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Multiplexed I/O

Multiplexed I/O. Motivation. Consider a process that reads from multiple sources without knowing in advance which source will provide some input first Three solutions: alternate non-blocking reads on input sources (wasteful of CPU)

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Multiplexed I/O

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  1. Multiplexed I/O

  2. Motivation • Consider a process that reads from multiple sources without knowing in advance which source will provide some input first • Three solutions: • alternate non-blocking reads on input sources (wasteful of CPU) • fork a process for each input source, and each child can block on one specific input source (can be hard to coordinate/synchronize) • use the select() system call … (see next slide)

  3. select() (Wang, 12.14) • Usage: #include <sys/time.h> #include <sys/types.h> int select( int nfds, fd_set *readfds, fd_set *writefds, fd_set *exceptfds, struct timeval *timeout ); • where the three fd_set variables are file descriptor masks • fd_set is defined in <sys/select.h>, which is included by <sys/types.h>

  4. Details • The first argument (nfds) represents the number of bits in the masks that will be processed. Typically, this is 1 + the value of the highest fd • The three fd_set arguments are bit masks … their manipulation is discussed on the next slide • The last argument specifies the amount of time the select call should wait before completing its action and returning: • if NULL, select will wait (block) indefinitely until one of the file descriptors is ready for i/o • if tv_sec and tv_usec are zero, select will return immediately • if timeval members are non-zero, the system will wait the specified time or until a file descriptor is ready for i/o • select() returns the number or file descriptors ready for i/o

  5. “FD_” macros • Useful macros defined in <sys/select.h> to manage the masks: void FD_ZERO( fd_set &fdset ); void FD_SET( int fd, fd_set &fdset ); void FD_CLR( int fd, fd_set &fdset ); int FD_ISSET( int fd, fd_set &fdset ); • Note that each macro is passed the address of the file descriptor mask

  6. Example #include <sys/types.h> fd_set rmask; int fd; /* a socket or file descriptor */ FD_ZERO( &rmask ); FD_SET( fd, &rmask ); FD_SET( fileno(stdin), &rmask ); for(;;) { select( fd+1, &rmask, NULL, NULL, NULL ); if( FD_ISSET( fileno(stdin, &rmask ) ) /* read from stdin */ if( FD_ISSET( fd, &rmask ) ) /* read from descriptor fd */ FD_SET( fd, &rmask); FD_SET( fileno(stdin), &rmask ); }

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