fset.cpp

#include <stdio.h>
#include <string.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <sys/time.h>
#include <stdlib.h>

#define __NFDBITS (8 * sizeof(unsigned long))                //每个ulong型可以表示多少个bit,
#define __FD_SETSIZE 1024                                          //socket最大取值为1024
#define __FDSET_LONGS (__FD_SETSIZE/__NFDBITS)     //bitmap一共有1024个bit，共需要多少个ulong
 
typedef struct {
    unsigned long fds_bits [__FDSET_LONGS];                 //用ulong数组来表示bitmap
} __kernel_fd_set;
 
typedef __kernel_fd_set   fd_set;

//每个ulong为32位，可以表示32个bit。
//fd  >> 5 即 fd / 32，找到对应的ulong下标i；fd & 31 即fd % 32，找到在ulong[i]内部的位置
 
#define __FD_SET(fd, fdsetp)   (((fd_set *)(fdsetp))->fds_bits[(fd) >> 5] |= (1<<((fd) & 31)))             //设置对应的bit
#define __FD_CLR(fd, fdsetp)   (((fd_set *)(fdsetp))->fds_bits[(fd) >> 5] &= ~(1<<((fd) & 31)))            //清除对应的bit
#define __FD_ISSET(fd, fdsetp)   ((((fd_set *)(fdsetp))->fds_bits[(fd) >> 5] & (1<<((fd) & 31))) != 0)     //判断对应的bit是否为1
#define __FD_ZERO(fdsetp)   (memset (fdsetp, 0, sizeof (*(fd_set *)(fdsetp))))                             //memset bitmap

int main(int argc ,char **argv)
{
    fd_set allset;
    cout << sizeof(allset.fds_bits[0]) << endl;
    FD_ZERO(&allset);
    for(int fd=0; fd<=10; fd++){

        FD_SET(fd, &allset);
        cout << fd << ":";
        for(int i=0; i<16;i++){
            printf("%llu ", allset.fds_bits[i]);
        }
        cout << endl;
    }

    return 0;
}

//8
//0:1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
//1:3 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
//2:7 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
//3:15 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
//4:31 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
//5:63 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
//6:127 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
//7:255 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
//8:511 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
//9:1023 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
//10:2047 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0