Imagine that we invented a new mode of operation for pseudo terminals called ‘‘packet mode plus.’’Inthis new mode, whenever a read is made on the slave side of a pseudo terminal, a message is sent to the master side ndicating the number of bytes the process wants to read. Can we use this new mode to support interactive program execution with the pty command? Explain your answer.
What will be an ideal response?
No. If we knew how many bytes the program wanted to read and distinguish between canonical mode and noncanonical mode processing in the pty command, we could send only VMIN bytes in noncanonical mode and an entireline in canonical mode. However,this still wouldn’t work with applications that display a prompt and use poll or select to determine when data can be read. In this case, the application would never call read unless we had already sent it some data, but we would only send data to the application after the application calls read.You might be tempted to try to trick the application by always indicating that the terminal is readable in this new mode, so that the application calls read.This won’t work when we have no data to send to the application: it will block trying to read from the stream and won’t be able to service any of its other ?le descriptors
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