fd究竟是何方神圣

#include <sys/stat.h>
#include <fcntl.h>
#include <stdlib.h>
#include <stdio.h>

 
 #define F_LEN 1024
 char filename[F_LEN]="temp";
 
 int
 main(void)
 {
    int fd;
    char buf[F_LEN];
 
    if((fd =open(filename, O_RDWR))< 0){
       perror("open error");
       exit(0);
    }
    snprintf(filename, F_LEN, "/proc/%ld/fd/%d",(long)getpid(), fd);
    if(readlink(filename, buf, F_LEN)< 0){
       perror("readlink() ");
       exit(0);
    }
    printf("In this process fd=%d<->filename=%s\n", fd, buf);
	pause(); 
    return 0;
 }