CS50 recover, when to use buffer and &buffer? - c

Question

I am solved the pset4 recover in the CS50. Although I solved the problem, I am confuse between "buffer" & "&buffer". Please pay attention to "LINE AAAAA" & "LINE BBBBB".
#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
// New type to store a byte of data
typedef uint8_t BYTE;
// Number of "block size" 512
const int BLOCK_SIZE = 512;
int main(int argc, char *argv[])
{
// Check for 2 command-line arguments
if (argc != 2)
{
printf("Usage: ./recover IMAGE\n");
return 1;
}
// Open card.raw file
FILE *input = fopen(argv[1],"r");
// Check for fail to open
if (input == NULL)
{
printf("Couldn't open the file.\n");
return 1;
}
// Read the first 4 bytes
BYTE buffer[BLOCK_SIZE];
// Count image
int count_image = 0;
// Assign NULL to output_file
FILE *output = NULL;
// Declare filename
char filename[8];
// LINE AAAAA
while (fread(buffer, sizeof(BYTE), BLOCK_SIZE, input) == BLOCK_SIZE)
{
// Check first 4 bytes for JPEG file format
if (buffer[0] == 0xff && buffer[1] == 0xd8 && buffer[2] == 0xff && (buffer[3] & 0xf0) == 0xe0)
{
// Only for first image, generate new filename and write into it
if (count_image == 0)
{
// Generate a new file with sequence name
sprintf(filename, "%03i.jpg", count_image);
// Open new file
output = fopen(filename, "w");
// LINE BBBBB
fwrite (&buffer, sizeof(BYTE), BLOCK_SIZE, output);
// Add filename counter
count_image++;
}
// For subsequence new repeat JPG images
// Close output file, generate new file, and write into it
else if (count_image > 0)
{
// fclose current writing files
fclose (output);
// Generate a new file with sequence name
sprintf(filename, "%03i.jpg", count_image);
// Open new file
output = fopen(filename, "w");
// LINE BBBBB
fwrite (&buffer, sizeof(BYTE), BLOCK_SIZE, output);
// Add filename counter
count_image++;
}
}
// Not fulfill the 4 bytes JPG condition, keep writing to the same filename
else if (count_image > 0)
{
// LINE BBBBB
fwrite (&buffer, sizeof(BYTE), BLOCK_SIZE, output);
}
}
fclose(output);
fclose(input);
}
Question:
Why do we use "&buffer" in LINE BBBBB instead of "buffer"?
I know LINE AAAAA is using "buffer" as BYTE buffer[BLOCK_SIZE] is an pointer or array. So "buffer" mean the location of the pointer.

Answer 1

When buffer is used, it often converts to the address of the first element. &buffer is the address of the array.
Those 2 addresses will compare equal, yet have different types. When the type is important, use the matching one. Enable all warnings to help identify incorrect type usage.
Since fread() use void *, either will work.
Conceptually use buffer in this case to match the sizeof(BYTE) and BLOCK_SIZE.
or
fread(&buffer, sizeof buffer, 1, input) == 1)

Answer 2

fwrite(buffer, ...) will work fine.