需要建议 pset4 恢复。尝试了一个星期，仍然出现分段错误

Question

输出：

:( recovers 000.jpg correctly
    failed to execute program due to segmentation fault
:( recovers middle images correctly
    failed to execute program due to segmentation fault
:( recovers 049.jpg correctly
    failed to execute program due to segmentation fault

代码：

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

int main(int argc, char *argv[])
{
    if (argc != 2) //to make sure that accept exactly one command-line argument
    {
        printf("Usage: ./recover key\n");
        return 1;
    }

    FILE *infile = fopen(argv[1], "r"); //open the file card.raw and creating a new file called f in read format
    if (infile == NULL)//if file cannot open then print below if can open just continue
    {
        printf("Cannot open file\n");
        return 2;
    }

    FILE *img; //img is the output
    int jpeg_counter = 0; // to count the no. of jpeg files
    uint8_t buffer[512]; //cos 512 bytes and the buffer is the temporary storage 
    char filename[8];


    while (fread(buffer, sizeof(buffer), 1, infile) == 512)
        //continue doing this loop if the while conditions are true. to repeat until end of card.like while the file you reading is true,
    {
        if (buffer[0] == 0xff && buffer[1] == 0xd8 && buffer[2] == 0xff && (buffer[3] & 0xf0) == 0xe0)
            //removing last 4 bits of the 8 bits, only looking at they first 4 which is e. setting all to 0
            //if start of new jpeg with above conditions 
        {
            if (jpeg_counter != 0) // telling them that you previously found jpeg
            {
                fclose(img);//else if never find before, tell them now that you have found it by making it true
            }

            sprintf(filename, "%03i.jpg", jpeg_counter); //%03i means print an integer with 3 digits
            jpeg_counter ++;
            img = fopen(filename , "w"); //open the new file w for writting 
            if (img == NULL) //see if can remove this
                return 3;
            fwrite(buffer, sizeof(buffer), 1, img);// writing new output file

        }    
        if (jpeg_counter != 0)
            fwrite(buffer, sizeof(buffer), 1, img);
    } 
    fclose(infile);
    fclose(img);
    return 0;
}

Answer 1

段错误是由 fclose(img) 引起的，img 是一个无效的 FILE 指针。问题是您的 while 循环条件永远不会为真，并且永远不会执行循环。您的 fread 永远不会 return 512，它 returns 1 就成功读取了。我已经为您修复了循环条件并添加了一些 printfs 以打印出更多信息，以便您更好地了解发生了什么。这里有一个link我们云IDE的固定代码，以后可以用它来调试segfault。

需要建议 pset4 恢复。尝试了一个星期，仍然出现分段错误

Need advice pset4 recover. Tried for a week and still getting segmentation fault

c

cs50

recover