c - 如何定位段错误的根源？ (CS50 : recover.c)

Question

我正在尝试为 CS50 编写一个 C 语言程序，从 .raw 文件中恢复 JPG(一次读取 512 个字节并查看它是否以 JPG 内容开头)，但它始终出现段错误。我如何知道问题的根源是什么？多谢你们! (这是我的代码供引用)

      /**
 * recover.c
 *
 * Computer Science 50
 * Problem Set 4
 *
 * Recovers JPEGs from a forensic image.
 */

 //0xff 0xd8 0xff 0xe0
 //0xff 0xd8 0xff 0xe1

#define BLOCK 512
#define START1END 0xe0
#define START2END 0xe1

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

//making variables
int found = 0; 
char* title;
FILE* img;
int ifopen = 1;

int main(int argc, char* argv[])
{
    //opening file
    FILE* inptr = fopen("card.raw", "r");
    //checking if file opening failed
    if (inptr == NULL)
    {
        return 2;
    }
    //sets the begins or jpgs
    uint8_t checkjpg1[4] = {0xff, 0xd8, 0xff, 0xe0};
    uint8_t checkjpg2[4] = {0xff, 0xd8, 0xff, 0xe1};

    //making buffer
    unsigned char buffer[512];

    //going through the file
    while(fread(&buffer,sizeof(char),BLOCK,inptr) == BLOCK)
    {
         //checking if begin == the possible begin of jpg    
         if ((buffer[0] == checkjpg1[0] && buffer[1] == checkjpg1[1] && buffer[2] == checkjpg1[2]) && 
         (buffer[3] == checkjpg1[3] || buffer[3] == checkjpg2[3]))
         {
            //if a jpg is not open
            if (ifopen == 1)
            {
                //make one
                found+=1;
                sprintf(title,"00%d",found);
                img = fopen(title,"a");
            }
            else//else
            {
                //end the one and open new one
                fclose(img);
                sprintf(title,"00%d",found);
                img = fopen(title,"a");
            }
         }
         else if(img != NULL)
         {
             fwrite(buffer,sizeof(char),BLOCK,img);
         }
    }

    fclose(inptr);
    free(buffer);
}

(抱歉，线路太长了!)

Answer 1

在这一行(和其他行)

sprintf(title,"00%d",found);

没有内存分配给title，已声明

char *title;

但仅此而已。

char title[BLOCK];

会更好。顺便说一句，在声明 buffer 时，您不使用 BLOCK ，它应该是

unsigned char buffer[BLOCK];

此外，您还需要另一个

found+=1;

在 else 代码块中打开 img 之前。