如何解决如何在循环内正确使用select?
我正在尝试修改发现的示例。
示例:
#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,&tv);
if (FD_ISSET(STDIN,&readfds))
printf("A key was pressed!\n");
else
printf("Timed out.\n");
return 0;
}
如果在2.5秒内没有发送消息,则打印超时,否则按下了打印键。
我试图将其放入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,&tv) == -1)
printf("err");
if (FD_ISSET(STDIN,&temp))
{
printf("A key was pressed!\n");
}
else
printf("Timed out.\n");
}
return 0;
}
在此代码中,当我输入一个键时,它将一直打印一个键,直到永远被按下。
我在线阅读,每次都必须设置电视变量,但仍然无济于事。
我是否需要临时fd_set
?我怎么了?
解决方法
#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,&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;
}
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