The Assembly Programming Master Book

All information is stored in files. File management is a vital component of practically any program. Beginners pay excessive attention to the design windows , toolbars , and nonstandard buttons . Unfortunately, they often neglect file operations. However files must be paid the attention they requireactually, mastering operations overfiles is the key to mastery of programming.

With the arrival of large and fast hard disks, the importance of files has grown considerably. Using API functions for file management can make your program more efficient and ensure its high performance. Most programs presented in this chapter are console applications. This is because console applications are the most suitable for demonstrating file processing. File processing is the method of hiding from users and programmers all the events that take place when information is read from or written to the storage media. Operating systems of the Windows NT family work with two types of file systemsFile Allocation Table 32 (FAT32) and the New Technology File System (NTFS). FAT32 is the direct descendant of the two file systems, FAT12 and FAT16. It has inherited most of their drawbacks. Microsoft continues to support this file system for backward compatibility with previous versions of its file systems, although in future versions of Windows, FAT32 support will be reduced to the capability of reading files from FAT volumes . On the contrary, NTFS is generally considered as one of the most perfect file systems. In the following few sections, I will cover the basic concepts of these file systems, which are required for efficient programming.

File Characteristics

When providing descriptions of file characteristics, I'll base them on the parameters that API functions manipulate. The types of file systems and their structure will be covered later. First, I'll list and briefly describe the main file attributes. Most of them are present in NTFS, and a considerable part is also present in FAT32.

File Attributes

The attribute is an integer of the DWORD type. It defines how the operating system treats the file.

File attributes can be changed using the SetFileAttributes function. To read file attributes, use the GetFileAttributes function. The values of the following attributes can be set only using the DeviceIoControl function:

Note that if the operating system doesn't impose any limitations on the possibility of changing file attributes, then the attributes themselves lose sense. For example, if the read-only attribute can be removed any time, then any user will be able to do with the file whatever he or she pleases.

The file has three time characteristics: creation time, last modification time, and last access time. The time is counted in nanoseconds, starting from 12 p.m., January 1, 1600; and is stored in two 32-bit values that can be represented by the following structure:

FILETIME STRUC dwLowDateTime DW ? dwLowHighTime DW ? FILETIME ENDS

It is necessary to point out that time is stored in universal coordinates and must be converted to the local time (this is achieved using the FileTimeToLocalFileTime function). To get all three time values, use the GetFileTime function. For output and manipulations with the file, it is expedient to invent a more convenient structure than two 32-bit values. There is such a structure SYSTIME . It has the following format:

SYSTIME STRUC wYear DW ? wMonth DW ? wDayOfWeek DW ? wDay DW ? wHour DW ? wMinute DW ? wSecund DW ? wMilliseconds DW ? SYSTIME ENDS

For converting the structure obtained using the GetFileTime function into the SYSTIME Structure, use the FileTimeToSystemTime API function.

To set the time characteristics of a file, the SetFileTime function is used. For defining the time, it is convenient to use the SYSTIME structure and then convert it into the SetFileTime format using the SystemTimeToFileTime function. Later in this chapter, I'll provide an example illustrating how to obtain time characteristics of the file (see Listing 11.6).

File length in bytes is usually stored in two 32-bit values or in one 64-bit value. If 32-bit values are designated as ll (least significant part) and 12 (most significant part), then a 64-bit value will be obtained according to the following formula: 12*0FFFFH+11 . The file size can be obtained using the GetFileSize function. This function returns the least significant part of the file length, which is enough in most cases. The second argument of this function is the pointer to the most significant part of the file length. However, the GetFileSizeEx function is more convenient for determining the file length. The second argument of this function is the address of the following structure:

FSIZE STRUC LOWPART DW ? HIGHPART DW ? FSISE ENDS

This structure is the one that gets the file length.

It is necessary to note that functions able to be used for obtaining file characteristics receive the file descriptor as their first argument. In other words, to get these characteristics, it is first necessary to open the file (see the description of the CreateFile function later in this chapter). An alternative method of receiving these parameters is using the FindFirstFile function (see the File Search section later in this chapter).

In addition to the previously described characteristics, the file, naturally, has a name. Note that it is necessary to distinguish between the long name and the short name of the file. Also, it is necessary to distinguish the fully qualified pathname (with all long names) and the abbreviated pathname (in which all long names are replaced by abbreviated ones). An abbreviated pathname is needed because some older programs interpret blank characters as parameter delimiters. To convert the long name into the short one, use the GetShortPathName function, which is capable of converting both names and paths. The inverse operation can be carried out using the GetFullPathName function.

In this book, I don't even touch direct disk access. However, you may still ask about the structure of the directory records. This is quite natural because, with the migration from FAT (MS-DOS) to FAT321 (Windows 95), there appeared the possibility of storing long filenames. In addition to the time and date of modification, there appeared such characteristics as time and date of creation and last access. Where are all these characteristics stored? This question will be answered in the next section.

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