判断图片格式

string getImageFileFormat(string fileName)
{
    unsigned short  BMP=0x4D42,
            JPG=0xD8FF,
            PNG[4]={0x5089,0x474E,0x0A0D,0x0A1A},
            GIF[3]={0x4947,0x3846,0x6139};

    FILE *fis;
    unsigned short pis[8];
    string fileFormat = "null";
    if((fis=fopen(fileName.c_str(),"r"))==NULL)
    {
        cout << getTimeZZ() << "can not read " << fileName << endl;
        return "null";
    }
    fread(pis,8,1,fis);
    if(pis[0]==BMP)
    {
        cout << getTimeZZ() << "it is a bmp "<< endl;
        fileFormat =  "bmp";
    }
    else if(pis[0]==JPG)
    {
        cout << getTimeZZ() << "it is a jpg "<< endl;
        fileFormat = "jpg";
    }
    else if(PNG[0]==pis[0]&&PNG[1]==pis[1]&&PNG[2]==pis[2]&&PNG[3]==pis[3])
    {
        cout << getTimeZZ() << "it is a png"<< endl;
        fileFormat = "png";
    }
    else if(GIF[0]==pis[0]&&GIF[1]==pis[1]&&GIF[2]==pis[2])
    {
        cout << getTimeZZ() << "it is a gif"<< endl;
        fileFormat = "gif";
    }
    return fileFormat;
}