PSET4: Recover, unable to solve the errors

Question

Questions:

1. I don't know why I'm getting the first error, I used unsigned char for file1.
2. I didn't understand how sprintf(file1, "%03i.jpg",2); FILE *img = fopen(file1, "w"); will create a jpg file.

Please help me out with these questions. Thank you! :)

//I'm trying to print out one jpg image first and then worry about the looping and all.

#include <stdio.h>
#include <stdlib.h>

#include "bmp.h"

int main(int argc, char *argv[])
{
    //these are the initial checks to check if the arguments are correct and if the file exists.
    if (argc != 2)
    {
        fprintf(stderr, "Usage: ./recover file_name\n");
        return 1;
    }

    JPEGFILE buffer; //buffer inptr
    FILE *corruptptr = fopen(argv[1], "r");

    if (corruptptr == NULL)
    {
        fprintf(stderr, "Error: File '%s' not found.\n", argv[1]);
        return 2;
    }
    int check_return = -15; // 1 if it reads, 0 if it doesn't, so we need some other number.
    int ctr = 0, y = 0;
    do
    {
        //Doing this in two loops. First time, I'll just check malloc size.
        check_return = fread(&buffer, sizeof(JPEGFILE), 1, corruptptr);

        if (buffer.b1 == 0xff && buffer.b2 == 0xd8 && buffer.b3 == 0xff && (buffer.b4 & 0xf0) == 0xe0)
            y ++;

        if (y == 1)
            ctr++; //ctr checks the number of 512 byte blocks we have to copy.

    }while(y != 2); //I'm just trying to get one picture, will try to loop over later.

    fseek(corruptptr, -(ctr * sizeof(JPEGFILE)), SEEK_CUR); //we're taking the pointer back to rerun the second time.
    unsigned char* file1 = malloc(ctr * sizeof(JPEGFILE)); //

    sprintf(file1, "%03i.jpg",2);
    FILE *img = fopen(file1, "w");

    //second do-while is to write (first was to get the size for malloc)
    for (int i = 0; i < ctr; i--)
    {
        fread (&buffer, sizeof(JPEGFILE), 1, corruptptr);
        fwrite (&img, sizeof(JPEGFILE), 1, buffer);
    }


    //do the print of JPEG.

    printf("%d\n",check_return);
    fclose(corruptptr);

    printf("All's good.\n");
    free(file1);
}

Errors:

~/workspace/pset4/recover/ $ make recover
clang -fsanitize=signed-integer-overflow -fsanitize=undefined -ggdb3 -O0 -std=c11 -Wall -Werror -Wextra
-Wno-sign-compare -Wshadow    recover.c  -lcrypt -lcs50 -lm -o recover recover.c:44:13: error: passing 'unsigned char *' to
parameter of type 'char *' converts between pointers to integer types with different sign
  [-Werror,-Wpointer-sign]
sprintf(file1, "%03i.jpg",2);
        ^~~~~
/usr/include/stdio.h:364:38: note: passing argument to parameter '__s' here
extern int sprintf (char *__restrict __s,
                                 ^
recover.c:45:23: error: passing 'unsigned char *' to parameter of type 'const char *' converts between pointers to integer
types with different
  sign [-Werror,-Wpointer-sign]
FILE *img = fopen(file1, "w");
                  ^~~~~
/usr/include/stdio.h:272:44: note: passing argument to parameter '__filename' here
extern FILE *fopen (const char *__restrict __filename,
                                       ^
recover.c:51:44: error: passing 'JPEGFILE' to parameter of incompatible type 'FILE *' (aka 'struct _IO_FILE *')
    fwrite (&img, sizeof(JPEGFILE), 1, buffer);
                                       ^~~~~~
/usr/include/stdio.h:716:38: note: passing argument to parameter '__s' here
                  size_t __n, FILE *__restrict __s);
                                               ^
3 errors generated.
make: *** [recover] Error 1

Answer 1

sprintf(file1, "%03i.jpg",2);
FILE *img = fopen(file1, "w");

These two lines will open a jpeg file for output. Each has a separate purpose.

The first line does nothing more than build a char string that will be the file name. It really doesn't have anything to do with a file yet. file1 is a var that will contain the string name. "%03i.jpg" is the pattern used to fabricate the actual string. %03i is both a placeholder and a pattern descriptor. It says to build a number with leading zeros that is 3 digits long, using an int. The int comes from the 2, or from an int var that you could replace the 2 with.

Once the filename is built and stored in file1, the next line opens a file with the name stored in file1. It is opened in w, or write, mode.

When file1 is declared, it only needs to be declared as a char array with enough length to hold the 7 chars needed for the file name pattern plus 1 for the end of string marker. The current declaration in the code above is far too much space.

There are more issues, but this answers your question.

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