C - 20 Line Pager w/Line Numbers and Continue/Quit Control

#include <stdio.h>
#include <string.h>
#include <ctype.h>
#include <termios.h>

#define MAXC 1024       /* if you need a constant, #define one (or more) */
#define LINES  20

/* avoid use of global variables, okay when used as flag or in callback functions.
 * you could pass address of both struct termios variables as parameters if desired.
 */
static struct termios cooked, raw;  /* keyboard modes cooked (normal) and raw */
static int inrawmode;
static int modesfilled;
static int linenumbers;

/** set (toggle) keyboard between cooked (normal buffered mode) and
 *  raw (unbuffered / noncanonical) mode each time the function is called.
 *  on first call, the termios structs cooked & raw are filled. for all
 *  remaining calls, tcsetattr either sets raw mode or restores cooked
 *  mode based on the flag 'inrawmode'.
 */
int kbd_toggle_raw_mode (void)
{
    if (inrawmode) {                        /* if in raw mode, switch back to cooked */
        if (tcsetattr (0, TCSANOW, &cooked)) {  /* validate change */
            fprintf (stderr, "%s() error: tcsetattr failed.\n", __func__);
            return -1;
        }
        inrawmode = 0;                      /* set in raw mode flag false */
    }
    else {                                  /* in cooked mode, switch to raw mode */
        if (!modesfilled) {                 /* if first time called */
            if (tcgetattr (0, &cooked)) {   /* save orig settings / validate */
                fprintf (stderr, "%s() error: tcgetattr failed.\n", __func__);
                return -1;
            }
            /* copy cooked to raw to initialize raw struct */
            memcpy (&raw, &cooked, sizeof(struct termios));

            raw.c_lflag &= ~(ICANON | ECHO);  /* set flags for raw mode */
            raw.c_cc[VTIME] = 0;
            raw.c_cc[VMIN] = 1;

            modesfilled = 1;                /* set flag indicating both structs filled */
        }
        if (tcsetattr (0, TCSANOW, &raw)) { /* set raw mode / validate */
            fprintf (stderr, "%s() error: tcsetattr failed.\n", __func__);
            return -1;
        }
        inrawmode = 1;                      /* set in raw mode flag true */
    }

    return 0;
}

/** requires filename to page as first argument, any second
 *  argument will enable line number of output.
 *  pressing any whitespace (space, tab, newline) will page
 *  next 20 lines, 'c' will continue to end, 'q' will quit
 *  at that point.
 */
int main (int argc, char **argv) {

    char line[MAXC];                    /* buffer to hold each line */
    int n = 0, pager = 1;               /* line counter, flag to page/continue to end */
    /* use filename provided as 1st argument (stdin by default)
     * allows piping output or redirecting file to program
     * (no line numbering control in that case of input on stdin)
     */
    FILE *fp = argc > 1 ? fopen (argv[1], "r") : stdin;

    if (!fp) {  /* validate file open for reading */
        perror ("file open failed");
        return 1;
    }

    if (argc > 2)                               /* if addition argument given */
        linenumbers = 1;                        /* enable line numbers */

    while (fgets (line, MAXC, fp)) {            /* read each line into line */
        n += 1;                                 /* increment line number counter */
        if (linenumbers)                        /* if line number flag set */
            printf ("%3d  ", n);                /* output line numbers */
        fputs (line, stdout);                   /* output line */
        if (pager && n % LINES == 0) {          /* pager set && LINES no. of lines out */
            int c, quit = 0;                    /* char to read, quit flag */
            if (kbd_toggle_raw_mode() == -1)    /* if error in setting raw mode, warn */
                fputs ("\nerror: unable to change to raw keyboard mode.\n\n", stderr);
            /* read user input, check if not space and not EOF */
            for (c = getchar(); !isspace(c) && c != EOF; c = getchar())
                if (c == 'c') {                 /* if user input 'c' */
                    pager = 0;                  /* set pager flag false */
                    break;
                }
                else if (c == 'q') {            /* if user input 'q' */
                    quit = 1;                   /* set quit flag true */
                    break;
                }
            if (kbd_toggle_raw_mode() == -1)    /* if error switing to cooked, warn */
                fputs ("\nerror: unable to restore keyboard mode.\n\n", stderr);
            if (c == EOF || quit)               /* if c is EOF, manual EOF from user */
                break;                          /* Ctrl+d Unix, (Ctrl+z on windows) */
        }
    }

    if (fp != stdin)   /* close file if not stdin */
        fclose (fp);

    return 0;
}