pset4 cs50 recover.c 指导

Question

我可以使用 cs50 中的这个问题集的一些帮助。每当我的代码遇到 while 循环时，fread 函数就会返回 0。我似乎无法理解为什么会发生这种情况。甚至在我遇到这个问题之前，我的代码并没有按我的意愿工作，所以如果有任何额外的提示，那真的很有帮助。

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

//PROTOTYPES 
bool check(uint8_t block[]);

int main(int argc, char* argv[])
{
    FILE* inptr = fopen("card.raw", "r"); //open up the card.raw file for reading

    if(inptr == NULL)
    {
        fclose(inptr);
        printf("Could not open the card data.\n");
        return 1;
    }

    FILE* outptr;

    int jpegCount = 0;
    bool foundFirstJpg = false;// flag to denote wheather the first jpg has been found

    char str[10]; //to hold the jpg name
    sprintf(str, "%i.jpg", jpegCount); //puts the image name in str
    jpegCount ++;
    outptr = fopen(str, "w"); //Create a new jpeg for each image

    uint8_t block[512]; // a tempory block to hold 512 bytes of info 

    while(fread(block, sizeof(block), 1, inptr) == 1) //read 512 bytes of info at a time from the inptr)
    {
        printf("hello\n");
        if(check(block)) //if I have found a jpg
        {
            if(foundFirstJpg) //if its not my first jpg
            {
                fclose(outptr); //close the previous jpg file
                sprintf(str, "%i.jpg", jpegCount); //puts the image name in str
                outptr = fopen(str, "w"); //Create a new jpeg for each image
                jpegCount ++;
                fwrite(block, sizeof(block), 1, outptr); //write the block to the image file 
            }
            else
            {
                fwrite(block, sizeof(block), 1, outptr); //write first block to the image file 
                foundFirstJpg = true; 
            }
        }
        else //if this is not a jpg
        {
            if(foundFirstJpg) //check if we have found our first jpg
            {
                 fwrite(block, sizeof(block), 1, outptr); //write 512 bytes to the current image file
            }
        }
    } 
    if (outptr)
    {
         fclose(outptr);
    }
    fclose(inptr);
    return 0;
}

//function to check if this is the start or end of a jpg
bool check(uint8_t block[])
{
    bool isJpg = true; //boolean value to be returned
    if (block[0] != 0xff || block[1] != 0xd8 || block[2] != 0xff) //checks if the first 3 bytes are those that represent a jpg
    {
        isJpg = false;
    }
    if (block[3] < 0xe0 || block[3] > 0xef) //checks if the fourth byte also represents a jpg
    {
        isJpg = false;
    }
    return isJpg;
}

Answer 1

您应该以二进制模式打开这些文件：

FILE *inptr = fopen("card.raw", "rb");
...
outptr = fopen(str, "wb");

如果card.raw 少于512 字节，fread 将返回0。