COMS W4118 Operating Systems I

Nonblocking I/O and I/O multiplexing

Nonblocking I/O

• Two ways to make “slow” systems calls nonblocking:

  1. call open() with O_NONBLOCK
  2. call fcntl() to turn on O_NONBLOCK file status flag
     • file status flag is part of file table entry (the middle layer)

• Nonblocking slow system call returns -1 with errno set to EAGAIN if it would have blocked

• set_fl() and clr_fl() wrappers on fcntl() from APUE:

  #include "apue.h"
  #include <fcntl.h>
  
  void set_fl(int fd, int flags) /* flags are file status flags to turn on */
  {
          int		val;
  
          if ((val = fcntl(fd, F_GETFL, 0)) < 0)
                  err_sys("fcntl F_GETFL error");
  
          val |= flags;		/* turn on flags */
  
          if (fcntl(fd, F_SETFL, val) < 0)
                  err_sys("fcntl F_SETFL error");
  }
  
  void clr_fl(int fd, int flags) /* flags are file status flags to turn off */
  {
          int		val;
  
          if ((val = fcntl(fd, F_GETFL, 0)) < 0)
                  err_sys("fcntl F_GETFL error");
  
          val &= ~flags;		/* turn flags off */
  
          if (fcntl(fd, F_SETFL, val) < 0)
                  err_sys("fcntl F_SETFL error");
  }
  

• Nonblocking write example:

  #include "apue.h"
  #include <errno.h>
  #include <fcntl.h>
  
  char    buf[500000];
  
  int
  main(void)
  {
      int     ntowrite, nwrite;
      char    *ptr;
  
      ntowrite = read(STDIN_FILENO, buf, sizeof(buf));
      fprintf(stderr, "read %d bytes\n", ntowrite);
  
      set_fl(STDOUT_FILENO, O_NONBLOCK);  /* set nonblocking */
  
      ptr = buf;
      while (ntowrite > 0) {
          errno = 0;
          nwrite = write(STDOUT_FILENO, ptr, ntowrite);
          fprintf(stderr, "nwrite = %d, errno = %d\n", nwrite, errno);
  
          if (nwrite > 0) {
              ptr += nwrite;
              ntowrite -= nwrite;
          }
      }
  
      clr_fl(STDOUT_FILENO, O_NONBLOCK);  /* clear nonblocking */
  
      exit(0);
  }
  

I/O multiplexing

• Consider the read-write loop in cat:

  while ((n = read(STDIN_FILENO, buf, BUFSIZ)) > 0)
      if (write(STDOUT_FILENO, buf, n) != n)
          err_sys("write error");
  

  How would netcat (nc) perform two such loops at the same time?

• select() API for I/O multiplexing:

  #include <sys/select.h>
  
  int select(int maxfdp1, // max fd plus 1, or simply pass FD_SETSIZE
  
             fd_set *restrict readfds,   // see if they're ready for reading
             fd_set *restrict writefds,  // see if they're ready for writing
             fd_set *restrict exceptfds, // see if exceptional condition occurred
                                         // ex) urgent out-of-band data in TCP
  
             struct timeval *restrict tvptr); // timeout
  
          // Returns: count of ready descriptors, 0 on timeout,–1 on error
  
  int FD_ISSET(int fd, fd_set *fdset);
  
          // Returns: nonzero if fd is in set, 0 otherwise
  
  void FD_CLR(int fd, fd_set *fdset);
  void FD_SET(int fd, fd_set *fdset);
  void FD_ZERO(fd_set *fdset);
  

• Things to note well on select() API:
  • stupid maxfdp1 parameter - just pass FD_SETSIZE, which is 1024
  • stupid input-and-output fds parameters
  • poll() does the same thing and has a better API
  • select() (as well as poll()) will always get interrupted on signals, even when the SA_RESTART flag was used

• Nonblocking write example, augmented with select():

  ptr = buf;
  while (ntowrite > 0) {
          errno = 0;
  
          // Wait until stdout is ready before we call write()
          fd_set write_fds;
          FD_ZERO(&write_fds);
          FD_SET(STDOUT_FILENO, &write_fds);
          select(STDOUT_FILENO + 1, NULL, &write_fds, NULL, NULL);
  
          nwrite = write(STDOUT_FILENO, ptr, ntowrite);
          fprintf(stderr, "nwrite = %d, errno = %d\n", nwrite, errno);
          if (nwrite > 0) {
                  ptr += nwrite;
                  ntowrite -= nwrite;
          }
  }
  

• Example: Single-thread Multi-Server

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Last updated: 2023-02-09