some misunderstanding with the molloc function. At first I used the malloc function to allocate space for the header part as below:
// TODO: Copy header from input file to output file
uint8_t *header = malloc(HEADER_SIZE);
if (header == NULL)
{
return 2;
}
fread(header, sizeof(header), 1, input);
fwrite(header, sizeof(header), 1, output);
free(header);
The program managed to compile but I think it didn't copy the header properly. Then I tried this:
// TODO: Copy header from input file to output file
uint8_t header[HEADER_SIZE];
fread(header, sizeof(header), 1, input);
fwrite(header, sizeof(header), 1, output);
And that worked, but I don't know what changes I have made by doing so. I just see this two kind of codes as having the same effect. Can somebody tell me the difference and what mistakes I made? I mean if I really want to use malloc function, what the codes should be like? Below is my full successful code for volumn.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]);
// TODO: Copy header from input file to output file
uint8_t header[HEADER_SIZE];
fread(header, sizeof(header), 1, input);
fwrite(header, sizeof(header), 1, output);
// TODO: Read samples from input file and write updated data to output file
int16_t buffer;
while (fread(&buffer, sizeof(buffer), 1, input))
{
buffer *= factor;
fwrite(&buffer, sizeof(buffer), 1, output);
}
// Close files
fclose(input);
fclose(output);
}

Answer 1

While working with this lab I also encountered similar problem. I was able to solve it without using malloc but I wanted to write another solution incorporating it.
The problem with your malloc is that you are not providing the correct argument.
It is given that each byte of header is an uint8_t (8-bit unsigned int). So the size should be sizeof(uint8_t) * HEADER_SIZE. Check out an example here.
Therefore the correct way is:
uint8_t *header = malloc(sizeof(uint8_t) * HEADER_SIZE);
instead of uint8_t *header = malloc(HEADER_SIZE);
Also you should not check for header == NULL in the next line as we just initialized this pointer and it doesn't matter what's present there.
Thanks for the question and hope my answer helps!