1、获取文件的flags，即open函数的第二个参数:

flags = fcntl(fd,F_GETFL,0);

2、设置文件的flags:

fcntl(fd,F_SETFL,flags);

3、增加文件的某个flags，比如文件是阻塞的，想设置成非阻塞:

flags = fcntl(fd,F_GETFL,0);   flags |= O_NONBLOCK;  fcntl(fd,F_SETFL,flags);

4、取消文件的某个flags，比如文件是非阻塞的，想设置成为阻塞:

flags = fcntl(fd,F_GETFL,0);  flags &= ~O_NONBLOCK;  fcntl(fd,F_SETFL,flags);

获取和设置文件flags举例::

#include 
    
     #include 
     
      #include 
      
       #include 
       
        #include 
        
         char buf[500000];int main(int argc,char *argv[]){        int ntowrite,nwrite;        const char *ptr ;        int flags;            ntowrite = read(STDIN_FILENO,buf,sizeof(buf));        if(ntowrite <0)         {                   perror("read STDIN_FILENO fail:");                exit(1);        }           fprintf(stderr,"read %d bytes\n",ntowrite);            if((flags = fcntl(STDOUT_FILENO,F_GETFL,0))==-1)        {                   perror("fcntl F_GETFL fail:");                exit(1);        }           flags |= O_NONBLOCK;        if(fcntl(STDOUT_FILENO,F_SETFL,flags)==-1)        {                   perror("fcntl F_SETFL fail:");                exit(1);        }               ptr = buf;        while(ntowrite > 0)        {                   nwrite = write(STDOUT_FILENO,ptr,ntowrite);                if(nwrite == -1)                 {                          perror("write file fail:");                }                   if(nwrite > 0)                {                           ptr += nwrite;                        ntowrite -= nwrite;                }           }               flags &= ~O_NONBLOCK;        if(fcntl(STDOUT_FILENO,F_SETFL,flags)==-1)        {                   perror("fcntl F_SETFL fail2:");        }           return 0;}