Linux：fd_set 结构体定义及使用

在使用select函数时，fd_set结构体是很重要的。

想正确使用select函数，理解fd_set是必不可少的。

<sys/select.h>

下面给出<sys/select.h>头文件的全部内容：

/* `fd_set' type and related macros, and `select'/`pselect' declarations.Copyright (C) 1996-2016 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/ormodify it under the terms of the GNU Lesser General PublicLicense as published by the Free Software Foundation; eitherversion 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 ofMERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNULesser General Public License for more details.You should have received a copy of the GNU Lesser General PublicLicense 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/sigset.h>#ifndef __sigset_t_defined
# define __sigset_t_defined
typedef __sigset_t sigset_t;
#endif/* Get definition of timer specification structures.  */
#define __need_time_t
#ifdef __USE_XOPEN2K
# define __need_timespec
#endif
#include <time.h>
#define __need_timeval
#include <bits/time.h>#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 namefrom 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 readreadiness, in WRITEFDS (if not NULL) for write readiness, and in EXCEPTFDS(if not NULL) for exceptional conditions.  If TIMEOUT is not NULL, time outafter waiting the interval specified therein.  Returns the number of readydescriptors, 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 higherresolution and a sigmask which is been set temporarily.  This versionshould 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 */

fd_set结构体

由上面可以看到，fd_set结构体的定义为：

/* fd_set for select and pselect.  */
typedef struct{/* XPG4.2 requires this member name.  Otherwise avoid the namefrom 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;

其中 __fd_mask 为long int 类型，查看select.h文件第56行：

/* The fd_set member is required to be an array of longs.  */
typedef long int __fd_mask;

和select模型紧密相关的四个宏：

/* 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)

<bits/select.h>

定义在<bits/select.h>头文件中，该文件内容如下：

/* Copyright (C) 1997-2016 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/ormodify it under the terms of the GNU Lesser General PublicLicense as published by the Free Software Foundation; eitherversion 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 ofMERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNULesser General Public License for more details.You should have received a copy of the GNU Lesser General PublicLicense along with the GNU C Library; if not, see<http://www.gnu.org/licenses/>.  */#ifndef _SYS_SELECT_H
# error "Never use <bits/select.h> directly; include <sys/select.h> instead."
#endif/* We don't use `memset' because this would require a prototype andthe array isn't too big.  */
#define __FD_ZERO(s) \do {                                                                        \unsigned int __i;                                                         \fd_set *__arr = (s);                                                      \for (__i = 0; __i < sizeof (fd_set) / sizeof (__fd_mask); ++__i)          \__FDS_BITS (__arr)[__i] = 0;                                            \} while (0)
#define __FD_SET(d, s) \((void) (__FDS_BITS (s)[__FD_ELT(d)] |= __FD_MASK(d)))
#define __FD_CLR(d, s) \((void) (__FDS_BITS (s)[__FD_ELT(d)] &= ~__FD_MASK(d)))
#define __FD_ISSET(d, s) \((__FDS_BITS (s)[__FD_ELT (d)] & __FD_MASK (d)) != 0)

宏的定义如上所示。具体功能介绍，参考下面链接：

https://blog.csdn.net/starflame/article/details/7860091

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Linux：fd_set 结构体定义及使用

<sys/select.h>

fd_set结构体

<bits/select.h>

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