select(2)

select, pselect, FD_CLR, FD_ISSET, FD_SET, FD_ZERO - synchronous I/O multiplexing

SYNOPSIS

#include <sys/select.h>

int select(int nfds, fd_set *restrict readfds,
           fd_set *restrict writefds, fd_set *restrict exceptfds,
           struct timeval *restrict timeout);

void FD_CLR(int fd, fd_set *set);
int  FD_ISSET(int fd, fd_set *set);
void FD_SET(int fd, fd_set *set);
void FD_ZERO(fd_set *set);

int pselect(int nfds, fd_set *restrict readfds,
            fd_set *restrict writefds, fd_set *restrict exceptfds,
            const struct timespec *restrict timeout,
            const sigset_t *restrict sigmask);

NOTE:

一、通过原文的"File descriptor sets"、"NOTES"、"BUGS"可以总结出select设计上的缺陷:

1、fd_set 的大小是固定的，这就导致了select需要逐个遍历来找到用户设置的需要监控的file descriptor，显然这浪费了空间、时间

2、fd_set 的大小是固定的，注定到了select无法解决C10K问题，因为当用户量非常大的时候，线性遍历是非常耗时的

3、如"File descriptor sets"段中所描绘的:

"Thus, if using select() within a loop, the sets must be reinitialized before each call. The implementation of the fd_set arguments as value-result arguments is a design error that is avoided in poll(2) and epoll(7)."

这就需要用户进行频繁的copy(在Redis中，演示了实现方式)，显然这就浪费了时间、导致了编程的麻烦

File descriptor sets

The principal arguments of select() are three "sets" of file descriptors (declared with the type fd_set), which allow the caller to wait for three classes of events on the specified set of file descriptors. Each of the fd_set arguments may be specified as NULL if no file descriptors are to be watched for the corresponding class of events.

Note well: Upon return, each of the file descriptor sets is modified in place to indicate which file descriptors are currently "ready". Thus, if using select() within a loop, the sets must be reinitialized before each call. The implementation of the fd_set arguments as value-result arguments is a design error that is avoided in poll(2) and epoll(7).

NOTES

An fd_set is a fixed size buffer. Executing FD_CLR() or FD_SET() with a value of fd that is negative or is equal to or larger than FD_SETSIZE will result in undefined behavior. Moreover, POSIX requires fd to be a valid file descriptor.

NOTE:

结合后面的"BUGS" 段，就可知select(2)的设计是多么地不合理

The self-pipe trick

BUGS

POSIX allows an implementation to define an upper limit, advertised via the constant FD_SETSIZE, on the range of file descriptors that can be specified in a file descriptor set. The Linux kernel imposes no fixed limit, but the glibc implementation makes fd_set a fixed-size type, with FD_SETSIZE defined as 1024, and the FD_*() macros operating according to that limit. To monitor file descriptors greater than 1023, use poll(2) or epoll(7) instead.