如何在循环中正确使用select（）？

我正在尝试修改我找到的一个示例

例如：

#include <stdio.h>
#include <sys/time.h>
#include <sys/types.h>
#include <unistd.h>

#define STDIN 0  // file descriptor for standard input

int main(void)
{
   struct timeval tv;
   fd_set readfds;

   tv.tv_sec = 2;
   tv.tv_usec = 500000;

   FD_ZERO(&readfds);
   FD_SET(STDIN, &readfds);

   // don't care about writefds and exceptfds:
   select(STDIN+1, &readfds, NULL, NULL, &tv);

   if (FD_ISSET(STDIN, &readfds))
       printf("A key was pressed!\n");
   else
       printf("Timed out.\n");

   return 0;
}

如果2.5秒后未发送消息，则打印超时，否则按打印键

我试着把它放在一个while循环中：

#include <stdio.h>
#include <sys/time.h>
#include <sys/types.h>
#include <unistd.h>

#define STDIN 0  // file descriptor for standard input

int main(void)
{
   fd_set readfds, temp;
   struct timeval tv;

   FD_ZERO(&readfds);
   FD_ZERO(&temp);
   FD_SET(STDIN, &readfds);

   while(1){
       
   temp = readfds;     
   tv.tv_sec = 2;
   tv.tv_usec = 500000;
   // don't care about writefds and exceptfds:
   if (select(STDIN+1, &temp, NULL, NULL, &tv) == -1)
       printf("err");

   if (FD_ISSET(STDIN, &temp))
   {
       printf("A key was pressed!\n");
   }
   else
       printf("Timed out.\n");
   }
   return 0;
} 

在这段代码中，当我输入一个键时，它会一直打印一个永远按下的键

我在网上读到，我每次都要设置电视变量，但仍然没有帮助

我需要一个临时fd_设置吗

---

? 我错了什么？

您的代码缺乏正确的缩进可能会让oyu绊倒。请修复它。查找从select获得的返回值以及它对temp内容的含义。另外，-1返回不是错误。您从未从fd读取数据，因此它当然是可读的。

#include <stdio.h>
#include <sys/time.h>
#include <sys/types.h>
#include <unistd.h>
#include <errno.h>

                // Note: STDIN_FILENO, or fileno(stdin)
#define STDIN 0  // file descriptor for standard input

int main(void)
{
   fd_set readfds, temp;
   struct timeval tv;
   int ret,ch;

   FD_ZERO(&readfds);
   FD_ZERO(&temp);
   FD_SET(STDIN, &readfds);

   while(1){

        temp = readfds;
        tv.tv_sec = 2;
        tv.tv_usec = 500000;
        // don't care about writefds and exceptfds:
        ret = select(STDIN+1, &temp, NULL, NULL, &tv) ;
        if (ret == -1) {
                if (errno == EAGAIN) continue; // These are NOT Errors, but natural occuring events
                if (errno == EINTR) continue; // The are reported to avoid your select() to block for too long
                        perror("erreur");
                break;
                }
        else if (ret ==0) {
           printf("Timed out.\n");
                continue;
                }
                // Ok: select has returned > 0; there must be something to read
        if (FD_ISSET(STDIN, &temp)) {
                ch = getc(stdin); // Lookout: stdin is line-buffered
                printf("A key was pressed: %d!\n", ch);
                }
        }
   return 0;
}