select statement

The SELECT System Call
The select system call accepts 5 arguments:

int select(int max_fd, fd_set *input, fd_set *output, fd_set *error,
           struct timeval *timeout);
The max_fd argument specifies the highest numbered file descriptor in the input, output, and error sets. The input, output, and error arguments specify sets of file descriptors for pending input, output, or error conditions; specify NULL to disable monitoring for the corresponding condition. These sets are initialized using three macros:

FD_ZERO(fd_set);
FD_SET(fd, fd_set);
FD_CLR(fd, fd_set);
The FD_ZERO macro clears the set entirely. The FD_SET and FD_CLR macros add and remove a file descriptor from the set, respectively.

The timeout argument specifies a timeout value which consists of seconds (timeout.tv_sec) and microseconds (timeout.tv_usec ). To poll one or more file descriptors, set the seconds and microseconds to zero. To wait indefinitely specify NULL for the timeout pointer.

The select system call returns the number of file descriptors that have a pending condition, or -1 if there was an error.

Using the SELECT System Call
Suppose we are reading data from a serial port and a socket. We want to check for input from either file descriptor, but want to notify the user if no data is seen within 10 seconds. To do this we'll need to use the select system call:

Listing 8 - Using SELECT to process input from more than one source.

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

int            n;
int            socket;
int            fd;
int            max_fd;
fd_set         input;
struct timeval timeout;

/* Initialize the input set */
FD_ZERO(input);
FD_SET(fd, input);
FD_SET(socket, input);

max_fd = (socket > fd ? socket : fd) + 1;

/* Initialize the timeout structure */
timeout.tv_sec  = 10;
timeout.tv_usec = 0;

/* Do the select */
n = select(max_fd,  NULL, NULL, ;

/* See if there was an error */
if (n 0)
  perror("select failed");
else if (n == 0)
  puts("TIMEOUT");
else
{
  /* We have input */
  if (FD_ISSET(fd, input))
    process_fd();
  if (FD_ISSET(socket, input))
    process_socket();
}
You'll notice that we first check the return value of the select system call. Values of 0 and -1 yield the appropriate warning and error messages. Values greater than 0 mean that we have data pending on one or more file descriptors.

To determine which file descriptor(s) have pending input, we use the FD_ISSET macro to test the input set for each file descriptor. If the file descriptor flag is set then the condition exists (input pending in this case) and we need to do something.