Re^2: Problem with passing my script's STDIN to child

Does this have advantages over the IO::Pipe method? (genuinly curious, not being difficult :)

It's been a good 10 years since I covered this in my OS classes and I haven't really had to use it since.

Comment on Re^2: Problem with passing my script's STDIN to child

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Re^3: Problem with passing my script's STDIN to child by ambrus (Abbot) on Aug 21, 2004 at 19:08 UTC
Yes, certainly. If you copy the input from the stdin through a pipe, that's trivially less efficent than if you dupe the filehandle. In the second case, the new program just reads directly from whatever file stdin was connected to. With a pipe, you need a separate process that has to copy data from stdin to the pipe every time the second program needs data; under linux a pipe has a buffer of length 4096 by default, so the os has to do a task switch to the first process and back to the second one for every 4K bytes read (it is possible that some of the overhead can be avoided with the sendfile syscall). Also, the first process occupies memory and a process table entry. In addittion, you have to take care not to block data with too much buffering; also the pipe solution might not even work on win32.	[reply]

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Re^3: Problem with passing my script's STDIN to child
by ambrus (Abbot) on Aug 21, 2004 at 19:08 UTC

Yes, certainly.

If you copy the input from the stdin through a pipe, that's trivially less efficent than if you dupe the filehandle. In the second case, the new program just reads directly from whatever file stdin was connected to. With a pipe, you need a separate process that has to copy data from stdin to the pipe every time the second program needs data; under linux a pipe has a buffer of length 4096 by default, so the os has to do a task switch to the first process and back to the second one for every 4K bytes read (it is possible that some of the overhead can be avoided with the sendfile syscall). Also, the first process occupies memory and a process table entry. In addittion, you have to take care not to block data with too much buffering; also the pipe solution might not even work on win32.

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