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Efficient Multiplexed I/O: Utilizing the Select System Call for Managing Multiple Input Sources

In this guide, we address the challenge of reading from multiple input sources without prior knowledge of which source will provide input first. Three proposed solutions are discussed: non-blocking reads, forking a process for each input source, and using the `select()` system call. We delve into the details of the `select()` function, including its parameters, which manage file descriptor sets and timeout periods. Relevant macros are also introduced for handling these sets effectively, providing a structured approach to efficiently monitor and process multiple inputs in concurrent programming.

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Efficient Multiplexed I/O: Utilizing the Select System Call for Managing Multiple Input Sources

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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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