PSet4, recover.c - file returns NULL even if the program is working

Question

I'm on PSet4 right now, implementing recovery.c. It seems to work all right but the only problem I'm having is that when checking if the opened "card.raw" returns NULL, the condition returns true. However, the file seems to be open as the program generates 16 JPGs just as described in PSet. Not really sure what's the cause and how to fix this. Please take a look at my code:

#include<stdio.h>
#include <sys/stat.h>
#include <stdint.h>
#include <stdlib.h>



// struct to check file size
struct stat st;


int main(int argc, char* argv[])
{
    FILE* card = fopen("card.raw", "r");
    if (card == NULL);
    {
        printf("Could not open the file.\n");
    }
    unsigned char buffer[512];
    int filecount = 0;

    // check filesize in bytes
    stat("card.raw", &st);
    int fileLength = st.st_size;

    int blocksNumber;

    // set a number of blocks to search in
    if (fileLength % 512 == 0)
        blocksNumber = fileLength / 512;
    else
        blocksNumber = fileLength / 512 + 1;

    int found = 0;
    FILE* recovered;
    char filename[8];



    for (int i = 0; i <= blocksNumber; i++)
    {
        fread(&buffer, 512, 1, card);

        // if that's a consecutive file found
        if (((buffer[0] == 0xff && buffer[1] == 0xD8 && buffer[2] == 0xff && buffer[3] == 0xe0) || ((buffer[0] == 0xff && buffer[1] == 0xD8 && buffer[2] == 0xff && buffer[3] == 0xe1))) && found == 1)
        {
            fclose(recovered);
            sprintf(filename, "%03d.jpg", filecount);
            recovered  = fopen(filename, "w");
            filecount += 1; 
        }

        // if that's the first file found
        if (((buffer[0] == 0xff && buffer[1] == 0xD8 && buffer[2] == 0xff && buffer[3] == 0xe0) || ((buffer[0] == 0xff && buffer[1] == 0xD8 && buffer[2] == 0xff && buffer[3] == 0xe1))) && found == 0)
        {   
            sprintf(filename, "%03d.jpg", filecount);
            recovered = fopen(filename, "w");
            found = 1;
            filecount += 1;
        }



        if (found == 1)
            fwrite(&buffer, 512, 1, recovered);
    }

    fclose(recovered);
}

Thanks a lot!

Answer 1

Easy fix. Look at this line: if (card == NULL); You have a stray semicolon at the end. That basically means 'if(something is true) do nothing' because the semicolon terminates the if statement. The code in the curly braces that follow will execute no matter what because it is just the next block of code. It has no if statement controlling it. Remove the semicolon to fix it.

Also, remember to add a return statement inside the if() code block to terminate the program on failure.

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