PSET 4 恢复分段故障 CS50答案

【问题标题】：PSET 4 Recover SEGMENTATION FAULT CS50PSET 4 恢复分段故障 CS50
【发布时间】：2020-07-10 09:20:50
【问题描述】：

我在读取文件和写入文件方面有点吃力。我已经在这个问题上工作了几天，但我似乎无法理解。该程序应该从存储卡中恢复一些 jpeg 文件。使用以下代码，我不断收到分段错误。你们能看出我哪里错了吗？下面是我的代码：

希望大家帮忙！

#include <stdio.h>
#include <stdlib.h>
#include <cs50.h>

int main(int argc, char *argv[])
{

    //Ensure only one command line argument//
    if (argc != 2)
    {
        printf("Usage: ./recover filename\n");
        return 1;
    }

    //Open File//
    FILE *file = fopen(argv[1], "r");
    if (file == NULL)
    {
        printf("File not working\n");
        return 1;
    }

    //Create buffer of size 512 bytes to store data from file that is being read//
    unsigned char buffer[512];
    //Set Bool value of already found first jpeg to false//
    bool already_found_jpeg = false; 
    //Create filename length of 8 that will be of new jpegs
    char filename[8];
    FILE *img = NULL;
    //num_of_imgs counter//
    int num_of_imgs = 0;
    while(fread(buffer, 512, 1, file) == 1)
    {
        //If start of new jpeg//
        if (buffer[0] == 0xff && buffer[1] == 0xd8 && buffer[2] == 0xff && (buffer[3] & 0xf0) == 0xe0)
        {
            //If first jpeg, start writing the very first file// 
            if (already_found_jpeg == false) 
            {
                sprintf(filename, "%03i.jpg", num_of_imgs);
                img = fopen(filename, "w");
                if (img == NULL)
                {
                    return 3;
                }
                //Write from buffer into new file//
                fwrite(&buffer, 512, 1, img);
                num_of_imgs ++;
                already_found_jpeg = true;
            }
            //Else if already found a jpeg meaning it's not the first jpeg then close file, so you can open up a new file that can be written too
            else if (already_found_jpeg == true)
            {
                fclose(img);
                sprintf(filename, "%03i.jpg", num_of_imgs);
                img = fopen(filename, "w");
                if (img == NULL)
                {
                    return 3;
                }
                //Write from buffer into new file//
                fwrite(&buffer, 512, 1, img);
                num_of_imgs ++;
            }
        }
        //else if not found the jpeg headers and already found first jpeg keep writing to file
        else 
        {
            if (already_found_jpeg == true)
            {
                fwrite(&buffer, 512, 1, img);
            }
        }
        //Close all files//
        fclose(file);
        fclose(img);
    }
}

【问题讨论】：

建议debug50。断点if (buffer[0] .... 并单步执行代码。很快就能找到漏洞。

标签： c segmentation-fault cs50

【解决方案1】：

问题

在 while 循环的第一次迭代结束时，您将关闭 file 和 img。这意味着这些指针的值变为NULL。在下一次迭代中，缓冲区从 NULL 指针读取（因为我们关闭了file，它没有指向任何内容）。基本上，取消引用空指针会导致分段错误。

解决方案

如果您将这段代码移到 while 循环之外，它将起作用。（使用 CS50 IDE 测试）

//Close all files//
fclose(file);
fclose(img);

这对我来说是一个很好的练习来回顾我在 CS50 中所做的事情 - 恢复。 :)

【讨论】：