对于STDIN_FILENO和STDOUT_FILENO，非阻塞I / O行为很奇怪(nonblocking I/O behavior is weird for STDIN_FILENO and STDOUT_FILENO)

 我有以下代码： 
 
void
set_fl(int fd, int flags) /* flags are file status flags to turn on */
{
    int val;

    if ((val = fcntl(fd, F_GETFL, 0)) < 0)
        err_sys("fcntl F_GETFL error");

    val |= flags;       /* turn on flags */

    if (fcntl(fd, F_SETFL, val) < 0)
        err_sys("fcntl F_SETFL error");
}

int
main(void)
{
    char buf[BUFSIZ];
    set_fl(STDOUT_FILENO, O_NONBLOCK);  //set STDOUT_FILENO to nonblock
    if(read(STDIN_FILENO, buf, BUFSIZ)==-1) { //read from STDIN_FILENO
        printf("something went wrong with read()! %s\n", strerror(errno));
    }
}
 
 如您所见，我将STDOUT_FILENO设置为非阻塞模式，但似乎STDIN_FILENO上的读操作立即完成。 为什么？ 
 
$ ./testprog
something went wrong with read()! Resource temporarily unavailable
 
 谢谢 

I have the following code:
 
void
set_fl(int fd, int flags) /* flags are file status flags to turn on */
{
    int val;

    if ((val = fcntl(fd, F_GETFL, 0)) < 0)
        err_sys("fcntl F_GETFL error");

    val |= flags;       /* turn on flags */

    if (fcntl(fd, F_SETFL, val) < 0)
        err_sys("fcntl F_SETFL error");
}

int
main(void)
{
    char buf[BUFSIZ];
    set_fl(STDOUT_FILENO, O_NONBLOCK);  //set STDOUT_FILENO to nonblock
    if(read(STDIN_FILENO, buf, BUFSIZ)==-1) { //read from STDIN_FILENO
        printf("something went wrong with read()! %s\n", strerror(errno));
    }
}
 
As you can see, I set STDOUT_FILENO to non-blocking mode but it seems the read operation on STDIN_FILENO finished immediately. Why?
 
$ ./testprog
something went wrong with read()! Resource temporarily unavailable
 
Thanks

原文：https://stackoverflow.com/questions/9676991