Why does my program keep getting stuck while running the mandelbrot brainf*** program?

Question

I wanted to improve my C skills, so I search some program's ideas.

Someone propose to create a simple Brainf*** interpreter and then a compiler. So here I am.

I created the interpreter and it works as expected, except with the Mandelbrot program:

A mandelbrot set fractal viewer in brainfuck written by Erik Bosman
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>>>>>>>>>]+[<<<<<<<<<]>>>>>>>>>-]+[>>>>>>>>[-]>]<<<<<<<<<[<<<<<<<<<]>>>>>>>>[-]+
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>>>>>>>>[-<<<<<<<<+>[<->-<<+>>]<[->+<]>>>>>>>>]<<<<<<<[->>>>>>>+<<<<<<<]<+<<<<<<
<<<]>>>>>>>>-<<<<<[-]+<<<]+>>>>>>>>[-<<<<<<<<->>>>>>>>]+<<<<<<<<[->>>>>>>>->[>>>
>>>[->>+<<]>>>]<<<<<<<<<[>[-]<->>>>>>>>[-<<<<<<<<+>[<->-<<+>>]<[->+<]>>>>>>>>]<<
<<<<<[->>>>>>>+<<<<<<<]<+<<<<<<<<<]>+++++[-[->>>>>>>>>+<<<<<<<<<]>>>>>>>>>]>>>>>
+>>>>>>>>>>>>>>>>>>>>>>>>>>>+<<<<<<[<<<<<<<<<]>>>>>>>>>[>>>>>>[-<<<<<<->>>>>>]+<
<<<<<[->>>>>>->>[-<<<<<<<<+>>>>>>>>]<<<<<<<<[->>>>>>>>+<<<<<<<<<<<<<<<<<[<<<<<<<
<<]>>>>[-]+>>>>>[>>>>>>>>>]>+<]]+>>>>>>>>[-<<<<<<<<->>>>>>>>]+<<<<<<<<[->>>>>>>>
-<<[-<<<<<<+>>>>>>]<<<<<<[->>>>>>+<<<<<<<<<<<<<<<[<<<<<<<<<]>>>[-]+>>>>>>[>>>>>>
>>>]>[-]+<]]+>[-<[>>>>>>>>>]<<<<<<<<]>>>>>>>>]<<<<<<<<<[<<<<<<<<<]>>>>[-]<<<++++
+[-[->>>>>>>>>+<<<<<<<<<]>>>>>>>>>]>>>>>->>>>>>>>>>>>>>>>>>>>>>>>>>>-<<<<<<[<<<<
<<<<<]]>>>]

I don't understand why, but my program keeps getting stuck in some sort of endless loop. I try to debug it using gdb but it's hard to find the right breakpoint in order to see why it stuck.

The program is as follows:

#include <stdio.h>
#include <stdlib.h>

// Brainfuck instructions
//  > Math stuff
#define INCR '+' // +1 on current MS
#define DECR '-' // -1 on current MS
//  > Memory slot stuff
#define FORW '>' // Go to next MS
#define BACK '<' // Go to previous MS
//  > Logic stuff
#define SJMP '[' // Loop till current MS value is equal to zero
#define EJMP ']' // Jump to the beginning of the loop
//  > I/O stuff
#define PRTC '.' // Print character with MS value as ASCII code
#define GETC ',' // Get an user input ASCII character code

typedef unsigned char byte;

// The brainfuck program struct
typedef struct {
      byte *values;
      int size;
      int index;

      byte *loops_starts;
      int loop_size;
      int loop_index;

} BrainfuckProgram;

void initialize(BrainfuckProgram *bfp) {
      bfp->values = malloc(bfp->size * sizeof(*bfp->values));
      bfp->loops_starts = malloc(bfp->size * sizeof(*bfp->loops_starts));
}

int allocate_values(BrainfuckProgram *bfp) {
      byte *newMem = realloc(bfp->values, bfp->size * sizeof(byte));

      if (!newMem) {
            return 1;
      }
      bfp->values = newMem;

      return 0;
}

int allocate_new_loop(BrainfuckProgram *bfp) {
      byte *newMem = realloc(bfp->loops_starts, bfp->loop_size * sizeof(byte));

      if (!newMem) {
            return 1;
      }
      bfp->loops_starts = newMem;

      return 0;
}

void set_value(BrainfuckProgram *bfp, byte value) {
      int tempValue = value < 0 ? 255: value % 256;
      *(bfp->values + bfp->index) = tempValue;
}

int get_value(BrainfuckProgram *bfp) {
      return *(bfp->values + bfp->index);
}

// FILE functions
int get_file_name(int argc, char *argv[], char** fname) {
     if (argc == 1) {
           printf("File name is missing\n");
           return 1;
     }

     *fname = argv[1];
     return 0;
}

int get_file_size(FILE *file) {
    fseek(file, 0L, SEEK_END);
    int size = ftell(file);
    rewind(file);
    return size;
}

// Main function
int main(int argc, char *argv[]) {
      // Vector part
      BrainfuckProgram bfp = {
            .values = NULL,
            .size = 1,
            .index = 0,

            .loops_starts = NULL,
            .loop_index = 0,
            .loop_size = 0

      };


      initialize(&bfp);

      // FILE Part
    char* fname = NULL;
      int error = get_file_name(argc, argv, &fname);
      if (error) {
            goto exit;
      }

    // Create file variable
    FILE *fin = NULL;
    fin = fopen(fname, "r");

    int size = get_file_size(fin);

    // Create a char array of the right size
    byte *prog = NULL;
    prog = malloc(size * sizeof(*prog));
      // Read 1 byte size times
    fread(prog, 1, size, fin);

    for (int i = 0; i < size; i++) {
            byte current =  *(prog+i);
        switch (current) {
            case INCR:
                        set_value(&bfp, get_value(&bfp) + 1);
                break;

            case DECR:
                        set_value(&bfp, get_value(&bfp) - 1);
                break;

            case FORW:
                        // if index+1 == size then we are at the
                        // last element of the vector cause
                        // size = index - 1
                        if (bfp.index+1 == bfp.size) {
                              bfp.size++;

                              int error = allocate_values(&bfp);
                              if (error) {
                                    printf("an error occured while forwarding pointer");
                                    goto close;
                              }
                        }
                        bfp.index++;

                break;

            case BACK:
                        // bfp.index == 0 then !bfp.index == true
                        if (!bfp.index) {
                             printf("can go back from index 0\n");
                             goto close;
                        }
                        bfp.index--;

                break;

            case SJMP:
                        if (bfp.loop_index == bfp.size) {
                              bfp.loop_size++;

                              int error = allocate_new_loop(&bfp);
                              if (error) {
                                    printf("an error occured while forwarding pointer");
                                    goto close;
                              }

                        }
                        bfp.loop_index++;
                        *(bfp.loops_starts + bfp.loop_index) = i;

                break;

            case EJMP:
                        if (bfp.loop_index == -1) {
                              printf("cannot go back");
                              goto close;
                        }

                        if (get_value(&bfp) != 0) {
                              i = *(bfp.loops_starts + bfp.loop_index);

                        } else {
                              bfp.loop_index--;

                        }

                break;

            case PRTC:
                        printf("%c", get_value(&bfp));

                break;

            case GETC:
                        ; // Semicolon here because of the label error
                        byte buf;
                        scanf("%c", &buf);
                        set_value(&bfp, buf);

                break;

        }
    }

      close:
            fclose(fin);

      exit:
            ;

    return 0;
}

I'd like to have some C related advice on what I have done wrong too.

Answer 1

The Mandelbrot program takes a decent time to run on a good interpreter. Yours, well, it isn't that good. That's why it "hangs" at the start.

Some good ideas that you had, performance-wise:

• precalculating loops

Some bad ideas, performance-wise:

• Using abstracted allocation functions instead of calling realloc
• using scanf and printf instead of getchar and putchar

Some odd things that you did:

• Manually truncating an unsigned char when wraparound is implicit
• checking if an unsigned char is less than 0
• using *(x + y) syntax instead of array indexing x[y]
• I don't think you actually use those precalculated loops, if you do then probably not correctly
• You also probably create a new precalculated loop every time you go over the loop again
• Using the get_value and set_value functions instead of pointers (this will be a performance issue if you don't compile with optimizations)

A couple suggestions:

• Try to avoid calling functions when you don't need to as much as possible.
• Write a BrainFuck->C compiler using this reference.

On second thought, maybe you should try to debug this program instead.

You may also use my interpreter as a reference if you're OK with GPLv3. Note that it doesn't precalculate loops.