Small but rather quick brainfuck interpreter.

deadbeef.c

/* This is the deadbeef brainfuck interpreter.
 * Robert de Bath (c) 2014-2019 GPL v2 or later.  */

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <limits.h>

void run(void);
struct bfi { int mov; int cmd; int arg; } *pgm = 0;
int pgmlen = 0, on_eof = 1, debug = 0;
int main(int argc, char **argv)
{
   FILE * ifd;
   char * fn = 0, *ar0 = argv[0];
   int ch, p= -1, n= -1, j= -1, i = 0;
   for (; argc > 1; argc--, argv++) {
      if (!strcmp(argv[1], "-e")) on_eof = -1;
      else if (!strcmp(argv[1], "-z")) on_eof = 0;
      else if (!strcmp(argv[1], "-n")) on_eof = 1;
      else if (!strcmp(argv[1], "-x")) on_eof = 2;
      else if (!strcmp(argv[1], "-d")) debug = 1;
      else if ((argv[1][0] == '-' && argv[1][1] != '\0') || fn) {
	 fprintf(stderr, "%s: Unexpected argument: '%s'\n", ar0, argv[1]);
	 exit(1);
      } else fn = argv[1];
   }
   ifd = fn && strcmp(fn, "-") ? fopen(fn, "r") : stdin;
   if(!ifd) { perror(fn); exit(1); } else {
      while((ch = getc(ifd)) != EOF && (ifd!=stdin || ch!='!' || j>=0 || !i)) {
	 int r = (ch == '<' || ch == '>' || ch == '+' || ch == '-');
	 if (r || ch == '[' || ch == ',' || ch == '.' ||
	       (debug && ch == '#') || (ch == ']' && j>=0)) {
	    if (ch == '<') { ch = '>'; r = -r; }
	    if (ch == '-') { ch = '+'; r = -r; }
	    if (r && p>=0 && pgm[p].cmd == ch) { pgm[p].arg += r; continue; }
	    if (p>=0 && pgm[p].cmd == '=' && ch == '+')
		{ pgm[p].arg += r; continue; }
	    if (p>=0 && pgm[p].cmd == '>') { pgm[p].mov = pgm[p].arg; }
	    else {
	       n++;
	       if (n>= pgmlen-2) pgm = realloc(pgm, (pgmlen=n+99)*sizeof*pgm);
	       if (!pgm) { perror("realloc"); exit(1); }
	       pgm[n].mov = 0;
	    }
	    pgm[n].cmd = ch; pgm[n].arg = r; p = n;
	    if (pgm[n].cmd == '[') { pgm[n].cmd=' '; pgm[n].arg=j; j = n; }
	    else if (pgm[n].cmd == ']') {
	       pgm[n].arg=j; j=pgm[r=j].arg; pgm[r].arg=n; pgm[r].cmd='[';
	       if (  pgm[n].mov == 0 && pgm[n-1].mov == 0 &&
		     pgm[n-1].cmd == '+' && (pgm[n-1].arg&1) == 1 &&
		     pgm[n-2].cmd == '[') {
		  n -= 2; pgm[p=n].cmd = '='; pgm[n].arg = 0;
	       } else if (pgm[n-1].cmd == '[') {
		  n--; pgm[p=n].cmd = (pgm[n].arg = pgm[n+1].mov)?'?':'!';
	       }
	    } else if (pgm[n].cmd == ',') i = 1;
	 }
      }
      if (ifd!=stdin) fclose(ifd);
      if (isatty(fileno(stdout))) setbuf(stdout, NULL);
      if (pgm) { pgm[n+1].cmd = 0; run(); }
   }
   return 0;
}

void run(void)
{
   static unsigned char t[(sizeof(int)>sizeof(short))+USHRT_MAX];
   unsigned short m = 0;
   int n, ch;
   for(n=0; ; n++) {
      m += pgm[n].mov;
      switch(pgm[n].cmd)
      {
	 case 0:    return;
	 case '=':  t[m] = pgm[n].arg; break;
	 case '+':  t[m] += pgm[n].arg; break;
	 case '[':  if (t[m] == 0) n=pgm[n].arg; break;
	 case ']':  if (t[m] != 0) n=pgm[n].arg; break;
	 case '?':  while(t[m]) m += pgm[n].arg; break;
	 case '>':  m += pgm[n].arg; break;
	 case '.':  putchar(t[m]); break;
	 case ',':  t[m] = ((ch=getchar())!=EOF)?ch:on_eof<1?on_eof:t[m];
		    if (ch == EOF && on_eof == 2)
			exit((fprintf(stderr, "^D\n"),0));
		    break;
	 case '!':  if(t[m]) exit(2); break;
	 case '#':
	    fprintf(stderr, "\n%3d %3d %3d %3d %3d %3d %3d %3d %3d %3d\n%*s\n",
	       t[0],t[1],t[2],t[3],t[4],t[5],t[6],t[7],t[8],t[9],4*m+3,"^");
	    break;
      }
   }
}

Thanks for the clarification!