I wanted to implement a short and functional brainfuck interpreter with minimal code repetition. It uses a little bit of macro magic, and does nested loops iteratively rather than using a cache, but it seems to do fine for most tasks I've given it.

One of my goals was to use the absolute minimum in external libraries, so I even made by own (length limited) strcpy.

Very open to style/functionality critiques, especially as it pertains to catching incorrect programs (eg [[[)

/* brainfuck interpreter */
#include <stdio.h>

#define WORKSPACE_SIZE (65535)

void die(const char *msg)
    int x = *(int *)0;

 * dp is a pointer to the data pointer; that way we can manipulate the data
 * pointer itself, and not just the data it points to. This allows us to
 * easily implement both '>' and '+' instructions.
 * pc is similar, it's a pointer to the program counter, which we use to index
 * into the program text itself.
void do_fsm(unsigned char **dp, const char **pc)
#define print_byte(b_) do {\
  fprintf(stderr, #b_ ": '%c', %02x\n", b_, b_);\
  } while(0)

 * starting at program counter, look in direction for matching
 * branch character, accounting for nested loops.
 * needs to have a check for overrunning past program text, in case of
 * unmatched braces
 * This is not quite as efficient as doing a single pass to note down
 * locations of all open braces and their matching closing brace, but
 * it's quick to write and this interpreter is meant to be simple, not fast.
#define BRANCH_CASE(branch_char, matched_char, direction, dp_negation) \
    case branch_char:\
        if(dp_negation **dp) {\
            loop_level++; \
            while(loop_level) {\
                (*pc) += direction;\
                working_byte = **pc;\
                if(working_byte == branch_char)\
                if(working_byte == matched_char)\

    int loop_level = 0;
    int working_byte = 0;

    switch(**pc) {
    case '>':
    case '<':
    case '+':
    case '-':
    case '.':
    case ',':
        **dp = getchar();
    BRANCH_CASE('[', ']',  1,  !)
    BRANCH_CASE(']', '[', -1, !!)

void cpy(unsigned char *dst, unsigned char *src, unsigned int len)
    for(;len--; *dst++ = *src++);

#define OFST(p2, p1) ((unsigned long int) (p2) - (unsigned long int) (p1))

#define dbg(pc, dp, prgm) printf("pc %08x; dp %08x; *dp %02x '%c'\n", OFST(pc,prgm), OFST(dp,prgm), *dp, (char) 'x')

#undef dbg
#define dbg(...)

#define BOUNDED_BY(start, ptr, end) ((ptr >= start) && (ptr < end))

#define WARN_UNBOUNDED(start, ptr, end) do {\
if(!BOUNDED_BY(start, ptr, end)) puts("[WARNING] " #ptr " has stepped outside its allowed bounds!");\
   } while(0)

void run(const char *prgm)
    unsigned char data[WORKSPACE_SIZE] = {0};
    unsigned char *dp = data;
    const char *pc = prgm;

    dbg(pc, dp, prgm);
    while(BOUNDED_BY(prgm, pc, prgm + WORKSPACE_SIZE) && BOUNDED_BY(data, dp, data + WORKSPACE_SIZE) && *pc) {
        do_fsm(&dp, &pc);
        dbg(pc, dp, prgm);

void xcpy(char *dest, const char *src)
    unsigned int len = 0;
    for(; len < WORKSPACE_SIZE && *src; *dest = *src, src++, dest++, len++);
    // zero out the rest to prevent multiple programs from cluttering the program buffer
    for(; len < WORKSPACE_SIZE; *dest = 0, dest++, len++);

int main(void)
    char prgm [WORKSPACE_SIZE] = {0};

    // Hello, World!
    xcpy(prgm, ">++++++++[<+++++++++>-]<.>++++[<+++++++>-]<+.+++++++..+++.>>++++++[<+++++++>-]<++.------------.>++++++[<+++++++++>-]<+.<.+++.------.--------.>>>++++[<++++++++>-]<+.");

    // Game of Life
    xcpy(prgm, ">>>->+>+++++>(++++++++++)[[>>>+<<<-]>+++++>+>>+[<<+>>>>>+<<<-]<-]>>>>[\n"
"  [>>>+>+<<<<-]+++>>+[<+>>>+>+<<<-]>>[>[[>>>+<<<-]<]<<++>+>>>>>>-]<-\n"
"  [+++++++++.-------->>>]+[-<<<]>>>[>>,----------[>]<]<<[\n"
"    <<<[\n"
"      >--[<->>+>-<<-]<[[>>>]+>-[+>>+>-]+[<<<]<-]>++>[<+>-]\n"
"      >[[>>>]+[<<<]>>>-]+[->>>]<-[++>]>[------<]>+++[<<<]>\n"
"    ]<\n"
"  ]>[\n"
"    -[+>>+>-]+>>+>>>+>[<<<]>->+>[\n"
"      >[->+>+++>>++[>>>]+++<<<++<<<++[>>>]>>>]<<<[>[>>>]+>>>]\n"
"      <<<<<<<[<<++<+[-<<<+]->++>>>++>>>++<<<<]<<<+[-<<<+]+>->>->>\n"
"    ]<<+<<+<<<+<<-[+<+<<-]+<+[\n"
"      ->+>[-<-<<[<<<]>[>>[>>>]<<+<[<<<]>-]]\n"
"      <[<[<[<<<]>+>>[>>>]<<-]<[<<<]]>>>->>>[>>>]+>\n"
"    ]>+[-<<[-]<]-[\n"
"      [>>>]<[<<[<<<]>>>>>+>[>>>]<-]>>>[>[>>>]<<<<+>[<<<]>>-]>\n"
"    ]<<<<<<[---<-----[-[-[<->>+++<+++++++[-]]]]<+<+]>\n"
"  ]>>\n"
    // Sierpinski
    //xcpy(prgm, "++++++++[>+>++++<<-]>++>>+<[-[>>+<<-]+>>]>+[-<<<[->[+[-]+>++>>>-<<]<[<]>>++++++[<<+++++>>-]+<<++.[-]<<]>.>+[>>]>+]");

    //xcpy(prgm, ">++++++++[<+++++++++>-]<.>++++[<+++++++>-]<+.+++++++..+++.>>++++++[<+++++++>-]<++.------------.>++++++[<+++++++++>-]<+.<.+++.------.--------.>>>++++[<++++++++>-]<+.");

    // For now, just run a simple "Hello, World!" program every time.
    run("[[["); // should segfault
    return 0;

1 Answer 1


One of my goals was to use the absolute minimum in external libraries

Why? It's going to make your code slower in all likelihood, it's not helping you learn the standard libraries, and it's making your code longer, more complex and less legible. There is no win here.

so I even made by own (length limited) strcpy.

Your cpy? It's a slower and less-standard memcpy.


  • die attempts to abort the process by deliberately dereferencing a null pointer, but there are no guarantees that anything sane (such as clean program termination) will happen. This is undefined behaviour. Your C ABI is allowed to destroy the entire contents of the operating system's memory and force your computer to reboot - or do nothing - if it wants.
  • print_byte isn't used and can be deleted
  • do / while 0 is a macro antipattern that I see floating around the web a lot. Bare brace-surrounded {} blocks are entirely legal and are a better fit.
  • 1
    \$\begingroup\$ Regarding do-while-0 vs {} I beg to differ. MACRO(args...); should always be one single (compound if unavoidable) statement, not two. Though OP is overdoing it, as MACRO(args...) being a fully-braced single expression is superior to even that. \$\endgroup\$ Commented May 26, 2021 at 12:40
  • 1
    \$\begingroup\$ "o / while 0 is a macro antipattern that I see floating around the web a lot. Bare brace-surrounded {} blocks are entirely legal and are a better fit." Hear, hear! I wrote a self-answered Q&A about the subject here: What is do { } while(0) in macros and should we use it?. We also managed to convince the MISRA-C committee to drop this requirement in the latest version of their guidelines - using do while(0) used to be a required rule in that document but they removed it completely. \$\endgroup\$
    – Lundin
    Commented May 27, 2021 at 11:05

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