c - 读取二进制文件并确定文件类型的程序

Question

我编写了这段代码，它读取文件二进制并确定其文件类型(用于一些测试文件结尾)。它适用于 PDF、MP3，但不适用于 jpg。

有什么问题吗？对于 jpg，行 printf("%s[%d]: %x\n", "Buffer", j, buffer[j]); 显示多个字节(即 ffffff 而不是一个字节)字节)

#include <stdio.h>


const int header[6][8] =    { 
                            {0x89,0x50,0x4E,0x47,0x0D,0x0A,0x1A,0x0A},
                            {0xFF,0xD8,0x00,0x00,0x00,0x00,0x00,0x00},
                            {0xFF,0xFB,0x00,0x00,0x00,0x00,0x00,0x00},
                            {0x49,0x44,0x33,0x00,0x00,0x00,0x00,0x00},
                            {0x25,0x50,0x44,0x46,0x2D,0x00,0x00,0x00},
                            {0x42,0x4C,0x45,0x4E,0x44,0x45,0x52,0x00} 
                            };

const char* filetype[6] = {"PNG","JPG","MP3","MP3v2","PDF","Blender"};

int main()
{
    FILE *fd;
    char buffer[8];


    if ((fd = fopen("C:\\Users\\***\\Desktop\\Unnamed.jpg", "rb")) == NULL) {
        return -1;
    }

    //fread(buffer, sizeof(char), 8, fd);
    fread(buffer, sizeof(buffer), 1, fd);

    for (int i = 0; i < 6; i++) {
        for(int j = 0; j < 8; j++){
            printf("%s[%d][%d]: %x\n","Header",i,j,header[i][j]);
            printf("%s[%d]: %x\n", "Buffer", j, buffer[j]);

            if (header[i][j] == 0x00) {
                printf("%s: %s","Found file type",filetype[i]);
                return 1;
            }
            if (header[i][j] != buffer[j]) {
                break;
            }
        }
    }
    printf("%s", "Couldn't determine filetype - Not in library");
    return 0;
}

Answer 1

您忘记将缓冲区设置为unsigned char，因此您会在值上获得符号扩展。

显然，内置魔术签名表也应该是 const unsigned char，并且应该通过单个 memcmp() 调用进行比较。