3.6 Selecting sequencing methods

This program uses a few techniques when it is discovered that many passes are needed to comply with the request. For example, suppose that four elementary steps were selected at recoding path optimisation time. Then Recode will split itself into four different interconnected tasks, logically equivalent to:

     step1 <input | step2 | step3 | step4 >output

The splitting into subtasks is often done using Unix pipes. But the splitting may also be completely avoided, and rather simulated by using memory buffer, or intermediate files. The various ‘--sequence=strategy’ options gives you control over the flow methods, by replacing strategy with ‘memory’, ‘pipe’ or ‘files’. So, these options may be used to override the default behaviour, which is also explained below.

‘--sequence=memory’

When the recoding requires a combination of two or more elementary recoding steps, this option forces many passes over the data, using in-memory buffers to hold all intermediary results.

‘-i’

‘--sequence=files’

When the recoding requires a combination of two or more elementary recoding steps, this option forces many passes over the data, using intermediate files between passes. This is the default behaviour when files are recoded over themselves. If this option is selected in filter mode, that is, when the program reads standard input and writes standard output, it might take longer for programs further down the pipe chain to start receiving some recoded data.

‘-p’

‘--sequence=pipe’

When the recoding requires a combination of two or more elementary recoding steps, this option forces the program to fork itself into a few copies interconnected with pipes, using the pipe(2) system call. All copies of the program operate in parallel. This is the default behaviour in filter mode. If this option is used when files are recoded over themselves, this should also save disk space because some temporary files might not be needed, at the cost of more system overhead.

If, at installation time, the pipe(2) call is said to be unavailable, selecting option ‘-p’ is equivalent to selecting option ‘-i’. (This happens, for example, on MS-DOS systems.)