I have no idea why it's broken

brainfuck.c

// Copyright Killaship (C) 2022
// Informally, this is under the MIT licence, where basically you can do whatever, just give me credit.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define PROG_SIZE 30000
unsigned char dcells[30000];
unsigned char program[PROG_SIZE];
int data = 0;
int instr = 0;
int stack[PROG_SIZE];
int targets[PROG_SIZE];
int sp = 0;
/*
BRAINFART: a brainf*ck interpreter by a guy who has had their brain fucked over by having shitty programming skills :D
*/


void loadfart(char *file) { // this has got to be the funniest function name I've written, although it just loads the program
	    FILE *prog_file  = fopen(file, "r");
        // test for files not existing.
        if(prog_file == NULL) {
        		printf("Error! Failed to load file\n");
                exit(-1);
        }
     	char op = 0;
        int counter = 0;
        while(fscanf(prog_file, "%c", &op) == 1) {
                program[counter] = op;
                counter++;
        }

        for(int i = 0; i < PROG_SIZE; i++) {
        	if((int) program[i] == 0) {
        		break;
        	}  	
        	printf("%c", program[i]);
        }
        printf("\n");
        for(int codep=0; codep < PROG_SIZE; codep++){
  			if (program[codep]=='[') {
  				stack[sp++]=codep;//put each '[' on the stack
  			}
            if (program[codep]==']') {
            	if(sp==0){ //and there is no '[' left on the stack, it's an error.
                	fprintf(stderr,"Unmatched ']' at byte %d.", codep), exit(1);
                } else {
           		 --sp; //if there is one, we take the matching '[' from the stack top,
            	targets[codep] = stack[sp]; //save it as the match for the current ']',
            	targets[stack[sp]] = codep; //and save the current ']' as the match for it.
            	}
            }
       }
       if(sp>0){ //Any unmatched '['s still left on the stack are an error too.
       		fprintf(stderr,"Unmatched '[' at byte %d.", stack[--sp]), exit(1);
      	}
            	                             

}    


int main(int argc, char **argv) {
	if(argc != 2) {
		printf("usage: brainfart /path/to/program \n");
		exit(-1);
	}
	loadfart(argv[1]);

	while(instr <= PROG_SIZE) {
	//	char input;
		switch(program[instr]) {
			case '>':
				data++;
			case '<':
				data--;
			case '+':
				if(dcells[data] == 255) {
					dcells[data] = 0;
				}
				else {
					dcells[data]++;
				}
				break;
			case '-':
				if(dcells[data] == 0) {
					dcells[data] = 255;
				}
				else { 
					dcells[data]--;
				}
				break;
			case '.': // output syscall
				putchar(dcells[data]);
				break;
			case ',': //input syscall
				// skipped over, for now
				break;
			case '[':
				printf("openingbracket\n");
				if(dcells[targets[instr]] == 0) {
					instr = targets[instr];
					instr++;
				}
				break;
			case ']':			
				printf("closingbracket\n");	
				if(dcells[targets[instr]] != 0) {
					instr = targets[instr];
					instr--;
				}
				break;
			default:
				// do nothing
				break;
		}
		instr++;
	}
	return 0;
}

help