BF Interpreter in C... Because I can. And I was a bit bored.

bf.c

/**
 * usage: ./bf "<program>"
 * example: ./bf ",[>++<-]>. ; multiplies a number by 2"
 *
 * Brainfuck Interpreter
 *
 * BF has the following commands:
 * > Move to next memory position
 * < Move to prev memory position
 * + Increment current memory position
 * - Decrement current memory position
 * [ Begin loop frame
 * ] End loop frame
 * , Read value from stdin
 * . Print value to stdout
 *
 */
#include<stdio.h>
#include<stdlib.h>
#include<string.h>

#define D_SEG_SIZE 64
#define INC(a, b) (a + 1 < b ? a + 1 : b)
#define DEC(a) (a > 0 ? a - 1 : a)

typedef struct _STACK {
    struct _STACK *next;
    unsigned int start;
} Stack;

typedef struct _VM {
    unsigned int ip;                // Instruction pointer
    char *is;                       // Instruction segment
    unsigned int is_len;            // Instruction segment length
    unsigned int dp;                // Data pointer
    unsigned int ds[D_SEG_SIZE];    // Data Segment
    Stack *frames;                  // Loop frames
} State;

void push_frame(State *state) {
#ifdef DEBUG
    printf("pushstack(%d)\n", state->ip);
#endif
    Stack *stack = (Stack *) malloc(sizeof(Stack));
    stack->next = state->frames;
    stack->start = state->ip;
    state->frames = stack;
}

void pop_frame(State *state) {
#ifdef DEBUG
    printf("popstack(%d)\n", state->ip);
#endif
    Stack *stack = state->frames;
    state->frames = state->frames->next;
    free(stack);
}

int perform_action(State *state) {
    switch (state->is[state->ip]) {
        case '>':       // Move to position right
            state->dp = INC(state->dp, D_SEG_SIZE - 1);
            break;
        case '<':       // Move to position left
            state->dp = DEC(state->dp);
            break;
        case '+':       // Increment current position
            state->ds[state->dp]++;
            break;
        case '-':       // Decrement current position
            state->ds[state->dp] = DEC(state->ds[state->dp]);
            break;
        case '[':       // Begin loop frame
            if (state->frames == NULL || state->frames->start != state->dp)
                push_frame(state);
            break;
        case ']':       // End loop frame
            if (state->ds[state->dp] == 0)
                pop_frame(state);
            else {
                state->ip = state->frames->start;
            }
            break;
        case ',':       // Read value from stdin
            scanf("%d", (state->ds + state->dp));
            break;
        case '.':       // Print current location to screen
            printf("%d", *(state->ds + state->dp));
            break;
        default:        // Ignore anything else
            break;
    }
#ifdef DEBUG
    printf("BF LOG :: ip(%d) dp(%d) :: is(%c) ds(%d) \n", state->ip, state->dp, state->is[state->ip], state->ds[state->dp]);
#endif
    state->ip++;
    return state->ip < state->is_len;
}

int main(int argc, char **argv) {

    State state;
    memset(&state, 0, sizeof(state));
    state.is = argv[1];
    state.is_len = strlen(argv[1]);
    while(perform_action(&state)) ;

    while(state.frames != NULL) { pop_frame(&state); }

    return EXIT_SUCCESS;
}