#include <sys/select.h> #include <sys/time.h> int select(int maxfd1, fd_set *readset, fd_set *writeset, fd_set *execpset, const struct timeval *timeout);
（1）timeout参数
该参数告知内核等待所指定描述符中的任何一个就绪可花多长时间；其timeval结构用于指定这段时间的秒数和微秒数
struct timeval { long tv_sec; /* seconds */ long tv_usec; /* microseconds */ };
这个参数有以下三种可能：
i. 永远等待下去：仅在有一个描述符准备好I/O时才返回；为此，可以把参数设置为空指针
ii. 等待一段固定时间：在有一个描述符准备好I/O时才返回，但是不超过由该参数所指向的timeval结构中指定的秒数和微秒数
iii. 根本不等到：检查描述符后立即返回，这成为轮询；为此，该参数必须指向一个timeval结构，而且其中的定时器值必须为0
前两种情形的等待通常会被进程在等待期间捕获的信号中断，并从信号处理函数返回
（2）操作fd_set的四个宏
void FD_ZERO(fd_set *fset); /* clear all bits fdset */ void FD_SET(int fd, fd_set *fdset); /* turn on the bit for fd in fdset */ void FD_CLR(int fd, fd_set *fdset); /* turn off the bit for fd in fdset */ void FD_ISSET(int fd, fd_set *fdset); /* is the bit for fd on in fdset ? */
（3）定义fd_set结构的文件select.h(ubuntu 18.04)
/* `fd_set' type and related macros, and `select'/`pselect' declarations. Copyright (C) 1996-2018 Free Software Foundation, Inc. This file is part of the GNU C Library. The GNU C Library is free software; you can redistribute it and/or modify it under the terms of the GNU Lesser General Public License as published by the Free Software Foundation; either version 2.1 of the License, or (at your option) any later version. The GNU C Library is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public License for more details. You should have received a copy of the GNU Lesser General Public License along with the GNU C Library; if not, see <http://www.gnu.org/licenses/>. */ /* POSIX 1003.1g: 6.2 Select from File Descriptor Sets <sys/select.h> */ #ifndef _SYS_SELECT_H #define _SYS_SELECT_H 1 #include <features.h> /* Get definition of needed basic types. */ #include <bits/types.h> /* Get __FD_* definitions. */ #include <bits/select.h> /* Get sigset_t. */ #include <bits/types/sigset_t.h> /* Get definition of timer specification structures. */ #include <bits/types/time_t.h> #include <bits/types/struct_timeval.h> #ifdef __USE_XOPEN2K # include <bits/types/struct_timespec.h> #endif #ifndef __suseconds_t_defined typedef __suseconds_t suseconds_t; # define __suseconds_t_defined #endif /* The fd_set member is required to be an array of longs. */ typedef long int __fd_mask; /* Some versions of <linux/posix_types.h> define this macros. */ #undef __NFDBITS /* It's easier to assume 8-bit bytes than to get CHAR_BIT. */ #define __NFDBITS (8 * (int) sizeof (__fd_mask)) #define __FD_ELT(d) ((d) / __NFDBITS) #define __FD_MASK(d) ((__fd_mask) (1UL << ((d) % __NFDBITS))) /* fd_set for select and pselect. */ typedef struct { /* XPG4.2 requires this member name. Otherwise avoid the name from the global namespace. */ #ifdef __USE_XOPEN __fd_mask fds_bits[__FD_SETSIZE / __NFDBITS]; # define __FDS_BITS(set) ((set)->fds_bits) #else __fd_mask __fds_bits[__FD_SETSIZE / __NFDBITS]; # define __FDS_BITS(set) ((set)->__fds_bits) #endif } fd_set; /* Maximum number of file descriptors in `fd_set'. */ #define FD_SETSIZE __FD_SETSIZE #ifdef __USE_MISC /* Sometimes the fd_set member is assumed to have this type. */ typedef __fd_mask fd_mask; /* Number of bits per word of `fd_set' (some code assumes this is 32). */ # define NFDBITS __NFDBITS #endif /* Access macros for `fd_set'. */ #define FD_SET(fd, fdsetp) __FD_SET (fd, fdsetp) #define FD_CLR(fd, fdsetp) __FD_CLR (fd, fdsetp) #define FD_ISSET(fd, fdsetp) __FD_ISSET (fd, fdsetp) #define FD_ZERO(fdsetp) __FD_ZERO (fdsetp) __BEGIN_DECLS /* Check the first NFDS descriptors each in READFDS (if not NULL) for read readiness, in WRITEFDS (if not NULL) for write readiness, and in EXCEPTFDS (if not NULL) for exceptional conditions. If TIMEOUT is not NULL, time out after waiting the interval specified therein. Returns the number of ready descriptors, or -1 for errors. This function is a cancellation point and therefore not marked with __THROW. */ extern int select (int __nfds, fd_set *__restrict __readfds, fd_set *__restrict __writefds, fd_set *__restrict __exceptfds, struct timeval *__restrict __timeout); #ifdef __USE_XOPEN2K /* Same as above only that the TIMEOUT value is given with higher resolution and a sigmask which is been set temporarily. This version should be used. This function is a cancellation point and therefore not marked with __THROW. */ extern 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); #endif /* Define some inlines helping to catch common problems. */ #if __USE_FORTIFY_LEVEL > 0 && defined __GNUC__ # include <bits/select2.h> #endif __END_DECLS #endif /* sys/select.h */