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

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, "i.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, "i.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.

CodePudding user response：

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

CodePudding user response：

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.

fread(&buffer, sizeof buffer, 1, input) == 1)