Tape Input and Output

There are several functions that applications can use to perform input and output (I/O) on a tape drive. Tape I/O is similar to I/O performed on a communications device.

When performing tape I/O, some tape drives store tape firmware information in the first few blocks on a tape, typically using some portion of the first 100 blocks. Applications should not use those blocks. More specific information on this subject is available from individual tape system manufacturers. In general, an application that skips the first 100 blocks on a tape will avoid tape drive idiosyncrasies.

The GetTapePosition and SetTapePosition functions retrieve and move the current tape position. The WriteTapemark function writes a specified number of setmarks, filemarks, short filemarks, and long filemarks. The EraseTape function erases all or part of a tape.

The ReadFile and WriteFile functions read and write file data from and to the tape. The data is read and written in complete blocks. If the tape's block size is 512 bytes, all read and write operations must use buffers that are simple integer multiples of that block size: 512, 1024, 1536, 2048, and so on. Most, if not all, drives only allow a write operation after the tape is rewound or after a read operation produces an end-of-data error message.

To read or write file data to or from a tape in variable-length block mode, perform the following steps:

1. Determine whether the tape drive supports variable-length block mode by calling the GetTapeParameters function and checking the TAPE_DRIVE_VARIABLE_BLOCK bit of the FeaturesLow member of the returned TAPE_GET_DRIVE_PARAMETERS structure.
2. Specify the variable block size mode by calling the SetTapeParameters function, setting the BlockSize member of the TAPE_SET_MEDIA_PARAMETERS structure to zero. Then, use ReadFile or WriteFile to read or write the file data.

If ReadFile encounters a filemark, the data up to the filemark is read and the function fails. (The GetLastError function returns an error code indicating the type of filemark that was encountered.) The operating system moves the tape past the filemark, and an application can call ReadFile again to continue reading.

ReadFile and WriteFile read and write only the data stream. The BackupRead and BackupWrite functions read and write all the streams associated with a file. These include data, extended attributes, security, and alternative data streams. The security and alternate data streams are relevant only on the NTFS file system partition.

The BackupSeek function seeks forward in a file initially accessed by BackupRead or BackupWrite. This function enables an application to skip information that causes access errors.

If an application needs to access only the file data, it should use ReadFile and WriteFile. These functions can also read alternative data streams if the streams were created by using the CreateFile function.

A tape-backup application must use BackupRead and BackupWrite to copy all information pertaining to a file. However, these functions do not read or write file characteristics such as attributes, file creation time, and so on. Applications must use the file input and output functions, such as GetFileAttributes and SetFileAttributes, to retrieve and set those values.