This is for a post on Qiita https://qiita.com/takl/items/6ffe14db22974b1f74ce

bf.c

//
// This file consists of 3 parts.
// The first 2 parts are from yohhoy's C11 <threads.h> emulation library.
// The original files are from https://gist.github.com/yohhoy/2223710
// I, takl, commented out 2 lines.
// These 2 parts are ditributed under the Boost Software License, Version 1.0.
// (See the header of these parts for detail)
//
// The last part is written by takl.
// This part is licensed under CC0.
// https://creativecommons.org/publicdomain/zero/1.0/deed.ja
//

//
// threads.h by yohhoy.
//

/*
 * C11 <threads.h> emulation library
 *
 * (C) Copyright yohhoy 2012.
 * Distributed under the Boost Software License, Version 1.0.
 * (See copy at http://www.boost.org/LICENSE_1_0.txt)
 */
#ifndef EMULATED_THREADS_H_INCLUDED_
#define EMULATED_THREADS_H_INCLUDED_

#include <time.h>

#if defined(_WIN32) && defined(_MSC_VER)
#include <windows.h>

// check configuration
#if defined(EMULATED_THREADS_USE_NATIVE_CALL_ONCE) && (_WIN32_WINNT < 0x0600)
#error EMULATED_THREADS_USE_NATIVE_CALL_ONCE requires _WIN32_WINNT>=0x0600
#endif

#if defined(EMULATED_THREADS_USE_NATIVE_CV) && (_WIN32_WINNT < 0x0600)
#error EMULATED_THREADS_USE_NATIVE_CV requires _WIN32_WINNT>=0x0600
#endif


/*---------------------------- macros ----------------------------*/
#ifdef EMULATED_THREADS_USE_NATIVE_CALL_ONCE
#define ONCE_FLAG_INIT INIT_ONCE_STATIC_INIT
#else
#define ONCE_FLAG_INIT {0}
#endif
#define TSS_DTOR_ITERATIONS 1

/*---------------------------- types ----------------------------*/
typedef struct cnd_t {
#ifdef EMULATED_THREADS_USE_NATIVE_CV
    CONDITION_VARIABLE condvar;
#else
    int blocked;
    int gone;
    int to_unblock;
    HANDLE sem_queue;
    HANDLE sem_gate;
    CRITICAL_SECTION monitor;
#endif
} cnd_t;

typedef HANDLE thrd_t;

typedef DWORD tss_t;

typedef struct mtx_t {
    CRITICAL_SECTION cs;
} mtx_t;

#ifdef EMULATED_THREADS_USE_NATIVE_CALL_ONCE
typedef INIT_ONCE once_flag;
#else
typedef struct once_flag_t {
    volatile LONG status;
} once_flag;
#endif

#elif defined(__unix__) || defined(__unix)
#include <pthread.h>

/*---------------------------- macros ----------------------------*/
#define ONCE_FLAG_INIT PTHREAD_ONCE_INIT
#ifdef INIT_ONCE_STATIC_INIT
#define TSS_DTOR_ITERATIONS PTHREAD_DESTRUCTOR_ITERATIONS
#else
#define TSS_DTOR_ITERATIONS 1  // assume TSS dtor MAY be called at least once.
#endif

/*---------------------------- types ----------------------------*/
typedef pthread_cond_t  cnd_t;
typedef pthread_t       thrd_t;
typedef pthread_key_t   tss_t;
typedef pthread_mutex_t mtx_t;
typedef pthread_once_t  once_flag;

#else
#error Not supported on this platform.
#endif


/*---------------------------- types ----------------------------*/
typedef void (*tss_dtor_t)(void*);
typedef int (*thrd_start_t)(void*);

struct xtime {
    time_t sec;
    long nsec;
};
typedef struct xtime xtime;


/*-------------------- enumeration constants --------------------*/
enum {
    mtx_plain     = 0,
    mtx_try       = 1,
    mtx_timed     = 2,
    mtx_recursive = 4
};

enum {
    thrd_success = 0, // succeeded
    thrd_timeout,     // timeout
    thrd_error,       // failed
    thrd_busy,        // resource busy
    thrd_nomem        // out of memory
};


/*-------------------------- functions --------------------------*/
void call_once(once_flag *flag, void (*func)(void));

int cnd_broadcast(cnd_t *cond);
void cnd_destroy(cnd_t *cond);
int cnd_init(cnd_t *cond);
int cnd_signal(cnd_t *cond);
int cnd_timedwait(cnd_t *cond, mtx_t *mtx, const xtime *xt);
int cnd_wait(cnd_t *cond, mtx_t *mtx);

void mtx_destroy(mtx_t *mtx);
int mtx_init(mtx_t *mtx, int type);
int mtx_lock(mtx_t *mtx);
int mtx_timedlock(mtx_t *mtx, const xtime *xt);
int mtx_trylock(mtx_t *mtx);
int mtx_unlock(mtx_t *mtx);

int thrd_create(thrd_t *thr, thrd_start_t func, void *arg);
thrd_t thrd_current(void);
int thrd_detach(thrd_t thr);
int thrd_equal(thrd_t thr0, thrd_t thr1);
void thrd_exit(int res);
int thrd_join(thrd_t thr, int *res);
void thrd_sleep(const xtime *xt);
void thrd_yield(void);

int tss_create(tss_t *key, tss_dtor_t dtor);
void tss_delete(tss_t key);
void *tss_get(tss_t key);
int tss_set(tss_t key, void *val);

int xtime_get(xtime *xt, int base);
// enum { TIME_UTC = 1 };


#endif /* EMULATED_THREADS_H_INCLUDED_ */

//
// threads_posix.c by yohhoy
//

/*
 * C11 <threads.h> emulation library
 *
 * (C) Copyright yohhoy 2012.
 * Distributed under the Boost Software License, Version 1.0.
 * (See copy at http://www.boost.org/LICENSE_1_0.txt)
 */
#include <stdlib.h>
#include <assert.h>
#include <limits.h>
#include <errno.h>
#include <unistd.h>
#include <sched.h>

/*
Configuration macro:

  EMULATED_THREADS_USE_NATIVE_TIMEDLOCK
    Use pthread_mutex_timedlock() for `mtx_timedlock()'
    Otherwise use mtx_trylock() + *busy loop* emulation.
*/
#if !defined(__CYGWIN__)
#define EMULATED_THREADS_USE_NATIVE_TIMEDLOCK
#endif

//#include "threads.h"


/*
Implementation limits:
  - Conditionally emulation for "mutex with timeout"
    (see EMULATED_THREADS_USE_NATIVE_TIMEDLOCK macro)
*/
struct impl_thrd_param {
    thrd_start_t func;
    void *arg;
};

void *impl_thrd_routine(void *p)
{
    struct impl_thrd_param pack = *((struct impl_thrd_param *)p);
    free(p);
    return (void*)pack.func(pack.arg);
}


/*--------------- 7.25.2 Initialization functions ---------------*/
// 7.25.2.1
void call_once(once_flag *flag, void (*func)(void))
{
    pthread_once(flag, func);
}


/*------------- 7.25.3 Condition variable functions -------------*/
// 7.25.3.1
int cnd_broadcast(cnd_t *cond)
{
    if (!cond) return thrd_error;
    pthread_cond_broadcast(cond);
    return thrd_success;
}

// 7.25.3.2
void cnd_destroy(cnd_t *cond)
{
    assert(cond);
    pthread_cond_destroy(cond);
}

// 7.25.3.3
int cnd_init(cnd_t *cond)
{
    if (!cond) return thrd_error;
    pthread_cond_init(cond, NULL);
    return thrd_success;
}

// 7.25.3.4
int cnd_signal(cnd_t *cond)
{
    if (!cond) return thrd_error;
    pthread_cond_signal(cond);
    return thrd_success;
}

// 7.25.3.5
int cnd_timedwait(cnd_t *cond, mtx_t *mtx, const xtime *xt)
{
    struct timespec abs_time;
    int rt;
    if (!cond || !mtx || !xt) return thrd_error;
    rt = pthread_cond_timedwait(cond, mtx, &abs_time);
    if (rt == ETIMEDOUT)
        return thrd_busy;
    return (rt == 0) ? thrd_success : thrd_error;
}

// 7.25.3.6
int cnd_wait(cnd_t *cond, mtx_t *mtx)
{
    if (!cond || !mtx) return thrd_error;
    pthread_cond_wait(cond, mtx);
    return thrd_success;
}


/*-------------------- 7.25.4 Mutex functions --------------------*/
// 7.25.4.1
void mtx_destroy(mtx_t *mtx)
{
    assert(mtx);
    pthread_mutex_destroy(mtx);
}

// 7.25.4.2
int mtx_init(mtx_t *mtx, int type)
{
    pthread_mutexattr_t attr;
    if (!mtx) return thrd_error;
    if (type != mtx_plain && type != mtx_timed && type != mtx_try
      && type != (mtx_plain|mtx_recursive)
      && type != (mtx_timed|mtx_recursive)
      && type != (mtx_try|mtx_recursive))
        return thrd_error;
    pthread_mutexattr_init(&attr);
    if ((type & mtx_recursive) != 0) {
#if defined(__linux__) || defined(__linux)
        pthread_mutexattr_settype(&attr, PTHREAD_MUTEX_RECURSIVE_NP);
#else
        pthread_mutexattr_settype(&attr, PTHREAD_MUTEX_RECURSIVE);
#endif
    }
    pthread_mutex_init(mtx, &attr);
    pthread_mutexattr_destroy(&attr);
    return thrd_success;
}

// 7.25.4.3
int mtx_lock(mtx_t *mtx)
{
    if (!mtx) return thrd_error;
    pthread_mutex_lock(mtx);
    return thrd_success;
}

// 7.25.4.4
int mtx_timedlock(mtx_t *mtx, const xtime *xt)
{
    if (!mtx || !xt) return thrd_error;
    {
#ifdef EMULATED_THREADS_USE_NATIVE_TIMEDLOCK
    struct timespec ts;
    int rt;
    ts.tv_sec = xt->sec;
    ts.tv_nsec = xt->nsec;
    rt = pthread_mutex_timedlock(mtx, &ts);
    if (rt == 0)
        return thrd_success;
    return (rt == ETIMEDOUT) ? thrd_busy : thrd_error;
#else
    time_t expire = time(NULL);
    expire += xt->sec;
    while (mtx_trylock(mtx) != thrd_success) {
        time_t now = time(NULL);
        if (expire < now)
            return thrd_busy;
        // busy loop!
        thrd_yield();
    }
    return thrd_success;
#endif
    }
}

// 7.25.4.5
int mtx_trylock(mtx_t *mtx)
{
    if (!mtx) return thrd_error;
    return (pthread_mutex_trylock(mtx) == 0) ? thrd_success : thrd_busy;
}

// 7.25.4.6
int mtx_unlock(mtx_t *mtx)
{
    if (!mtx) return thrd_error;
    pthread_mutex_unlock(mtx);
    return thrd_success;
}


/*------------------- 7.25.5 Thread functions -------------------*/
// 7.25.5.1
int thrd_create(thrd_t *thr, thrd_start_t func, void *arg)
{
    struct impl_thrd_param *pack;
    if (!thr) return thrd_error;
    pack = malloc(sizeof(struct impl_thrd_param));
    if (!pack) return thrd_nomem;
    pack->func = func;
    pack->arg = arg;
    if (pthread_create(thr, NULL, impl_thrd_routine, pack) != 0) {
        free(pack);
        return thrd_error;
    }
    return thrd_success;
}

// 7.25.5.2
thrd_t thrd_current(void)
{
    return pthread_self();
}

// 7.25.5.3
int thrd_detach(thrd_t thr)
{
    return (pthread_detach(thr) == 0) ? thrd_success : thrd_error;
}

// 7.25.5.4
int thrd_equal(thrd_t thr0, thrd_t thr1)
{
    return pthread_equal(thr0, thr1);
}

// 7.25.5.5
void thrd_exit(int res)
{
    pthread_exit((void*)res);
}

// 7.25.5.6
int thrd_join(thrd_t thr, int *res)
{
    void *code;
    if (pthread_join(thr, &code) != 0)
        return thrd_error;
    if (res)
        *res = (int)code;
    return thrd_success;
}

// 7.25.5.7
void thrd_sleep(const xtime *xt)
{
    struct timespec req;
    assert(xt);
    req.tv_sec = xt->sec;
    req.tv_nsec = xt->nsec;
    nanosleep(&req, NULL);
}

// 7.25.5.8
void thrd_yield(void)
{
    sched_yield();
}


/*----------- 7.25.6 Thread-specific storage functions -----------*/
// 7.25.6.1
int tss_create(tss_t *key, tss_dtor_t dtor)
{
    if (!key) return thrd_error;
    return (pthread_key_create(key, dtor) == 0) ? thrd_success : thrd_error;
}

// 7.25.6.2
void tss_delete(tss_t key)
{
    pthread_key_delete(key);
}

// 7.25.6.3
void *tss_get(tss_t key)
{
    return pthread_getspecific(key);
}

// 7.25.6.4
int tss_set(tss_t key, void *val)
{
    return (pthread_setspecific(key, val) == 0) ? thrd_success : thrd_error;
}


/*-------------------- 7.25.7 Time functions --------------------*/
// 7.25.6.1
int xtime_get(xtime *xt, int base)
{
    if (!xt) return 0;
    if (base == TIME_UTC) {
        xt->sec = time(NULL);
        xt->nsec = 0;
        return base;
    }
    return 0;
}

//
// The rest of this file is written by takl.
// This part is licensed under CC0.
// https://creativecommons.org/publicdomain/zero/1.0/deed.ja
//

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

enum stack_command {
  STACK_COMMAND_NONE,
  STACK_COMMAND_PUSH,
  STACK_COMMAND_POP,
};

struct stack {
  thrd_t thrd;
  volatile enum stack_command command;
  volatile int argument;
  cnd_t cnd;
  mtx_t mtx;
};

void stack_not_empty(struct stack *stack, int cell) {
  stack->command = STACK_COMMAND_NONE;
  cnd_signal(&stack->cnd);

  while (1) {
    switch (stack->command) {
    case STACK_COMMAND_NONE:
      cnd_wait(&stack->cnd, &stack->mtx);
      break;
    case STACK_COMMAND_PUSH:
      stack_not_empty(stack, stack->argument);
      break;
    case STACK_COMMAND_POP:
      stack->argument = cell;
      stack->command = STACK_COMMAND_NONE;
      cnd_signal(&stack->cnd);
      return;
    }
  }
}

void stack_empty(struct stack *stack) {
  while (1) {
    switch (stack->command) {
    case STACK_COMMAND_NONE:
      cnd_wait(&stack->cnd, &stack->mtx);
      break;
    case STACK_COMMAND_PUSH:
      stack_not_empty(stack, stack->argument);
      break;
    case STACK_COMMAND_POP:
      stack->argument = 0;
      stack->command = STACK_COMMAND_NONE;
      cnd_signal(&stack->cnd);
      break;
    }
  }
}

int stack_start(void *stack_) {
  struct stack *stack = stack_;
  mtx_lock(&stack->mtx);
  stack_empty(stack);
  return 0;
}

void stack_init(struct stack *stack) {
  stack->command = STACK_COMMAND_NONE;
  cnd_init(&stack->cnd);
  mtx_init(&stack->mtx, mtx_plain);
  mtx_lock(&stack->mtx);
  thrd_create(&stack->thrd, stack_start, stack);
}

void stack_push(struct stack *stack, int i) {
  stack->command = STACK_COMMAND_PUSH;
  stack->argument = i;
  cnd_signal(&stack->cnd);
  cnd_wait(&stack->cnd, &stack->mtx);
}

int stack_pop(struct stack *stack) {
  stack->command = STACK_COMMAND_POP;
  cnd_signal(&stack->cnd);
  cnd_wait(&stack->cnd, &stack->mtx);
  return stack->argument;
}

const char * const prog = PROG;

const char *back(const char *ip) {
  int nest = 0;

  while (1) {
    if (ip == prog) {
      printf("invalid ']'\n");
      exit(EXIT_FAILURE);
    }
    
    switch (*--ip) {
    case '[':
      if (nest == 0) return ip;
      --nest;
      break;
    case ']':
      ++nest;
      break;
    default:
      break;
    }
  }
}

const char *forth(const char *ip) {
  int nest = 0;

  while (1) {
    switch (*ip++) {
    case '\0':
      printf("invalid '['\n");
      exit(EXIT_FAILURE);
    case '[':
      ++nest;
      break;
    case ']':
      if (nest == 0) return ip;
      --nest;
      break;
    default:
      break;
    }
  }
}

int main() {
  struct stack ls, rs;
  stack_init(&ls);
  stack_init(&rs);

  int cell = 0;
  const char *ip = prog;
  
  while (1) {
    switch(*ip++) {
    case '\0':
      return 0;
    case '+':
      ++cell;
      break;
    case '-':
      --cell;
      break;
    case ';':
      printf("[%d]", cell);
      fflush(stdout);
      break;
    case '.':
      printf("%c", cell);
      fflush(stdout);
      break;
    case ',':
      cell = getchar();
      break;
    case '>':
      stack_push(&ls, cell);
      cell = stack_pop(&rs);
      break;
    case '<':
      stack_push(&rs, cell);
      cell = stack_pop(&ls);
      break;
    case '[':
      if (cell == 0) ip = forth(ip);
      break;
    case ']':
      if (cell != 0) ip = back(ip - 1) + 1;
      break;
    default:
      break;
    }
  }
}