/*

    下面的函数是在vs中单步调试时得到的，具体操作是在fp.open()这里打个断点，然后单步调试(F11)

    这里体现了C中和C++打开文件的各种模式的对应关系，即mods和valid中的元素是一一对应的

*/

_CRTIMP2_PURE FILE *__CLRCALL_PURE_OR_CDECL _Fiopen(const _Sysch_t *filename,

    ios_base::openmode mode, int prot)

    {    // open a file with native name

    static const _Sysch_t *mods[] =

        {    // fopen mode strings corresponding to valid[i]

        _SYSCH("r"), _SYSCH("w"), _SYSCH("w"), _SYSCH("a"),

        _SYSCH("rb"), _SYSCH("wb"), _SYSCH("wb"), _SYSCH("ab"),

        _SYSCH("r+"), _SYSCH("w+"), _SYSCH("a+"),

        _SYSCH("r+b"), _SYSCH("w+b"), _SYSCH("a+b"),

        0};

    static const int valid[] =

        {    // valid combinations of open flags

        ios_base::in,

        ios_base::out,

        ios_base::out | ios_base::trunc,

        ios_base::out | ios_base::app,

        ios_base::in | ios_base::binary,

        ios_base::out | ios_base::binary,

        ios_base::out | ios_base::trunc | ios_base::binary,

        ios_base::out | ios_base::app | ios_base::binary,

        ios_base::in | ios_base::out,

        ios_base::in | ios_base::out | ios_base::trunc,

        ios_base::in | ios_base::out | ios_base::app,

        ios_base::in | ios_base::out | ios_base::binary,

        ios_base::in | ios_base::out | ios_base::trunc

            | ios_base::binary,

        ios_base::in | ios_base::out | ios_base::app

            | ios_base::binary,

        0};

    FILE *fp = 0;

    int n;

    ios_base::openmode atendflag = mode & ios_base::ate;

    ios_base::openmode norepflag = mode & ios_base::_Noreplace;

    if (mode & ios_base::_Nocreate)

        mode |= ios_base::in;    // file must exist

    if (mode & ios_base::app)

        mode |= ios_base::out;    // extension -- app implies out

    mode &= ~(ios_base::ate | ios_base::_Nocreate | ios_base::_Noreplace);

    for (n = 0; valid[n] != 0 && valid[n] != mode; ++n)

        ;    // look for a valid mode

    if (valid[n] == 0)

        return (0);    // no valid mode

    else if (norepflag && mode & (ios_base::out || ios_base::app)

        && (fp = _Xfsopen(filename, _SYSCH("r"), prot)) != 0)

        {    // file must not exist, close and fail

        fclose(fp);

        return (0);

        }

    else if (fp != 0 && fclose(fp) != 0)

        return (0);    // can't close after test open

    else if ((fp = _Xfsopen(filename, mods[n], prot)) == 0)

        return (0);    // open failed

    if (!atendflag || fseek(fp, 0, SEEK_END) == 0)

        return (fp);    // no need to seek to end, or seek succeeded

    fclose(fp);    // can't position at end

    return (0);

        }