Processes and threads provide a powerful structuring tool for implementing programs that would be much more complex

Question:

Processes and threads provide a powerful structuring tool for implementing programs that would be much more complex as simple sequential programs. An earlier construct that is instructive to examine is the coroutine. The purpose of this problem is to introduce coroutines and compare them to processes. Consider this simple problem from [CONW63]:
Read 80-column cards and print them on 125-character lines, with the following changes. After every card image an extra blank is inserted, and every adjacent pair of asterisks (**) on a card is replaced by the character( ).
a. Develop a solution to this problem as an ordinary sequential program. You will find that the program is tricky to write. The interactions among the various elements

of the program are uneven because of the conversion from a length of 80 to 125; furthermore, the length of the card image, after conversion, will vary depending on the number of double asterisk occurrences. One way to improve clarity, and to minimize the potential for bugs, is to write the application as three separate procedures. The first procedure reads in card images, pads each image with a blank, and writes a stream of characters to a temporary file. After all of the cards have been read, the second procedure reads the temporary file, does the character substitution, and writes out a second temporary file. The third procedure reads the stream of characters from the second temporary file and prints lines of 125 characters each.
b. The sequential solution is unattractive because of the overhead of I/O and temporary files. Conway proposed a new form of program structure, the coroutine, that allows the application to be written as three programs connected by one-character buffers (see Figure 5.28). In a traditional procedure, there is a master/slave relationship between the called and calling procedures. The calling procedure may execute a call from any point in the procedure; the called procedure is begun at its entry point and returns to the calling procedure at the point of call. The coroutine exhibits a more symmetric relationship. As each call is made, execution takes up from the last active point in the called procedure. Because there is no sense in which a calling procedure is “higher” than the called, there is no return. Rather, any coroutine can pass control to any other coroutine with a resume command. The first time a coroutine is invoked, it is “resumed” at its entry point. Subsequently, the coroutine is reactivated at the point of its own last resume command. Note only one coroutine in a program can be in execution at one time, and the transition points are explicitly defined in the code, so this is not an example of concurrent processing. Explain the operation of the program in Figure 5.28.
c. The program does not address the termination condition. Assume that the I/O routine READCARD returns the value true if it has placed an 80-character image in inbuf; otherwise it returns false. Modify the program to include this contingency.
d. Rewrite the solution as a set of three processes using semaphores.

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