Week 11 #

Multiplexing Input and Output #

Handling multiple input/output clients with select and epoll

select #

int select(int n, fd_set *read_fds, fd_set *write_fds, NULL, struct timeval *wait_timeout);

• Blocks until fds are ready
• Returns 0 if reaches the timeout, else returns positive count if some fds are ready
• Modifies the given fd_sets, set them again before the next call
• fd_set: holds a set of file descriptors
• n: The highest fd you specify, + 1

fd_set functions #

void FD_ZERO(fd_set *s);           // empties the set

void FD_SET(int fd, fd_set *s);    // adds an fd to the set

void FD_CLR(int fd, fd_set *s);    // deletes an fd

int FD_ISSET(int fd, fd_set *s);   // queries if fd is in the set

Limitations #

• Max size for fd_set usually 1024
• Slow when using many file descriptors

epoll #

Only available on Linux

int epoll_create1(int flags);

• Creates a new epoll instance
• Returns the file descriptor of this instance (epfd)
• flags: 0 or FD_CLOEXEC
• Instance only useful for other epoll functions

int epol_ctl(int epfd, int op, int another_epfd, struct epoll_event *ev);

• Sets what the epoll instance will wait for
• Returns 0 if successful
• op: EPOLL_CTL_ADD, EPOLL_CTL_DEL, EPOLL_CTL_MOD
• ev: Events to wait for

int epoll_wait(int epfd, struct epoll_event *evs, int n, int timeout);

• evs: Array to recieve events
• n: Length of events array
• Returns count of ready file descriptors

struct epoll_event {
  uint32_t      events;
  epoll_data_t  data;
};

• You can combine events using bitwise operators

• events:

  • EPOLLIN: Ready to read

  • EPOLLOUT: Ready to write

  • EPOLLONESHOT: Monitor once only

  • EPOLLET: Notify whenever changes from not-ready to ready

  • EPOLLHUP: Other end of pipe/socket has closed

  • EPOLLERR: Error condition

typedef union epoll_data {
  void      *ptr;
  int       fd;
  uint32_t  u32;
  uint64_t  u64;
} epoll_data_t;

• You store to this data when calling epoll_ctl
• Stored data gets returned when you call epoll_wait
• Usually store the fd, or a pointer to some struct to keep track of history