How to use select() correctly inside a loop? - c

Question

I am trying to modify an example I found.
The example:
#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;
}
Which print time out if 2.5 seconds has passed without sending a message, otherwise printing a key was pressed.
I tried to put it inside a while loop:
#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;
}
In this code when I enter a key it keeps printing a key was pressed forever.
I read online the I have to set tv variable every time but still no help.
Do i need a temp fd_set
? what am I wrong ?

Answer 1

#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;
}