Question

我是一个非常新手的程序员，我遇到了一个我不理解的问题，同时处理优秀的cs50课程的问题集。我已经实现了一个程序来从存储卡的图像中恢复JPEG图像，并在文件结束时实现中断，如下所示：

if(file > 1)
    {
        if (fread(&buffer, 1, 512, in_pointer) != 512)
        {
            free(filename);
            return 0;
        }
        else 
            fseek(in_pointer, -512, SEEK_CUR); 
    }

（图片以512字节块填充卡片）。当我第一次实现它时，它打破了我的第一张图片（它是可识别但扭曲的）所以我通过第一个if语句排除它。然而现在该集合的中间文件略有偏离 - 它们仍然以Jpegs打开但我无法使其缩略图起作用。我的假设是我正在破坏JPEG文件格式标题。开始（包括集合的第一个和最后一个图像完美地工作）。我的问题是：

由于我的getto解决方案造成了麻烦，实现EOF中断的优雅方法是什么？
我创造的问题的可能性是什么（用外行的话来说）？

非常感谢，

吉洪

ps这就是整件事

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

int main(int argc, char *argv[])
{
// ensure proper usage
if (argc != 2)
{
    fprintf(stderr, "enter exactly two command line arguments: ./recover and destination of disc to scan\n");
    return 1;
}

//name the file
char *infile = argv[1];

//open card file and ensure proper format
FILE *in_pointer = fopen(infile, "r");
if (in_pointer == NULL)
{
    fprintf(stderr, "could not open %s\n", infile);
    return 2;
}

typedef uint8_t  BYTE;

BYTE buffer[512];
bool new_jpeg = false;
int block = 0;
int file = 0;
char *filename = malloc(3);
//sprintf(filename, "%03i.jpg",1);

do
{

    //read a 512 block of a jpeg
    fread(&buffer, 512, 1, in_pointer);

    //check for new jpeg
    if (buffer[0] == 0xff &&
        buffer[1] == 0xd8 &&
        buffer[2] == 0xff &&
        (buffer[3] & 0xf0) == 0xe0) // took me a while to figure this out
        {
    new_jpeg = true;
    //printf("jpeg found, block %i\n", block);
        }

    block++;



} while(new_jpeg == false);

do
{
            //set name of file to write to
            sprintf(filename, "%03i.jpg",file);
            file++;
            new_jpeg = false;

            // open output file
            FILE *img = fopen(filename, "w");
            if (img == NULL)
                {
                    fprintf(stderr, "Could not create %s.\n", filename);
                    return 3;
                }


        //add blocks to file while before we reach the nea JPEG.
        do
        {


                fwrite(&buffer, 1, 512, img);

                //read the next block
                fread(&buffer, 1, 512, in_pointer);

                //There MUST be a better way... Anyhow this checks for end of file but backtracks becouse the act of checking moved the file coursor forward... 
                if(file > 1)
                {
                    if (fread(&buffer, 1, 512, in_pointer) != 512)
                    {
                        free(filename);
                        return 0;
                    }
                    else 
                    fseek(in_pointer, -512, SEEK_CUR); 
                }


                block++; //we are reading off teh next block

                if (buffer[0] == 0xff &&
                    buffer[1] == 0xd8 &&
                    buffer[2] == 0xff &&
                    (buffer[3] & 0xf0) == 0xe0) // took me a while to figure this out
                {
                    new_jpeg = true;
                    //printf("jpeg %i found, block %i\n", file, block);
                }


        }while(new_jpeg == false);

}while(!feof(in_pointer));    

free(filename);
//ran valgrind no probs detected.
}

Answer 1

好的，我修好了。

我没有用完整的单独代码来检查eof，而是在一块石头上杀死了两只鸟，然后在检查eof时抓住文件：

export class HomePage {

  posts: any;

  constructor(public http: Http) {

    this.http.get('https://www.reddit.com/r/gifs/new/.json?limit=10').map(res => res.json()).subscribe(data => {
        this.posts = data.data.children;
    });

  }
}

我仍然不知道为什么我以前的方法不起作用，我会非常感激建议......

文件结束检测

1 个答案: