CS50 恢复不输出任何图像

Question

我正在研究 CS50 课程的恢复计划。以下是说明：

Implement your program in a file called recover.c in a directory called recover.

Your program should accept exactly one command-line argument, the name of a forensic image from which to recover JPEGs.

If your program is not executed with exactly one command-line argument, it should remind the user of correct usage, and main should return 1.

If the forensic image cannot be opened for reading, your program should inform the user as much, and main should return 1.

Your program, if it uses malloc, must not leak any memory.

我认为我的代码应该可以工作，但实际上没有。事实上，它根本不输出任何图像！这是代码：

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

int main(int argc, char *argv[])
{
    FILE * pFile = NULL;
    unsigned char *buffer = malloc(512);
    char* filename = NULL;
    int filenumber = 0;

    //If user didn't print 2 items
    if(argc != 2)
    {
        printf("Usage: ./recover image\n");
        return 1;
    }

    //Open the file
    pFile = fopen(argv[1], "r");
    if (!pFile)
    {
        fprintf(stderr, "File cannot be opened\n");
        return 2;
    }

    int j=0;
    
    // checking the card by 512b chunks 
    //loop (i=0,  i++);
    while (pFile)
    {
        int i =0;
        i++;

        //k=fread (buffer, 512, i, *file);
        int k = fread(buffer, 512, i, pFile);
        
        // if 512 byte block is jpeg, make new jpeg file
        if (buffer[0] == 0xff && buffer[1] == 0xd8 && buffer[2] == 0xff && (buffer[3] & 0xf0) == 0xe0)
        {
            // if it's not the first file, we should close the last one
            if (filename != NULL)
            {
                fclose(pFile);
            }

            //sprintf
            sprintf(filename, "%03i.jpg", 2);

            //FILE = fopen (W) 
            pFile = fopen(filename, "w");

            // fwrite (buffer, 512, j, *file1)
            fwrite (buffer, 512, j, pFile);

            //j=j+1
            j = j + 1;
        }

        // if k<512 - end of the loop
        if (k < 512)
        {
            return 0;
        }
    }
    free(buffer);
}

我不明白，但我的文件中没有新文件或 JPEG 弹出窗口。当我尝试双击名为 card.raw 的文件时，它不允许我打开它。

Answer 1

你有很多问题。运行您在调试器中的代码应该会在一秒钟内显示其中的大部分内容。

一起来看看：

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

int main(int argc, char *argv[])
{
    FILE * pFile = NULL;
    unsigned char *buffer = malloc(512);
    char* filename = NULL;  <<==== You never allocate any memory for this. Use an array.
    int filenumber = 0;

    //If user didn't print 2 items
    if(argc != 2)
    {
        printf("Usage: ./recover image\n");
        return 1;
    }

    //Open the file
    pFile = fopen(argv[1], "r");
    if (!pFile)
    {
        fprintf(stderr, "File cannot be opened\n");
        return 2;
    }

    int j=0;
    
    // checking the card by 512b chunks 
    //loop (i=0,  i++);   <<== No information provided by this comment.
    while (pFile)       <<== pFile is your input file. This should never change. ???
    {
        int i =0;
        i++;

        //k=fread (buffer, 512, i, *file);    <<== Useless comment. Nearly same as code below but causes compiler error
        int k = fread(buffer, 512, i, pFile); <<== i is always 1 and must be 1. Don't use variable.
                                              <<== BTW: You should check k **before** using the buffer.


        // if 512 byte block is jpeg, make new jpeg file
        if (buffer[0] == 0xff && buffer[1] == 0xd8 && buffer[2] == 0xff && (buffer[3] & 0xf0) == 0xe0)
        {
            // if it's not the first file, we should close the last one
            if (filename != NULL)
            {
                fclose(pFile);  <<== Yikes!!! This is your input file.
            }

            //sprintf  <<== Yes, that's obvious. Useless comment.
            sprintf(filename, "%03i.jpg", 2);  <<== Yikes!! You never allocate memory. NULL pointer!!
                                               <<== Why do you always print 2? you have a counter.

            //FILE = fopen (W)  <<== Again no useful information in comment
            pFile = fopen(filename, "w");  <<== Feed NULL into fopen and kill pFile.

            // fwrite (buffer, 512, j, *file1)  <<== you know what I mean...
            fwrite (buffer, 512, j, pFile);  <<== You only have 1 buffer, why write j blocks?

            //j=j+1  <<== obvious
            j = j + 1;
        }

        // if k<512 - end of the loop
        if (k < 512)   <<== fread returns number of elements, i.e. 1, not number of bytes.
        {
            << you return without
               - closing files
               - freeing buffer
            return 0;
        }
   <<== Now you go back to top of the loop and want to read next block from your raw file but pFile was killed in the loop.

    }
    free(buffer);
}

CS50 恢复不输出任何图像

CS50 recover doesn't output any images

c

cs50