c-PSET 4恢复分段故障CS50

Question

我在读文件和写文件方面有点吃力。我已经处理这个问题好几天了，但我似乎不明白。该程序应该从存储卡中恢复一些jpeg文件。使用以下代码，我不断地遇到分段错误。你们能看到我哪里错了吗？下面是我的代码：

希望你们能帮忙！

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

Answer 1

问题

在while循环的第一次迭代结束时，您正在关闭file和img。这意味着这些指针的值将变为NULL。在下一次迭代中，缓冲区将从NULL指针中读取(因为我们关闭了file，所以它不指向任何内容(。基本上，取消引用空指针会导致分段错误。

解决方案

如果将这段代码移到while循环之外，它就会工作。(用CS50 IDE测试(

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

这对我来说是一次很好的练习，可以回顾我在CS50中所做的事情——恢复。：(