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1. An interprocess communication mechanism.. wherefd is an integer,buf is an array of characters, andnbytes is another integer. If the call is made from the main program, the stack will be as shown in Fig. 2-17(a) before the call. To make the call, the caller pushes the parameters onto the stack in order, last one first, as shown in Fig. 2-17(b). (The reason that C compilers push the parameters in reverse order has to do withprintf— by doing so,printf can always locate its first parameter, the format string.) Afterread has finished running, it puts the return value in a register, removes the return address, and transfers control back to the caller. The caller then removes the parameters from the stack, returning it to the original state, as shown in Fig. 2-17(c).. With a single computer and a single clock, it does not matter much if this clock is off by a small amount. Since all processes on the machine use the same clock, they will still be internally consistent. For example, if the fileinput.c has time 2151 and fileinput.o has time 2150,make will recompile the source file, even if the clock is off by 2 and the true times are 2153 and 2152, respectively. All that really matters are the relative times.. [Картинка: any2fbimgloader65]. Fig. 3-21. Two-phase locking.. [Картинка: any2fbimgloader117]. Most distributed systems use some form of two-level naming. Files (and other objects) havesymbolic namessuch asprog.c, for use by people, but they can also have internal,binary namesfor use by the system itself. what directories in fact really do is provide a mapping between these two naming levels. It is convenient for people and programs to use symbolic (ASCII) names, but for use within the system itself, these names are too long and cumbersome. Thus when a user opens a file or otherwise references a symbolic name, the system immediately looks up the symbolic name in the appropriate directory to get the binary name that will be used to locate the file. Sometimes the binary names are visible to the users and sometimes they are not.. A more general naming scheme is to have the binary name indicate both a server and a specific file on that server. This approach allows a directory on one server to hold a file on a different server. An alternative way to do the same thing that is sometimes preferred is to use asymbolic link.A symbolic link is a directory entry that maps onto a (server, file name) string, which can be looked up on the server named to find the binary name. The symbolic link itself is just a path name.. Fig. 5-14. NFS layer structure.. 1. Reduce the amount of communication.. As an aside, note that a conflict may exist between choosing a replicated page (which may just be discarded) and choosing a page that has not been referenced in a long time (which may be the only copy). The same problem exists in traditional virtual memory systems, however, so the same compromises and heuristics apply.. Fig. 8-1. The abstract model for UNIX emulation using Mach.. [Картинка: any2fbimgloader210]. Groups are commonly used to provide reconfigurable services. Initially some set of servers belongs to the group, which provides some service. Clients can send messages to the group without having to know which servers are available to do the work. Later, new servers can join the group and old ones can leave without disrupting the services and without the clients even being aware that the system has been reconfigured.. The next group of calls (see Fig. 10-11) deals with killing threads and the threads’ ability to resist. Thecancel call tries to kill a thread, but sometimes killing a thread can have devastating effects, for example, if the thread has a mutex locked at the time. For this reason, threads can arrange for attempts to kill them to be enabled or disabled in various ways, very roughly analogous to the ability of UNIX processes to catch or ignore signals instead of being terminated by them..