# define os_file_read(file, buf, offset, offset_high, n)        \
    os_file_read_func(file, buf, offset, offset_high, n)

/*******************************************************************//**
NOTE! Use the corresponding macro os_file_read(), not directly this
function!
Requests a synchronous positioned read operation.
@return    TRUE if request was successful, FALSE if fail */
UNIV_INTERN
ibool
os_file_read_func(
/*==============*/
    os_file_t    file,    /*!< in: handle to a file */
    void*        buf,    /*!< in: buffer where to read */
    ulint        offset,    /*!< in: least significant 32 bits of file
                offset where to read */
    ulint        offset_high, /*!< in: most significant 32 bits of
                offset */
    ulint        n)    /*!< in: number of bytes to read */
{
#ifdef __WIN__
    BOOL        ret;
    DWORD        len;
    DWORD        ret2;
    DWORD        low;
    DWORD        high;
    ibool        retry;
#ifndef UNIV_HOTBACKUP
    ulint        i;
#endif /* !UNIV_HOTBACKUP */

    /* On 64-bit Windows, ulint is 64 bits. But offset and n should be
    no more than 32 bits. */
    ut_a((offset & 0xFFFFFFFFUL) == offset);
    ut_a((n & 0xFFFFFFFFUL) == n);

    os_n_file_reads++;
    os_bytes_read_since_printout += n;

try_again:
    ut_ad(file);
    ut_ad(buf);
    ut_ad(n > );

    low = (DWORD) offset;
    high = (DWORD) offset_high;

    os_mutex_enter(os_file_count_mutex);
    os_n_pending_reads++;
    os_mutex_exit(os_file_count_mutex);

#ifndef UNIV_HOTBACKUP
    /* Protect the seek / read operation with a mutex */
    i = ((ulint) file) % OS_FILE_N_SEEK_MUTEXES;

    os_mutex_enter(os_file_seek_mutexes[i]);
#endif /* !UNIV_HOTBACKUP */

    ret2 = SetFilePointer(file, low, &high, FILE_BEGIN);

    if (ret2 == 0xFFFFFFFF && GetLastError() != NO_ERROR) {

#ifndef UNIV_HOTBACKUP
        os_mutex_exit(os_file_seek_mutexes[i]);
#endif /* !UNIV_HOTBACKUP */

        os_mutex_enter(os_file_count_mutex);
        os_n_pending_reads--;
        os_mutex_exit(os_file_count_mutex);

        goto error_handling;
    }

    ret = ReadFile(file, buf, (DWORD) n, &len, NULL);

#ifndef UNIV_HOTBACKUP
    os_mutex_exit(os_file_seek_mutexes[i]);
#endif /* !UNIV_HOTBACKUP */

    os_mutex_enter(os_file_count_mutex);
    os_n_pending_reads--;
    os_mutex_exit(os_file_count_mutex);

    if (ret && len == n) {
        return(TRUE);
    }
#else /* __WIN__ */
    ibool    retry;
    ssize_t    ret;

    os_bytes_read_since_printout += n;

try_again:
    ret = os_file_pread(file, buf, n, offset, offset_high); //读取数据 详见    if ((ulint)ret == n) {

        return(TRUE);
    }

    fprintf(stderr,
        "InnoDB: Error: tried to read %lu bytes at offset %lu %lu.\n"
        "InnoDB: Was only able to read %ld.\n",
        (ulong)n, (ulong)offset_high,
        (ulong)offset, (long)ret);
#endif /* __WIN__ */
#ifdef __WIN__
error_handling:
#endif
    retry = os_file_handle_error(NULL, "read");

    if (retry) {
        goto try_again;
    }

    fprintf(stderr,
        "InnoDB: Fatal error: cannot read from file."
        " OS error number %lu.\n",
#ifdef __WIN__
        (ulong) GetLastError()
#else
        (ulong) errno
#endif
        );
    fflush(stderr);

    ut_error;

    return(FALSE);
}