ncurses in a multithreaded application - c

Question

I have a multi threaded application which uses ncurses on a single thread to report information to the user. My code basically looks like this:
const unsigned int refresh_cycle = 180;
unsigned int refresh_count = refresh_cycle;
while(killswitch != 1) {
if (refresh_count >= refresh_cycle) {
// critical section which obtains some data worked on by a thread. only does this once every refresh cycle times
// mtx lock, fetch, mtx unlock
refresh_count = 0;
}
refresh_count++;
// get input
// draw some stuff
// refresh
}
What I notice is that the ncurses window gets refreshed lots and lots of times. Way more than is really needed for a user who could probably get by with only 15-30 refreshes in a second.
But now I am worrying this might 'steal' unnecesary processing power from one of the threads that is doing work. Is this a reasonable assertion?
Should I build in a sort of frame limiter with usleep() or would that be going overboard?

Answer 1

As per the comments, if in between the refreshes you need to deal with user input, then the easiest way is probably to call select() on STDIN_FILENO with a suitably small timeout. When select() returns, either because there is user input or because it timed out, do a refresh at that point.
Here's an example that'll let you see how to set this up, and shows when when and how many times select() is returning so you can visualize what's going on. Try letting it sit and run for a while, and then try holding down a key, and watch how the select() has returned [n] times message behaves in each case. The comments in the code explain what's happening:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
#include <unistd.h>
#include <time.h>
#include <sys/select.h>
#include <ncurses.h>
/* struct to store curses info for cleanup */
struct curinfo {
WINDOW * main_window;
int old_cursor;
};
/* curses helper functions */
void start_curses(struct curinfo * info);
void stop_curses(struct curinfo * info);
/* main function */
int main(int argc, char * argv[])
{
/* Set default timeout */
int secs = 0;
int usecs = 500000;
/* Set timeout based on command line args, if provided */
if ( argc > 1 ) {
if ( !strcmp(argv[1], "veryshort") ) {
secs = 0;
usecs = 200000;
}
else if ( !strcmp(argv[1], "short") ) {
secs = 1;
usecs = 0;
}
else if ( !strcmp(argv[1], "medium") ) {
secs = 2;
usecs = 0;
}
else if ( !strcmp(argv[1], "long") ) {
secs = 5;
usecs = 0;
}
}
struct curinfo cinfo;
start_curses(&cinfo);
int input = '0'; /* Set to something printable */
int num_sel = 0; /* Number of times select() has returned */
while ( input != 'q' && input != 'Q' ) {
/* Output messages */
mvprintw(3, 3, "select() has returned %d times", num_sel);
mvprintw(4, 3, "Last character input was %c", input);
mvprintw(5, 3, "Press 'q' to quit");
refresh();
/* select() modifies the fd_sets passed to it,
* so zero and set them prior to each call. */
fd_set fds;
FD_ZERO(&fds);
FD_SET(STDIN_FILENO, &fds);
/* Same deal for the struct timeval, select() may
* modify it, it may not, so recreate to be portable. */
struct timeval tv;
tv.tv_sec = secs;
tv.tv_usec = usecs;
/* Store the return so we can check it */
int status = select(STDIN_FILENO + 1, &fds, NULL, NULL, &tv);
/* Check for error */
if ( status == -1 ) {
/* select() returned with an error. */
if ( errno != EINTR ) {
/* If interrupted by a signal, no problem,
* keep going. Otherwise, let's just quit. */
stop_curses(&cinfo);
perror("error calling select()");
return EXIT_FAILURE;
}
}
else if ( FD_ISSET(STDIN_FILENO, &fds) ) {
/* Only call getch() if input is ready.
* getch() will not block when we do it this way. */
if ( (input = getch()) == ERR ) {
stop_curses(&cinfo);
fprintf(stderr, "ERR returned from getch()\n");
return EXIT_FAILURE;
}
}
/* Increment number of times select() has returned */
++num_sel;
}
stop_curses(&cinfo);
return 0;
}
/* Starts curses and populates the passed struct */
void start_curses(struct curinfo * info)
{
if ( (info->main_window = initscr()) == NULL ) {
fprintf(stderr, "Error calling initscr()\n");
exit(EXIT_FAILURE);
}
keypad(stdscr, TRUE);
timeout(0);
raw();
nonl();
noecho();
info->old_cursor = curs_set(0);
refresh();
}
/* Stops curses and cleans up */
void stop_curses(struct curinfo * info)
{
delwin(info->main_window);
curs_set(info->old_cursor);
endwin();
refresh();
}