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	diff --git a/c_src/exile.c b/c_src/exile.c
	index 3d04087..e4af4f7 100644
	--- a/c_src/exile.c
	+++ b/c_src/exile.c
	@@ -1,719 +1,716 @@
	#ifndef _POSIX_C_SOURCE
	#define _POSIX_C_SOURCE 200809L
	#endif

	#include "erl_nif.h"
	#include <errno.h>
	#include <fcntl.h>
	#include <signal.h>
	#include <stdbool.h>
	#include <stdio.h>
	#include <string.h>
	#include <sys/types.h>
	#include <sys/wait.h>
	#include <unistd.h>

	#ifdef ERTS_DIRTY_SCHEDULERS
	#define USE_DIRTY_IO ERL_NIF_DIRTY_JOB_IO_BOUND
	#else
	#define USE_DIRTY_IO 0
	#endif

	//#define DEBUG

	#ifdef DEBUG
	#define debug(...) \
	do { \
	enif_fprintf(stderr, __VA_ARGS__); \
	enif_fprintf(stderr, "\n"); \
	} while (0)
	#define start_timing() ErlNifTime __start = enif_monotonic_time(ERL_NIF_USEC)
	#define elapsed_microseconds() (enif_monotonic_time(ERL_NIF_USEC) - __start)
	#else
	#define debug(...)
	#define start_timing()
	#define elapsed_microseconds() 0
	#endif

	#define error(...) \
	do { \
	enif_fprintf(stderr, __VA_ARGS__); \
	enif_fprintf(stderr, "\n"); \
	} while (0)

	#define GET_CTX(env, arg, ctx) \
	do { \
	ExilePriv *data = enif_priv_data(env); \
	if (enif_get_resource(env, arg, data->exec_ctx_rt, (void **)&ctx) == \
	false) { \
	return make_error(env, ATOM_INVALID_CTX); \
	} \
	} while (0);

	static const int PIPE_READ = 0;
	static const int PIPE_WRITE = 1;
	static const int PIPE_CLOSED = -1;
	static const int CMD_EXIT = -1;
	static const int MAX_ARGUMENTS = 50;
	static const int MAX_ARGUMENT_LEN = 1024;
	-static const int MAX_ENV_LEN = 1024;
	+static const int MAX_ENV_VAR_LEN = 1024;
	static const int UNBUFFERED_READ = -1;
	static const int PIPE_BUF_SIZE = 65535;

	/* We are choosing an exit code which is not reserved see:
	* https://www.tldp.org/LDP/abs/html/exitcodes.html. */
	static const int FORK_EXEC_FAILURE = 125;

	static ERL_NIF_TERM ATOM_TRUE;
	static ERL_NIF_TERM ATOM_FALSE;
	static ERL_NIF_TERM ATOM_OK;
	static ERL_NIF_TERM ATOM_ERROR;
	static ERL_NIF_TERM ATOM_UNDEFINED;
	static ERL_NIF_TERM ATOM_INVALID_CTX;
	static ERL_NIF_TERM ATOM_PIPE_CLOSED;
	static ERL_NIF_TERM ATOM_EAGAIN;
	static ERL_NIF_TERM ATOM_ALLOC_FAILED;

	/* command exit types */
	static ERL_NIF_TERM ATOM_EXIT;
	static ERL_NIF_TERM ATOM_SIGNALED;
	static ERL_NIF_TERM ATOM_STOPPED;

	enum exec_status {
	SUCCESS,
	PIPE_CREATE_ERROR,
	PIPE_FLAG_ERROR,
	FORK_ERROR,
	PIPE_DUP_ERROR,
	NULL_DEV_OPEN_ERROR,
	};

	enum exit_type { NORMAL_EXIT, SIGNALED, STOPPED };

	typedef struct ExilePriv {
	ErlNifResourceType *exec_ctx_rt;
	ErlNifResourceType *io_rt;
	} ExilePriv;

	typedef struct ExecContext {
	int cmd_input_fd;
	int cmd_output_fd;
	int exit_status; // can be exit status or signal number depending on exit_type
	enum exit_type exit_type;
	pid_t pid;
	// these are to hold enif_select resource objects
	int *read_resource;
	int *write_resource;
	} ExecContext;

	typedef struct StartProcessResult {
	bool success;
	int err;
	ExecContext context;
	} StartProcessResult;

	/* TODO: assert if the external process is exit (?) */
	static void exec_ctx_dtor(ErlNifEnv env, void obj) {
	ExecContext *ctx = obj;
	enif_release_resource(ctx->read_resource);
	enif_release_resource(ctx->write_resource);
	debug("Exile exec_ctx_dtor called");
	}

	static void exec_ctx_stop(ErlNifEnv env, void obj, int fd,
	int is_direct_call) {
	debug("Exile exec_ctx_stop called");
	}

	static void exec_ctx_down(ErlNifEnv env, void obj, ErlNifPid *pid,
	ErlNifMonitor *monitor) {
	debug("Exile exec_ctx_down called");
	}

	static ErlNifResourceTypeInit exec_ctx_rt_init = {exec_ctx_dtor, exec_ctx_stop,
	exec_ctx_down};

	static void io_resource_dtor(ErlNifEnv env, void obj) {
	debug("Exile io_resource_dtor called");
	}

	static void io_resource_stop(ErlNifEnv env, void obj, int fd,
	int is_direct_call) {
	debug("Exile io_resource_stop called %d", fd);
	}

	static void io_resource_down(ErlNifEnv env, void obj, ErlNifPid *pid,
	ErlNifMonitor *monitor) {
	debug("Exile io_resource_down called");
	}

	static ErlNifResourceTypeInit io_rt_init = {io_resource_dtor, io_resource_stop,
	io_resource_down};

	static inline ERL_NIF_TERM make_ok(ErlNifEnv *env, ERL_NIF_TERM term) {
	return enif_make_tuple2(env, ATOM_OK, term);
	}

	static inline ERL_NIF_TERM make_error(ErlNifEnv *env, ERL_NIF_TERM term) {
	return enif_make_tuple2(env, ATOM_ERROR, term);
	}

	static int set_flag(int fd, int flags) {
	return fcntl(fd, F_SETFL, fcntl(fd, F_GETFL) \| flags);
	}

	static void close_all(int pipes[2][2]) {
	for (int i = 0; i < 2; i++) {
	if (pipes[i][PIPE_READ] > 0)
	close(pipes[i][PIPE_READ]);
	if (pipes[i][PIPE_WRITE] > 0)
	close(pipes[i][PIPE_WRITE]);
	}
	}

	/* time is assumed to be in microseconds */
	static void notify_consumed_timeslice(ErlNifEnv *env, ErlNifTime start,
	ErlNifTime stop) {
	ErlNifTime pct;

	pct = (ErlNifTime)((stop - start) / 10);
	if (pct > 100)
	pct = 100;
	else if (pct == 0)
	pct = 1;
	enif_consume_timeslice(env, pct);
	}

	/* This is not ideal, but as of now there is no portable way to do this */
	static void close_all_fds() {
	int fd_limit = (int)sysconf(_SC_OPEN_MAX);
	for (int i = STDERR_FILENO + 1; i < fd_limit; i++)
	close(i);
	}

	static StartProcessResult start_process(char *const args[],
	bool stderr_to_console,
	const char dir[],
	char *const exec_env[]) {
	StartProcessResult result = {.success = false};
	pid_t pid;
	int pipes[2][2] = {{0, 0}, {0, 0}};

	if (pipe(pipes[STDIN_FILENO]) == -1 \|\| pipe(pipes[STDOUT_FILENO]) == -1) {
	result.err = errno;
	perror("[exile] failed to create pipes");
	close_all(pipes);
	return result;
	}

	const int r_cmdin = pipes[STDIN_FILENO][PIPE_READ];
	const int w_cmdin = pipes[STDIN_FILENO][PIPE_WRITE];

	const int r_cmdout = pipes[STDOUT_FILENO][PIPE_READ];
	const int w_cmdout = pipes[STDOUT_FILENO][PIPE_WRITE];

	if (set_flag(r_cmdin, O_CLOEXEC) < 0 \|\| set_flag(w_cmdout, O_CLOEXEC) < 0 \|\|
	set_flag(w_cmdin, O_CLOEXEC \| O_NONBLOCK) < 0 \|\|
	set_flag(r_cmdout, O_CLOEXEC \| O_NONBLOCK) < 0) {
	result.err = errno;
	perror("[exile] failed to set flags for pipes");
	close_all(pipes);
	return result;
	}

	switch (pid = fork()) {

	case -1:
	result.err = errno;
	perror("[exile] failed to fork");
	close_all(pipes);
	return result;

	case 0: // child

	if (dir[0] && chdir(dir) != 0) {
	perror("[exile] failed to change directory");
	_exit(FORK_EXEC_FAILURE);
	}

	close(STDIN_FILENO);
	close(STDOUT_FILENO);

	if (dup2(r_cmdin, STDIN_FILENO) < 0) {
	perror("[exile] failed to dup to stdin");

	/* We are assuming FORK_EXEC_FAILURE exit code wont be used by the command
	* we are running. Technically we can not assume any exit code here. The
	* parent can not differentiate between exit before `exec` and the normal
	* command exit.
	* One correct way to solve this might be to have a separate
	* pipe shared between child and parent and signaling the parent by
	* closing it or writing to it. */
	_exit(FORK_EXEC_FAILURE);
	}
	if (dup2(w_cmdout, STDOUT_FILENO) < 0) {
	perror("[exile] failed to dup to stdout");
	_exit(FORK_EXEC_FAILURE);
	}

	if (stderr_to_console != true) {
	close(STDERR_FILENO);
	int dev_null = open("/dev/null", O_WRONLY);

	if (dev_null == -1) {
	perror("[exile] failed to open /dev/null");
	_exit(FORK_EXEC_FAILURE);
	}

	if (dup2(dev_null, STDERR_FILENO) < 0) {
	perror("[exile] failed to dup stderr");
	_exit(FORK_EXEC_FAILURE);
	}

	close(dev_null);
	}

	close_all_fds();

	execve(args[0], args, exec_env);
	perror("[exile] execvp(): failed");

	_exit(FORK_EXEC_FAILURE);

	default: // parent
	/* close file descriptors used by child */
	close(r_cmdin);
	close(w_cmdout);

	result.success = true;
	result.context.pid = pid;
	result.context.cmd_input_fd = w_cmdin;
	result.context.cmd_output_fd = r_cmdout;

	return result;
	}
	}

	/* TODO: return appropriate error instead returning generic "badarg" error */
	static ERL_NIF_TERM execute(ErlNifEnv *env, int argc,
	const ERL_NIF_TERM argv[]) {
	char dir[1024] = {'\0'};
	ErlNifTime start;
	unsigned int args_len, env_len;
	ERL_NIF_TERM head, tail, list;

	start = enif_monotonic_time(ERL_NIF_USEC);

	if (enif_get_list_length(env, argv[0], &args_len) != true) {
	error("invalid command with arguments param");
	return enif_make_badarg(env);
	}

	if (args_len > MAX_ARGUMENTS) {
	error("command argument size exceeds limit: %d", MAX_ARGUMENTS);
	return enif_make_badarg(env);
	}

	- char _args_temp[MAX_ARGUMENTS][MAX_ARGUMENT_LEN + 1];
	- char *exec_args[MAX_ARGUMENTS + 1];
	+ char _args_temp[args_len][MAX_ARGUMENT_LEN + 1];
	+ char *exec_args[args_len + 1];

	list = argv[0];
	for (unsigned int i = 0; i < args_len; i++) {
	if (enif_get_list_cell(env, list, &head, &tail) != true)
	return enif_make_badarg(env);

	if (enif_get_string(env, head, _args_temp[i], MAX_ARGUMENT_LEN,
	ERL_NIF_LATIN1) < 1)
	return enif_make_badarg(env);
	exec_args[i] = _args_temp[i];
	list = tail;
	}
	exec_args[args_len] = NULL;

	if (enif_get_list_length(env, argv[1], &env_len) != true) {
	error("invalid env param");
	return enif_make_badarg(env);
	}

	- if (args_len > MAX_ARGUMENTS) {
	- error("env size exceeds limit: %d", MAX_ARGUMENTS);
	- return enif_make_badarg(env);
	- }
	+ debug("env size: %d", env_len);

	- char _env_temp[env_len][MAX_ENV_LEN];
	+ char _env_temp[env_len][MAX_ENV_VAR_LEN];
	char *exec_env[env_len + 1];

	list = argv[1];
	for (unsigned int i = 0; i < env_len; i++) {
	if (enif_get_list_cell(env, list, &head, &tail) != true)
	return enif_make_badarg(env);

	- if (enif_get_string(env, head, _env_temp[i], MAX_ENV_LEN, ERL_NIF_LATIN1) <
	+ if (enif_get_string(env, head, _env_temp[i], MAX_ENV_VAR_LEN, ERL_NIF_LATIN1) <
	1)
	return enif_make_badarg(env);
	exec_env[i] = _env_temp[i];
	list = tail;
	}
	exec_env[env_len] = NULL;

	bool stderr_to_console = true;
	int tmp_int;
	if (enif_get_int(env, argv[3], &tmp_int) != true)
	return enif_make_badarg(env);
	stderr_to_console = tmp_int == 1 ? true : false;

	if (enif_get_string(env, argv[2], dir, 1024, ERL_NIF_LATIN1) < 0) {
	return enif_make_badarg(env);
	}

	struct ExilePriv *data = enif_priv_data(env);
	StartProcessResult result =
	start_process(exec_args, stderr_to_console, dir, exec_env);
	ExecContext *ctx = NULL;
	ERL_NIF_TERM term;

	if (result.success) {
	ctx = enif_alloc_resource(data->exec_ctx_rt, sizeof(ExecContext));
	ctx->cmd_input_fd = result.context.cmd_input_fd;
	ctx->cmd_output_fd = result.context.cmd_output_fd;
	ctx->read_resource = enif_alloc_resource(data->io_rt, sizeof(int));
	ctx->write_resource = enif_alloc_resource(data->io_rt, sizeof(int));
	ctx->pid = result.context.pid;

	debug("pid: %d cmd_in_fd: %d cmd_out_fd: %d", ctx->pid, ctx->cmd_input_fd,
	ctx->cmd_output_fd);

	term = enif_make_resource(env, ctx);

	/* resource should be collected beam GC when there are no more references */
	enif_release_resource(ctx);

	notify_consumed_timeslice(env, start, enif_monotonic_time(ERL_NIF_USEC));

	return make_ok(env, term);
	} else {
	return make_error(env, enif_make_int(env, result.err));
	}
	}

	static int select_write(ErlNifEnv env, ExecContext ctx) {
	int retval = enif_select(env, ctx->cmd_input_fd, ERL_NIF_SELECT_WRITE,
	ctx->write_resource, NULL, ATOM_UNDEFINED);
	if (retval != 0)
	perror("select_write()");

	return retval;
	}

	static ERL_NIF_TERM sys_write(ErlNifEnv *env, int argc,
	const ERL_NIF_TERM argv[]) {
	if (argc != 2)
	enif_make_badarg(env);

	ErlNifTime start;
	start = enif_monotonic_time(ERL_NIF_USEC);

	ExecContext *ctx = NULL;
	GET_CTX(env, argv[0], ctx);

	if (ctx->cmd_input_fd == PIPE_CLOSED)
	return make_error(env, ATOM_PIPE_CLOSED);

	ErlNifBinary bin;
	if (enif_inspect_binary(env, argv[1], &bin) != true)
	return enif_make_badarg(env);

	if (bin.size == 0)
	return enif_make_badarg(env);

	/* should we limit the bin.size here? */
	ssize_t result = write(ctx->cmd_input_fd, bin.data, bin.size);
	int write_errno = errno;

	notify_consumed_timeslice(env, start, enif_monotonic_time(ERL_NIF_USEC));

	/* TODO: branching is ugly, cleanup required */
	if (result >= (ssize_t)bin.size) { // request completely satisfied
	return make_ok(env, enif_make_int(env, result));
	} else if (result >= 0) { // request partially satisfied
	int retval = select_write(env, ctx);
	if (retval != 0)
	return make_error(env, enif_make_int(env, retval));
	return make_ok(env, enif_make_int(env, result));
	} else if (write_errno == EAGAIN \|\| write_errno == EWOULDBLOCK) { // busy
	int retval = select_write(env, ctx);
	if (retval != 0)
	return make_error(env, enif_make_int(env, retval));
	return make_error(env, ATOM_EAGAIN);
	} else { // Error
	perror("write()");
	return make_error(env, enif_make_int(env, write_errno));
	}
	}

	static ERL_NIF_TERM sys_close(ErlNifEnv *env, int argc,
	const ERL_NIF_TERM argv[]) {
	ExecContext *ctx = NULL;
	GET_CTX(env, argv[0], ctx);

	int kind;
	enif_get_int(env, argv[1], &kind);

	int result;
	switch (kind) {
	case 0:
	if (ctx->cmd_input_fd == PIPE_CLOSED) {
	return ATOM_OK;
	} else {
	enif_select(env, ctx->cmd_input_fd, ERL_NIF_SELECT_STOP,
	ctx->write_resource, NULL, ATOM_UNDEFINED);
	result = close(ctx->cmd_input_fd);
	if (result == 0) {
	ctx->cmd_input_fd = PIPE_CLOSED;
	return ATOM_OK;
	} else {
	perror("cmd_input_fd close()");
	return make_error(env, enif_make_int(env, errno));
	}
	}
	case 1:
	if (ctx->cmd_output_fd == PIPE_CLOSED) {
	return ATOM_OK;
	} else {
	enif_select(env, ctx->cmd_output_fd, ERL_NIF_SELECT_STOP,
	ctx->read_resource, NULL, ATOM_UNDEFINED);
	result = close(ctx->cmd_output_fd);
	if (result == 0) {
	ctx->cmd_output_fd = PIPE_CLOSED;
	return ATOM_OK;
	} else {
	perror("cmd_output_fd close()");
	return make_error(env, enif_make_int(env, errno));
	}
	}
	default:
	debug("invalid file descriptor type");
	return enif_make_badarg(env);
	}
	}

	static int select_read(ErlNifEnv env, ExecContext ctx) {
	int retval = enif_select(env, ctx->cmd_output_fd, ERL_NIF_SELECT_READ,
	ctx->read_resource, NULL, ATOM_UNDEFINED);
	if (retval != 0)
	perror("select_read()");
	return retval;
	}

	static ERL_NIF_TERM sys_read(ErlNifEnv *env, int argc,
	const ERL_NIF_TERM argv[]) {
	if (argc != 2)
	enif_make_badarg(env);

	ErlNifTime start;
	start = enif_monotonic_time(ERL_NIF_USEC);

	ExecContext *ctx = NULL;
	GET_CTX(env, argv[0], ctx);

	if (ctx->cmd_output_fd == PIPE_CLOSED)
	return make_error(env, ATOM_PIPE_CLOSED);

	int size, request;

	enif_get_int(env, argv[1], &request);
	size = request;

	if (request == UNBUFFERED_READ) {
	size = PIPE_BUF_SIZE;
	} else if (request < 1) {
	enif_make_badarg(env);
	} else if (request > PIPE_BUF_SIZE) {
	size = PIPE_BUF_SIZE;
	}

	unsigned char buf[size];
	ssize_t result = read(ctx->cmd_output_fd, buf, size);
	int read_errno = errno;

	ERL_NIF_TERM bin_term = 0;
	if (result >= 0) {
	/* no need to release this binary */
	unsigned char *temp = enif_make_new_binary(env, result, &bin_term);
	memcpy(temp, buf, result);
	}

	notify_consumed_timeslice(env, start, enif_monotonic_time(ERL_NIF_USEC));

	if (result >= 0) {
	/* we do not 'select' if request completely satisfied OR EOF OR its
	* UNBUFFERED_READ */
	if (result == request \|\| result == 0 \|\| request == UNBUFFERED_READ) {
	return make_ok(env, bin_term);
	} else { // request partially satisfied
	int retval = select_read(env, ctx);
	if (retval != 0)
	return make_error(env, enif_make_int(env, retval));
	return make_ok(env, bin_term);
	}
	} else {
	if (read_errno == EAGAIN \|\| read_errno == EWOULDBLOCK) { // busy
	int retval = select_read(env, ctx);
	if (retval != 0)
	return make_error(env, enif_make_int(env, retval));
	return make_error(env, ATOM_EAGAIN);
	} else { // Error
	perror("read()");
	return make_error(env, enif_make_int(env, read_errno));
	}
	}
	}

	static ERL_NIF_TERM is_alive(ErlNifEnv *env, int argc,
	const ERL_NIF_TERM argv[]) {
	ExecContext *ctx = NULL;
	GET_CTX(env, argv[0], ctx);

	if (ctx->pid == CMD_EXIT)
	return make_ok(env, ATOM_TRUE);

	int result = kill(ctx->pid, 0);

	if (result == 0) {
	return make_ok(env, ATOM_TRUE);
	} else {
	return make_ok(env, ATOM_FALSE);
	}
	}

	static ERL_NIF_TERM sys_terminate(ErlNifEnv *env, int argc,
	const ERL_NIF_TERM argv[]) {
	ExecContext *ctx = NULL;
	GET_CTX(env, argv[0], ctx);
	if (ctx->pid == CMD_EXIT)
	return make_ok(env, enif_make_int(env, 0));

	return make_ok(env, enif_make_int(env, kill(ctx->pid, SIGTERM)));
	}

	static ERL_NIF_TERM sys_kill(ErlNifEnv *env, int argc,
	const ERL_NIF_TERM argv[]) {
	ExecContext *ctx = NULL;
	GET_CTX(env, argv[0], ctx);
	if (ctx->pid == CMD_EXIT)
	return make_ok(env, enif_make_int(env, 0));

	return make_ok(env, enif_make_int(env, kill(ctx->pid, SIGKILL)));
	}

	static ERL_NIF_TERM make_exit_term(ErlNifEnv env, ExecContext ctx) {
	switch (ctx->exit_type) {
	case NORMAL_EXIT:
	return make_ok(env, enif_make_tuple2(env, ATOM_EXIT,
	enif_make_int(env, ctx->exit_status)));
	case SIGNALED:
	/* exit_status here points to signal number */
	return make_ok(env, enif_make_tuple2(env, ATOM_SIGNALED,
	enif_make_int(env, ctx->exit_status)));
	case STOPPED:
	return make_ok(env, enif_make_tuple2(env, ATOM_STOPPED,
	enif_make_int(env, ctx->exit_status)));
	default:
	error("Invalid wait status");
	return make_error(env, ATOM_UNDEFINED);
	}
	}

	static ERL_NIF_TERM sys_wait(ErlNifEnv *env, int argc,
	const ERL_NIF_TERM argv[]) {
	ExecContext *ctx = NULL;
	GET_CTX(env, argv[0], ctx);

	if (ctx->pid == CMD_EXIT)
	return make_exit_term(env, ctx);

	int status;
	int wpid = waitpid(ctx->pid, &status, WNOHANG);

	if (wpid == ctx->pid) {
	ctx->pid = CMD_EXIT;

	if (WIFEXITED(status)) {
	ctx->exit_type = NORMAL_EXIT;
	ctx->exit_status = WEXITSTATUS(status);
	} else if (WIFSIGNALED(status)) {
	ctx->exit_type = SIGNALED;
	ctx->exit_status = WTERMSIG(status);
	} else if (WIFSTOPPED(status)) {
	ctx->exit_type = STOPPED;
	ctx->exit_status = 0;
	}

	return make_exit_term(env, ctx);
	} else if (wpid != 0) {
	perror("waitpid()");
	}
	ERL_NIF_TERM term = enif_make_tuple2(env, enif_make_int(env, wpid),
	enif_make_int(env, status));
	return make_error(env, term);
	}

	static ERL_NIF_TERM os_pid(ErlNifEnv *env, int argc,
	const ERL_NIF_TERM argv[]) {
	ExecContext *ctx = NULL;
	GET_CTX(env, argv[0], ctx);
	if (ctx->pid == CMD_EXIT)
	return make_ok(env, enif_make_int(env, 0));

	return make_ok(env, enif_make_int(env, ctx->pid));
	}

	static int on_load(ErlNifEnv env, void *priv, ERL_NIF_TERM load_info) {
	struct ExilePriv *data = enif_alloc(sizeof(struct ExilePriv));
	if (!data)
	return 1;

	data->exec_ctx_rt =
	enif_open_resource_type_x(env, "exile_resource", &exec_ctx_rt_init,
	ERL_NIF_RT_CREATE \| ERL_NIF_RT_TAKEOVER, NULL);
	data->io_rt =
	enif_open_resource_type_x(env, "exile_resource", &io_rt_init,
	ERL_NIF_RT_CREATE \| ERL_NIF_RT_TAKEOVER, NULL);

	ATOM_TRUE = enif_make_atom(env, "true");
	ATOM_FALSE = enif_make_atom(env, "false");
	ATOM_OK = enif_make_atom(env, "ok");
	ATOM_ERROR = enif_make_atom(env, "error");
	ATOM_UNDEFINED = enif_make_atom(env, "undefined");
	ATOM_INVALID_CTX = enif_make_atom(env, "invalid_exile_exec_ctx");
	ATOM_PIPE_CLOSED = enif_make_atom(env, "closed_pipe");
	ATOM_EXIT = enif_make_atom(env, "exit");
	ATOM_SIGNALED = enif_make_atom(env, "signaled");
	ATOM_STOPPED = enif_make_atom(env, "stopped");
	ATOM_EAGAIN = enif_make_atom(env, "eagain");
	ATOM_ALLOC_FAILED = enif_make_atom(env, "alloc_failed");

	priv = (void )data;

	return 0;
	}

	static void on_unload(ErlNifEnv env, void priv) {
	debug("exile unload");
	enif_free(priv);
	}

	static ErlNifFunc nif_funcs[] = {
	{"execute", 4, execute, USE_DIRTY_IO},
	{"sys_write", 2, sys_write, USE_DIRTY_IO},
	{"sys_read", 2, sys_read, USE_DIRTY_IO},
	{"sys_close", 2, sys_close, USE_DIRTY_IO},
	{"sys_terminate", 1, sys_terminate, USE_DIRTY_IO},
	{"sys_wait", 1, sys_wait, USE_DIRTY_IO},
	{"sys_kill", 1, sys_kill, USE_DIRTY_IO},
	{"alive?", 1, is_alive, USE_DIRTY_IO},
	{"os_pid", 1, os_pid, USE_DIRTY_IO},
	};

	ERL_NIF_INIT(Elixir.Exile.ProcessNif, nif_funcs, &on_load, NULL, NULL,
	&on_unload)
	diff --git a/lib/exile.ex b/lib/exile.ex
	index 925e103..7b1198f 100644
	--- a/lib/exile.ex
	+++ b/lib/exile.ex
	@@ -1,43 +1,57 @@
	defmodule Exile do
	@moduledoc """
	Exile is an alternative for beam ports with back-pressure and non-blocking IO
	"""

	+ use Application
	+
	+ @doc false
	+ def start(_type, _args) do
	+ opts = [
	+ name: Exile.WatcherSupervisor,
	+ strategy: :one_for_one
	+ ]
	+
	+ # we use DynamicSupervisor for cleaning up external processes on
	+ # :init.stop or SIGTERM
	+ DynamicSupervisor.start_link(opts)
	+ end
	+
	@doc """
	Runs the given command with arguments and return an Enumerable to read command output.

	First parameter must be a list containing command with arguments. example: `["cat", "file.txt"]`.

	### Options
	* `input` - Input can be either an `Enumerable` or a function which accepts `Collectable`.
	1. input as Enumerable:
	```elixir
	# List
	Exile.stream!(~w(bc -q), input: ["1+1\n", "2*2\n"]) \|> Enum.to_list()

	# Stream
	Exile.stream!(~w(cat), input: File.stream!("log.txt", [], 65536)) \|> Enum.to_list()
	```
	2. input as collectable:
	If the input in a function with arity 1, Exile will call that function with a `Collectable` as the argument. The function must push input to this collectable. Return value of the function is ignored.
	```elixir
	Exile.stream!(~w(cat), input: fn sink -> Enum.into(1..100, sink, &to_string/1) end)
	\|> Enum.to_list()
	```
	By defaults no input will be given to the command
	* `exit_timeout` - Duration to wait for external program to exit after completion before raising an error. Defaults to `:infinity`
	- * `chunk_size` - Size of each iodata chunk emitted by Enumerable stream. When set to `nil` the output is unbuffered and chunk size will be variable. Defaults to 65535
	+ * `chunk_size` - Size of each iodata chunk emitted by Enumerable stream. When set to `:unbuffered` the output is unbuffered and chunk size will be variable depending on the amount of data availble at that time. Defaults to 65535
	All other options are passed to `Exile.Process.start_link/3`

	### Examples

	``` elixir
	Exile.stream!(~w(ffmpeg -i pipe:0 -f mp3 pipe:1), input: File.stream!("music_video.mkv", [], 65535))
	\|> Stream.into(File.stream!("music.mp3"))
	\|> Stream.run()
	```
	"""
	def stream!(cmd_with_args, opts \\ []) do
	Exile.Stream.__build__(cmd_with_args, opts)
	end
	end
	diff --git a/lib/exile/process.ex b/lib/exile/process.ex
	index fb08cd3..5c6af87 100644
	--- a/lib/exile/process.ex
	+++ b/lib/exile/process.ex
	@@ -1,403 +1,395 @@
	defmodule Exile.Process do
	@moduledoc """
	GenServer which wraps spawned external command.

	One should use `Exile.stream!` over `Exile.Process`. stream internally manages this server for you. Use this only if you need more control over the life-cycle OS process.

	## Overview
	`Exile.Process` is an alternative primitive for Port. It has different interface and approach to running external programs to solve the issues associated with the ports.

	### When compared to Port
	* it is demand driven. User explicitly has to `read` output of the command and the progress of the external command is controlled using OS pipes. so unlike Port, this never cause memory issues in beam by loading more than we can consume
	* it can close stdin of the program explicitly
	* does not create zombie process. It always tries to cleanup resources

	At high level it makes non-blocking asynchronous system calls to execute and interact with the external program. It completely bypasses beam implementation for the same using NIF. It uses `select()` system call for asynchronous IO. Most of the system calls are non-blocking, so it does not has adverse effect on scheduler. Issues such as "scheduler collapse".
	"""

	alias Exile.ProcessNif
	+ require Exile.ProcessNif
	require Logger
	use GenServer

	- defmacro fork_exec_failure(), do: 125
	-
	# delay between exit_check when io is busy (in milliseconds)
	@exit_check_timeout 5

	@default_opts [stderr_to_console: false, env: []]

	@doc """
	Starts `Exile.ProcessServer`

	Starts external program using `cmd_with_args` with options `opts`

	`cmd_with_args` must be a list containing command with arguments. example: `["cat", "file.txt"]`.

	### Options
	* `cd` - the directory to run the command in
	* `env` - an enumerable of tuples containing environment key-value. These can be accessed in the external program
	* `stderr_to_console` - whether to print stderr output to console. Defaults to `false`
	"""
	def start_link(cmd_with_args, opts \\ []) do
	opts = Keyword.merge(@default_opts, opts)
	- GenServer.start(__MODULE__, %{cmd_with_args: cmd_with_args, opts: opts})
	+
	+ with {:ok, args} <- normalize_args(cmd_with_args, opts) do
	+ GenServer.start(__MODULE__, args)
	+ end
	end

	def close_stdin(process) do
	GenServer.call(process, :close_stdin, :infinity)
	end

	def write(process, iodata) do
	GenServer.call(process, {:write, IO.iodata_to_binary(iodata)}, :infinity)
	end

	- def read(process, size) when is_integer(size) or size == :unbuffered do
	+ def read(process, size) when (is_integer(size) and size > 0) or size == :unbuffered do
	GenServer.call(process, {:read, size}, :infinity)
	end

	def read(process) do
	GenServer.call(process, {:read, :unbuffered}, :infinity)
	end

	def kill(process, signal) when signal in [:sigkill, :sigterm] do
	GenServer.call(process, {:kill, signal}, :infinity)
	end

	def await_exit(process, timeout \\ :infinity) do
	GenServer.call(process, {:await_exit, timeout}, :infinity)
	end

	def os_pid(process) do
	GenServer.call(process, :os_pid, :infinity)
	end

	def stop(process), do: GenServer.call(process, :stop, :infinity)

	## Server

	defmodule Pending do
	defstruct bin: [], remaining: 0, client_pid: nil
	end

	defstruct [
	- :cmd_with_args,
	- :opts,
	+ :args,
	:errno,
	:context,
	:status,
	await: %{},
	pending_read: nil,
	pending_write: nil
	]

	alias __MODULE__

	def init(args) do
	- %{cmd_with_args: [cmd \| args], opts: opts} = args
	- path = System.find_executable(cmd)
	-
	- unless path do
	- raise "Command not found: #{cmd}"
	- end
	-
	- if opts[:cd] do
	- if !File.exists?(opts[:cd]) \|\| !File.dir?(opts[:cd]) do
	- raise ":dir is not a valid path"
	- end
	- end
	-
	state = %__MODULE__{
	- cmd_with_args: [path \| args],
	- opts: opts,
	+ args: args,
	errno: nil,
	status: :init,
	await: %{},
	pending_read: %Pending{},
	pending_write: %Pending{}
	}

	{:ok, state, {:continue, nil}}
	end

	def handle_continue(nil, state) do
	- exec_args = Enum.map(state.cmd_with_args, &to_charlist/1)
	- stderr_to_console = if state.opts[:stderr_to_console], do: 1, else: 0
	- cd = to_charlist(state.opts[:cd] \|\| "")
	- env = normalize_env(state.opts[:env] \|\| %{})
	+ %{cmd_with_args: cmd_with_args, cd: cd, env: env, stderr_to_console: stderr_to_console} =
	+ state.args

	- case ProcessNif.execute(exec_args, env, cd, stderr_to_console) do
	+ case ProcessNif.execute(cmd_with_args, env, cd, stderr_to_console) do
	{:ok, context} ->
	- start_watcher(context)
	+ {:ok, _} = Exile.Watcher.watch(self(), context)
	{:noreply, %Process{state \| context: context, status: :start}}

	{:error, errno} ->
	- raise "Failed to start command: #{state.cmd_with_args}, errno: #{errno}"
	+ raise "Failed to start command: #{cmd_with_args}, errno: #{errno}"
	end
	end

	def handle_call(:stop, _from, state) do
	# watcher will take care of termination of external process

	# TODO: pending write and read should receive "stopped" return
	# value instead of exit signal
	{:stop, :normal, :ok, state}
	end

	def handle_call(_, _from, %{status: {:exit, status}}), do: {:reply, {:error, {:exit, status}}}

	def handle_call({:await_exit, timeout}, from, state) do
	tref =
	if timeout != :infinity do
	Elixir.Process.send_after(self(), {:await_exit_timeout, from}, timeout)
	else
	nil
	end

	state = put_timer(state, from, :timeout, tref)
	check_exit(state, from)
	end

	def handle_call({:write, binary}, from, state) when is_binary(binary) do
	pending = %Pending{bin: binary, client_pid: from}
	do_write(%Process{state \| pending_write: pending})
	end

	def handle_call({:read, size}, from, state) do
	pending = %Pending{remaining: size, client_pid: from}
	do_read(%Process{state \| pending_read: pending})
	end

	def handle_call(:close_stdin, _from, state), do: do_close(state, :stdin)

	def handle_call(:os_pid, _from, state), do: {:reply, ProcessNif.os_pid(state.context), state}

	def handle_call({:kill, signal}, _from, state) do
	do_kill(state.context, signal)
	{:reply, :ok, %{state \| status: {:exit, :killed}}}
	end

	def handle_info({:check_exit, from}, state), do: check_exit(state, from)

	def handle_info({:await_exit_timeout, from}, state) do
	cancel_timer(state, from, :check)

	receive do
	{:check_exit, ^from} -> :ok
	after
	0 -> :ok
	end

	GenServer.reply(from, :timeout)
	{:noreply, clear_await(state, from)}
	end

	def handle_info({:select, _write_resource, _ref, :ready_output}, state), do: do_write(state)

	def handle_info({:select, _read_resource, _ref, :ready_input}, state), do: do_read(state)

	def handle_info(msg, _state), do: raise(msg)

	defp do_write(%Process{pending_write: %Pending{bin: <<>>}} = state) do
	GenServer.reply(state.pending_write.client_pid, :ok)
	{:noreply, %{state \| pending_write: %Pending{}}}
	end

	defp do_write(%Process{pending_write: pending} = state) do
	case ProcessNif.sys_write(state.context, pending.bin) do
	{:ok, size} ->
	if size < byte_size(pending.bin) do
	binary = binary_part(pending.bin, size, byte_size(pending.bin) - size)
	{:noreply, %{state \| pending_write: %Pending{pending \| bin: binary}}}
	else
	GenServer.reply(pending.client_pid, :ok)
	{:noreply, %{state \| pending_write: %Pending{}}}
	end

	{:error, :eagain} ->
	{:noreply, state}

	{:error, errno} ->
	GenServer.reply(pending.client_pid, {:error, errno})
	{:noreply, %{state \| errno: errno}}
	end
	end

	defp do_read(%Process{pending_read: %Pending{remaining: :unbuffered} = pending} = state) do
	case ProcessNif.sys_read(state.context, -1) do
	{:ok, <<>>} ->
	GenServer.reply(pending.client_pid, {:eof, []})
	{:noreply, state}

	{:ok, binary} ->
	GenServer.reply(pending.client_pid, {:ok, binary})
	{:noreply, state}

	{:error, :eagain} ->
	{:noreply, state}

	{:error, errno} ->
	GenServer.reply(pending.client_pid, {:error, errno})
	{:noreply, %{state \| errno: errno}}
	end
	end

	defp do_read(%Process{pending_read: pending} = state) do
	case ProcessNif.sys_read(state.context, pending.remaining) do
	{:ok, <<>>} ->
	GenServer.reply(pending.client_pid, {:eof, pending.bin})
	{:noreply, %Process{state \| pending_read: %Pending{}}}

	{:ok, binary} ->
	if byte_size(binary) < pending.remaining do
	pending = %Pending{
	pending
	\| bin: [pending.bin \| binary],
	remaining: pending.remaining - byte_size(binary)
	}

	{:noreply, %Process{state \| pending_read: pending}}
	else
	GenServer.reply(pending.client_pid, {:ok, [state.pending_read.bin \| binary]})
	{:noreply, %Process{state \| pending_read: %Pending{}}}
	end

	{:error, :eagain} ->
	{:noreply, state}

	{:error, errno} ->
	GenServer.reply(pending.client_pid, {:error, errno})
	{:noreply, %{state \| pending_read: %Pending{}, errno: errno}}
	end
	end

	defp check_exit(state, from) do
	case ProcessNif.sys_wait(state.context) do
	- {:ok, {:exit, fork_exec_failure()}} ->
	+ {:ok, {:exit, ProcessNif.fork_exec_failure()}} ->
	GenServer.reply(from, {:error, :failed_to_execute})
	cancel_timer(state, from, :timeout)
	{:noreply, clear_await(state, from)}

	{:ok, status} ->
	GenServer.reply(from, {:ok, status})
	cancel_timer(state, from, :timeout)
	{:noreply, clear_await(state, from)}

	{:error, {0, _}} ->
	# Ideally we should not poll and we should handle this with SIGCHLD signal
	tref = Elixir.Process.send_after(self(), {:check_exit, from}, @exit_check_timeout)
	{:noreply, put_timer(state, from, :check, tref)}

	{:error, {-1, status}} ->
	GenServer.reply(from, {:error, status})
	cancel_timer(state, from, :timeout)
	{:noreply, clear_await(state, from)}
	end
	end

	defp do_kill(context, :sigkill), do: ProcessNif.sys_kill(context)

	defp do_kill(context, :sigterm), do: ProcessNif.sys_terminate(context)

	defp do_close(state, type) do
	- case ProcessNif.sys_close(state.context, stream_type(type)) do
	+ case ProcessNif.sys_close(state.context, ProcessNif.to_process_fd(type)) do
	:ok ->
	{:reply, :ok, state}

	{:error, errno} ->
	raise errno
	{:reply, {:error, errno}, %Process{state \| errno: errno}}
	end
	end

	defp clear_await(state, from) do
	%Process{state \| await: Map.delete(state.await, from)}
	end

	defp cancel_timer(state, from, key) do
	case get_timer(state, from, key) do
	nil -> :ok
	tref -> Elixir.Process.cancel_timer(tref)
	end
	end

	defp put_timer(state, from, key, timer) do
	if Map.has_key?(state.await, from) do
	await = put_in(state.await, [from, key], timer)
	%Process{state \| await: await}
	else
	%Process{state \| await: %{from => %{key => timer}}}
	end
	end

	defp get_timer(state, from, key), do: get_in(state.await, [from, key])

	- # Try to gracefully terminate external process if the genserver associated with the process is killed
	- defp start_watcher(context) do
	- process_server = self()
	- watcher_pid = spawn(fn -> watcher(process_server, context) end)
	+ defp normalize_cmd(cmd) do
	+ path = System.find_executable(cmd)

	- receive do
	- {^watcher_pid, :done} -> :ok
	+ if path do
	+ {:ok, to_charlist(path)}
	+ else
	+ {:error, "command not found: #{inspect(cmd)}"}
	end
	end

	- defp stream_type(:stdin), do: 0
	- defp stream_type(:stdout), do: 1
	-
	- defp process_exit?(context) do
	- match?({:ok, _}, ProcessNif.sys_wait(context))
	+ defp normalize_cmd_args(args) do
	+ if is_list(args) do
	+ {:ok, Enum.map(args, &to_charlist/1)}
	+ else
	+ {:error, "command arguments must be list of strings. #{inspect(args)}"}
	+ end
	end

	- defp process_exit?(context, timeout) do
	- if process_exit?(context) do
	- true
	+ defp normalize_cd(nil), do: {:ok, ''}
	+
	+ defp normalize_cd(cd) do
	+ if File.exists?(cd) && File.dir?(cd) do
	+ {:ok, to_charlist(cd)}
	else
	- :timer.sleep(timeout)
	- process_exit?(context)
	+ {:error, "`:cd` must be valid directory path"}
	end
	end

	+ defp normalize_env(nil), do: {:ok, []}
	+
	defp normalize_env(env) do
	- Enum.map(env, fn {key, value} ->
	- (String.trim(key) <> "=" <> String.trim(value))
	- \|> to_charlist()
	- end)
	+ user_env =
	+ Map.new(env, fn {key, value} ->
	+ {String.trim(key), String.trim(value)}
	+ end)
	+
	+ # spawned process env will be beam env at that time + user env.
	+ # this is similar to erlang behavior
	+ env_list =
	+ Map.merge(System.get_env(), user_env)
	+ \|> Enum.map(fn {k, v} ->
	+ to_charlist(k <> "=" <> v)
	+ end)
	+
	+ {:ok, env_list}
	end

	- # for proper process exit parent of the child must wait() for
	- # child processes termination exit and "pickup" after the exit
	- # (receive child exit_status). Resources acquired by child such as
	- # file descriptors won't be released even if the child process
	- # itself is terminated.
	- defp watcher(process_server, context) do
	- ref = Elixir.Process.monitor(process_server)
	- send(process_server, {self(), :done})
	-
	- receive do
	- {:DOWN, ^ref, :process, ^process_server, _reason} ->
	- try do
	- Logger.debug(fn -> "Stopping external program" end)
	-
	- # sys_close is idempotent, calling it multiple times is okay
	- ProcessNif.sys_close(context, stream_type(:stdin))
	- ProcessNif.sys_close(context, stream_type(:stdout))
	+ defp normalize_stderr_to_console(nil), do: {:ok, ProcessNif.nif_false()}

	- # at max we wait for 100ms for program to exit
	- process_exit?(context, 100) && throw(:done)
	+ defp normalize_stderr_to_console(term) do
	+ if term, do: {:ok, ProcessNif.nif_true()}, else: {:ok, ProcessNif.nif_false()}
	+ end

	- Logger.debug("Failed to stop external program gracefully. attempting SIGTERM")
	- ProcessNif.sys_terminate(context)
	- process_exit?(context, 100) && throw(:done)
	+ defp validate_opts_fields(opts) do
	+ {_, additional_opts} = Keyword.split(opts, [:cd, :stderr_to_console, :env])

	- Logger.debug("Failed to stop external program with SIGTERM. attempting SIGKILL")
	- ProcessNif.sys_kill(context)
	- process_exit?(context, 1000) && throw(:done)
	+ if Enum.empty?(additional_opts) do
	+ :ok
	+ else
	+ {:error, "invalid opts: #{inspect(additional_opts)}"}
	+ end
	+ end

	- Logger.error("[exile] failed to kill external process")
	- raise "Failed to kill external process"
	- catch
	- :done -> Logger.debug(fn -> "External program exited successfully" end)
	- end
	+ defp normalize_args([cmd \| args], opts) when is_list(opts) do
	+ with {:ok, cmd} <- normalize_cmd(cmd),
	+ {:ok, args} <- normalize_cmd_args(args),
	+ :ok <- validate_opts_fields(opts),
	+ {:ok, cd} <- normalize_cd(opts[:cd]),
	+ {:ok, stderr_to_console} <- normalize_stderr_to_console(opts[:stderr_to_console]),
	+ {:ok, env} <- normalize_env(opts[:env]) do
	+ {:ok,
	+ %{cmd_with_args: [cmd \| args], cd: cd, stderr_to_console: stderr_to_console, env: env}}
	end
	end
	+
	+ defp normalize_args(_, _), do: {:error, "invalid arguments"}
	end
	diff --git a/lib/exile/process_nif.ex b/lib/exile/process_nif.ex
	index 92d158f..aa10f57 100644
	--- a/lib/exile/process_nif.ex
	+++ b/lib/exile/process_nif.ex
	@@ -1,29 +1,37 @@
	defmodule Exile.ProcessNif do
	@moduledoc false
	-
	@on_load :load_nifs

	def load_nifs do
	nif_path = :filename.join(:code.priv_dir(:exile), "exile")
	:erlang.load_nif(nif_path, 0)
	end

	def execute(_cmd, _dir, _env, _stderr_to_console),
	do: :erlang.nif_error(:nif_library_not_loaded)

	def sys_write(_context, _bin), do: :erlang.nif_error(:nif_library_not_loaded)

	def sys_read(_context, _bytes), do: :erlang.nif_error(:nif_library_not_loaded)

	def sys_close(_context, _pipe), do: :erlang.nif_error(:nif_library_not_loaded)

	def sys_kill(_context), do: :erlang.nif_error(:nif_library_not_loaded)

	def sys_terminate(_context), do: :erlang.nif_error(:nif_library_not_loaded)

	def sys_wait(_context), do: :erlang.nif_error(:nif_library_not_loaded)

	def os_pid(_context), do: :erlang.nif_error(:nif_library_not_loaded)

	def alive?(_context), do: :erlang.nif_error(:nif_library_not_loaded)
	+
	+ # non-nif helper functions
	+ defmacro fork_exec_failure(), do: 125
	+
	+ defmacro nif_false(), do: 0
	+ defmacro nif_true(), do: 1
	+
	+ def to_process_fd(:stdin), do: 0
	+ def to_process_fd(:stdout), do: 1
	end
	diff --git a/lib/exile/stream.ex b/lib/exile/stream.ex
	index 79e01ed..7a77697 100644
	--- a/lib/exile/stream.ex
	+++ b/lib/exile/stream.ex
	@@ -1,132 +1,173 @@
	defmodule Exile.Stream do
	@moduledoc """
	- Defines a `Exile.Stream` struct returned by `Exile.stream!/3`.
	+ Defines a `Exile.Stream` struct returned by `Exile.stream!/3`.
	"""

	alias Exile.Process

	defmodule Sink do
	defstruct [:process]

	defimpl Collectable do
	def into(%{process: process} = stream) do
	collector_fun = fn
	:ok, {:cont, x} ->
	:ok = Process.write(process, x)

	:ok, :done ->
	:ok = Process.close_stdin(process)
	stream

	:ok, :halt ->
	:ok = Process.close_stdin(process)
	end

	{:ok, collector_fun}
	end
	end
	end

	defstruct [:process, :stream_opts]

	- @default_opts [exit_timeout: :infinity, chunk_size: 65535]
	-
	@type t :: %__MODULE__{}

	@doc false
	def __build__(cmd_with_args, opts) do
	{stream_opts, process_opts} = Keyword.split(opts, [:exit_timeout, :chunk_size, :input])
	- stream_opts = Keyword.merge(@default_opts, stream_opts)
	-
	- {:ok, process} = Process.start_link(cmd_with_args, process_opts)

	- start_input_streamer(%Sink{process: process}, stream_opts[:input])
	-
	- %Exile.Stream{process: process, stream_opts: stream_opts}
	+ with {:ok, stream_opts} <- normalize_stream_opts(stream_opts) do
	+ {:ok, process} = Process.start_link(cmd_with_args, process_opts)
	+ start_input_streamer(%Sink{process: process}, stream_opts.input)
	+ %Exile.Stream{process: process, stream_opts: stream_opts}
	+ else
	+ {:error, error} -> raise ArgumentError, message: error
	+ end
	end

	@doc false
	defp start_input_streamer(sink, input) do
	- cond do
	- is_nil(input) ->
	+ case input do
	+ :no_input ->
	:ok

	- !is_function(input) && Enumerable.impl_for(input) ->
	+ {:enumerable, enum} ->
	spawn_link(fn ->
	- Enum.into(input, sink)
	+ Enum.into(enum, sink)
	end)

	- is_function(input, 1) ->
	+ {:collectable, func} ->
	spawn_link(fn ->
	- input.(sink)
	+ func.(sink)
	end)
	-
	- true ->
	- raise ArgumentError,
	- message: ":input must be either Enumerable or a function with arity 1"
	end
	end

	defimpl Enumerable do
	def reduce(%{process: process, stream_opts: stream_opts}, acc, fun) do
	start_fun = fn -> :ok end

	next_fun = fn :ok ->
	- case Process.read(process, stream_opts[:chunk_size]) do
	+ case Process.read(process, stream_opts.chunk_size) do
	{:eof, []} ->
	{:halt, :normal}

	{:eof, x} ->
	# multiple reads on closed pipe always returns :eof
	{[IO.iodata_to_binary(x)], :ok}

	{:ok, x} ->
	{[IO.iodata_to_binary(x)], :ok}

	{:error, errno} ->
	raise "Failed to read from the process. errno: #{errno}"
	end
	end

	after_fun = fn exit_type ->
	try do
	# always close stdin before stoping to give the command chance to exit properly
	Process.close_stdin(process)
	- result = Process.await_exit(process, stream_opts[:exit_timeout])
	+ result = Process.await_exit(process, stream_opts.exit_timeout)

	case {exit_type, result} do
	{_, :timeout} ->
	Process.kill(process, :sigkill)
	raise "command fail to exit within timeout: #{stream_opts[:exit_timeout]}"

	{:normal, {:ok, {:exit, 0}}} ->
	:ok

	{:normal, {:ok, error}} ->
	raise "command exited with status: #{inspect(error)}"

	- {_, error} ->
	+ {exit_type, error} ->
	Process.kill(process, :sigkill)
	- raise "command exited with error: #{inspect(error)}"
	+
	+ raise "command exited with exit_type: #{inspect(exit_type)}, error: #{
	+ inspect(error)
	+ }"
	end
	after
	Process.stop(process)
	end
	end

	Stream.resource(start_fun, next_fun, after_fun).(acc, fun)
	end

	def count(_stream) do
	{:error, __MODULE__}
	end

	def member?(_stream, _term) do
	{:error, __MODULE__}
	end

	def slice(_stream) do
	{:error, __MODULE__}
	end
	end
	+
	+ defp normalize_input(term) do
	+ cond do
	+ is_nil(term) ->
	+ {:ok, :no_input}
	+
	+ !is_function(term) && Enumerable.impl_for(term) ->
	+ {:ok, {:enumerable, term}}
	+
	+ is_function(term, 1) ->
	+ {:ok, {:collectable, term}}
	+
	+ true ->
	+ {:error, "`:input` must be either Enumerable or a function which accepts collectable"}
	+ end
	+ end
	+
	+ defp normalize_chunk_size(nil), do: {:ok, 65536}
	+ defp normalize_chunk_size(:no_buffering), do: {:ok, :no_buffering}
	+
	+ defp normalize_chunk_size(chunk_size) do
	+ if is_integer(chunk_size) and chunk_size > 0,
	+ do: {:ok, chunk_size},
	+ else: {:error, ":exit_timeout must be either :infinity or a positive integer"}
	+ end
	+
	+ defp normalize_exit_timeout(term) when term in [nil, :infinity], do: {:ok, :infinity}
	+
	+ defp normalize_exit_timeout(term) do
	+ if is_integer(term),
	+ do: {:ok, term},
	+ else: {:error, ":exit_timeout must be either :infinity or an integer"}
	+ end
	+
	+ defp normalize_stream_opts(opts) when is_list(opts) do
	+ with {:ok, input} <- normalize_input(opts[:input]),
	+ {:ok, exit_timeout} <- normalize_exit_timeout(opts[:exit_timeout]),
	+ {:ok, chunk_size} <- normalize_chunk_size(opts[:chunk_size]) do
	+ {:ok, %{input: input, exit_timeout: exit_timeout, chunk_size: chunk_size}}
	+ end
	+ end
	+
	+ defp normalize_stream_opts(_), do: {:error, "stream_opts must be a keyword list"}
	end
	diff --git a/lib/exile/watcher.ex b/lib/exile/watcher.ex
	new file mode 100644
	index 0000000..a4cab72
	--- /dev/null
	+++ b/lib/exile/watcher.ex
	@@ -0,0 +1,81 @@
	+defmodule Exile.Watcher do
	+ use GenServer, restart: :temporary
	+ require Logger
	+ alias Exile.ProcessNif
	+
	+ def start_link(args) do
	+ {:ok, _pid} = GenServer.start_link(__MODULE__, args)
	+ end
	+
	+ def watch(pid, context) do
	+ spec = {Exile.Watcher, %{pid: pid, context: context}}
	+ DynamicSupervisor.start_child(Exile.WatcherSupervisor, spec)
	+ end
	+
	+ def init(args) do
	+ %{pid: pid, context: context} = args
	+ Process.flag(:trap_exit, true)
	+ ref = Elixir.Process.monitor(pid)
	+ {:ok, %{pid: pid, context: context, ref: ref}}
	+ end
	+
	+ def handle_info({:DOWN, ref, :process, pid, _reason}, %{pid: pid, context: context, ref: ref}) do
	+ attempt_graceful_exit(context)
	+ {:stop, :normal, nil}
	+ end
	+
	+ def handle_info({:EXIT, _, reason}, nil), do: {:stop, reason, nil}
	+
	+ # when watcher is attempted to be killed, we forcefully kill external os process.
	+ # This can happen when beam receive SIGTERM
	+ def handle_info({:EXIT, _, reason}, %{pid: pid, context: context}) do
	+ Logger.debug(fn -> "Watcher exiting. reason: #{inspect(reason)}" end)
	+ Exile.Process.stop(pid)
	+ attempt_graceful_exit(context)
	+ {:stop, reason, nil}
	+ end
	+
	+ # for proper process exit parent of the child must wait() for
	+ # child processes termination exit and "pickup" after the exit
	+ # (receive child exit_status). Resources acquired by child such as
	+ # file descriptors won't be released even if the child process
	+ # itself is terminated.
	+ defp attempt_graceful_exit(context) do
	+ try do
	+ Logger.debug(fn -> "Stopping external program" end)
	+
	+ # sys_close is idempotent, calling it multiple times is okay
	+ ProcessNif.sys_close(context, ProcessNif.to_process_fd(:stdin))
	+ ProcessNif.sys_close(context, ProcessNif.to_process_fd(:stdout))
	+
	+ # at max we wait for 100ms for program to exit
	+ process_exit?(context, 100) && throw(:done)
	+
	+ Logger.debug("Failed to stop external program gracefully. attempting SIGTERM")
	+ ProcessNif.sys_terminate(context)
	+ process_exit?(context, 100) && throw(:done)
	+
	+ Logger.debug("Failed to stop external program with SIGTERM. attempting SIGKILL")
	+ ProcessNif.sys_kill(context)
	+ process_exit?(context, 1000) && throw(:done)
	+
	+ Logger.error("[exile] failed to kill external process")
	+ raise "Failed to kill external process"
	+ catch
	+ :done -> Logger.debug(fn -> "External program exited successfully" end)
	+ end
	+ end
	+
	+ defp process_exit?(context) do
	+ match?({:ok, _}, ProcessNif.sys_wait(context))
	+ end
	+
	+ defp process_exit?(context, timeout) do
	+ if process_exit?(context) do
	+ true
	+ else
	+ :timer.sleep(timeout)
	+ process_exit?(context)
	+ end
	+ end
	+end
	diff --git a/mix.exs b/mix.exs
	index ff4e6ce..ffb9f00 100644
	--- a/mix.exs
	+++ b/mix.exs
	@@ -1,30 +1,31 @@
	defmodule Exile.MixProject do
	use Mix.Project

	def project do
	[
	app: :exile,
	version: "0.1.0",
	elixir: "~> 1.7",
	start_permanent: Mix.env() == :prod,
	compilers: [:elixir_make] ++ Mix.compilers(),
	make_targets: ["all"],
	make_clean: ["clean"],
	deps: deps()
	]
	end

	# Run "mix help compile.app" to learn about applications.
	def application do
	[
	+ mod: {Exile, []},
	extra_applications: [:logger]
	]
	end

	# Run "mix help deps" to learn about dependencies.
	defp deps do
	[
	{:elixir_make, "~> 0.6", runtime: false}
	]
	end
	end
	diff --git a/test/exile/process_test.exs b/test/exile/process_test.exs
	index 16767d6..890a520 100644
	--- a/test/exile/process_test.exs
	+++ b/test/exile/process_test.exs
	@@ -1,300 +1,333 @@
	defmodule Exile.ProcessTest do
	use ExUnit.Case, async: true
	alias Exile.Process

	test "read" do
	{:ok, s} = Process.start_link(~w(echo test))
	assert {:eof, iodata} = Process.read(s, 100)
	assert IO.iodata_to_binary(iodata) == "test\n"
	assert :ok == Process.close_stdin(s)
	assert {:ok, {:exit, 0}} == Process.await_exit(s, 500)
	+ Process.stop(s)
	end

	test "write" do
	{:ok, s} = Process.start_link(~w(cat))
	assert :ok == Process.write(s, "hello")
	assert {:ok, iodata} = Process.read(s, 5)
	assert IO.iodata_to_binary(iodata) == "hello"

	assert :ok == Process.write(s, "world")
	assert {:ok, iodata} = Process.read(s, 5)
	assert IO.iodata_to_binary(iodata) == "world"

	assert :ok == Process.close_stdin(s)
	assert {:eof, []} == Process.read(s)
	assert {:ok, {:exit, 0}} == Process.await_exit(s, 100)
	+ Process.stop(s)
	end

	test "stdin close" do
	logger = start_events_collector()

	# base64 produces output only after getting EOF from stdin. we
	# collect events in order and assert that we can still read from
	# stdout even after closing stdin
	{:ok, s} = Process.start_link(~w(base64))

	# parallel reader should be blocked till we close stdin
	start_parallel_reader(s, logger)
	:timer.sleep(100)

	assert :ok == Process.write(s, "hello")
	add_event(logger, {:write, "hello"})
	assert :ok == Process.write(s, "world")
	add_event(logger, {:write, "world"})
	:timer.sleep(100)

	assert :ok == Process.close_stdin(s)
	add_event(logger, :input_close)
	- assert {:ok, {:exit, 0}} == Process.await_exit(s, 50)
	+ assert {:ok, {:exit, 0}} == Process.await_exit(s, 100)
	+ Process.stop(s)

	assert [
	{:write, "hello"},
	{:write, "world"},
	:input_close,
	{:read, "aGVsbG93b3JsZA==\n"},
	:eof
	] == get_events(logger)
	end

	test "external command termination on stop" do
	{:ok, s} = Process.start_link(~w(cat))
	{:ok, os_pid} = Process.os_pid(s)
	assert os_process_alive?(os_pid)

	Process.stop(s)
	:timer.sleep(100)

	refute os_process_alive?(os_pid)
	end

	test "external command kill on stop" do
	# cat command hangs waiting for EOF
	{:ok, s} = Process.start_link([fixture("ignore_sigterm.sh")])

	{:ok, os_pid} = Process.os_pid(s)
	assert os_process_alive?(os_pid)
	Process.stop(s)

	if os_process_alive?(os_pid) do
	:timer.sleep(3000)
	refute os_process_alive?(os_pid)
	else
	:ok
	end
	end

	test "exit status" do
	{:ok, s} = Process.start_link(~w(sh -c "exit 2"))
	assert {:ok, {:exit, 2}} == Process.await_exit(s, 500)
	+ Process.stop(s)
	end

	test "writing binary larger than pipe buffer size" do
	large_bin = generate_binary(5 * 65535)
	{:ok, s} = Process.start_link(~w(cat))

	writer =
	Task.async(fn ->
	Process.write(s, large_bin)
	Process.close_stdin(s)
	end)

	:timer.sleep(100)

	{_, iodata} = Process.read(s, 5 * 65535)
	Task.await(writer)

	assert IO.iodata_length(iodata) == 5 * 65535
	assert {:ok, {:exit, 0}} == Process.await_exit(s, 500)
	+ Process.stop(s)
	end

	test "back-pressure" do
	logger = start_events_collector()

	# we test backpressure by testing if `write` is delayed when we delay read
	{:ok, s} = Process.start_link(~w(cat))

	large_bin = generate_binary(65535 * 5)

	writer =
	Task.async(fn ->
	Enum.each(1..10, fn i ->
	Process.write(s, large_bin)
	add_event(logger, {:write, i})
	end)

	Process.close_stdin(s)
	end)

	:timer.sleep(50)

	reader =
	Task.async(fn ->
	Enum.each(1..10, fn i ->
	Process.read(s, 5 * 65535)
	add_event(logger, {:read, i})
	# delay in reading should delay writes
	:timer.sleep(10)
	end)
	end)

	Task.await(writer)
	Task.await(reader)

	assert {:ok, {:exit, 0}} == Process.await_exit(s, 500)
	+ Process.stop(s)

	assert [
	write: 1,
	read: 1,
	write: 2,
	read: 2,
	write: 3,
	read: 3,
	write: 4,
	read: 4,
	write: 5,
	read: 5,
	write: 6,
	read: 6,
	write: 7,
	read: 7,
	write: 8,
	read: 8,
	write: 9,
	read: 9,
	write: 10,
	read: 10
	] == get_events(logger)
	end

	# this test does not work properly in linux
	@tag :skip
	test "if we are leaking file descriptor" do
	{:ok, s} = Process.start_link(~w(sleep 60))
	{:ok, os_pid} = Process.os_pid(s)

	# we are only printing FD, TYPE, NAME with respective prefix
	{bin, 0} = System.cmd("lsof", ["-F", "ftn", "-p", to_string(os_pid)])

	Process.stop(s)

	open_files = parse_lsof(bin)
	assert [%{fd: "0", name: _, type: "PIPE"}, %{type: "PIPE", fd: "1", name: _}] = open_files
	end

	test "process kill with pending write" do
	{:ok, s} = Process.start_link(~w(cat))
	{:ok, os_pid} = Process.os_pid(s)

	large_data =
	Stream.cycle(["test"]) \|> Stream.take(500_000) \|> Enum.to_list() \|> IO.iodata_to_binary()

	task =
	Task.async(fn ->
	try do
	Process.write(s, large_data)
	catch
	:exit, reason -> reason
	end
	end)

	:timer.sleep(200)
	Process.stop(s)
	:timer.sleep(3000)

	refute os_process_alive?(os_pid)
	assert {:normal, _} = Task.await(task)
	end

	test "cd" do
	parent = Path.expand("..", File.cwd!())
	{:ok, s} = Process.start_link(~w(sh -c pwd), cd: parent)
	{:ok, dir} = Process.read(s)
	assert String.trim(dir) == parent
	assert {:ok, {:exit, 0}} = Process.await_exit(s)
	+ Process.stop(s)
	end

	test "invalid path" do
	assert {:error, _} = Process.start_link(~w(sh -c pwd), cd: "invalid")
	end

	+ test "invalid opt" do
	+ assert {:error, "invalid opts: [invalid: :test]"} =
	+ Process.start_link(~w(cat), invalid: :test)
	+ end
	+
	test "env" do
	- assert {:ok, s} = Process.start_link([fixture("env.sh")], env: %{"TEST_ENV" => "test"})
	+ assert {:ok, s} = Process.start_link(~w(printenv TEST_ENV), env: %{"TEST_ENV" => "test"})
	+
	+ assert {:ok, "test\n"} = Process.read(s)
	+ assert {:ok, {:exit, 0}} = Process.await_exit(s)
	+ Process.stop(s)
	+ end

	- assert {:ok, "test"} = Process.read(s)
	+ test "if external process inherits beam env" do
	+ :ok = System.put_env([{"BEAM_ENV_A", "10"}])
	+ assert {:ok, s} = Process.start_link(~w(printenv BEAM_ENV_A))
	+
	+ assert {:ok, "10\n"} = Process.read(s)
	assert {:ok, {:exit, 0}} = Process.await_exit(s)
	+ Process.stop(s)
	+ end
	+
	+ test "if user env overrides beam env" do
	+ :ok = System.put_env([{"BEAM_ENV", "base"}])
	+
	+ assert {:ok, s} =
	+ Process.start_link(~w(printenv BEAM_ENV), env: %{"BEAM_ENV" => "overridden"})
	+
	+ assert {:ok, "overridden\n"} = Process.read(s)
	+ assert {:ok, {:exit, 0}} = Process.await_exit(s)
	+ Process.stop(s)
	end

	def start_parallel_reader(proc_server, logger) do
	spawn_link(fn -> reader_loop(proc_server, logger) end)
	end

	def reader_loop(proc_server, logger) do
	case Process.read(proc_server) do
	{:ok, data} ->
	add_event(logger, {:read, data})
	reader_loop(proc_server, logger)

	{:eof, []} ->
	add_event(logger, :eof)
	end
	end

	def start_events_collector do
	{:ok, ordered_events} = Agent.start(fn -> [] end)
	ordered_events
	end

	def add_event(agent, event) do
	:ok = Agent.update(agent, fn events -> events ++ [event] end)
	end

	def get_events(agent) do
	Agent.get(agent, & &1)
	end

	defp os_process_alive?(pid) do
	match?({_, 0}, System.cmd("ps", ["-p", to_string(pid)]))
	end

	defp fixture(script) do
	Path.join([__DIR__, "../scripts", script])
	end

	defp parse_lsof(iodata) do
	String.split(IO.iodata_to_binary(iodata), "\n", trim: true)
	\|> Enum.reduce([], fn
	"f" <> fd, acc -> [%{fd: fd} \| acc]
	"t" <> type, [h \| acc] -> [Map.put(h, :type, type) \| acc]
	"n" <> name, [h \| acc] -> [Map.put(h, :name, name) \| acc]
	_, acc -> acc
	end)
	\|> Enum.reverse()
	\|> Enum.reject(fn
	%{fd: fd} when fd in ["255", "cwd", "txt"] ->
	true

	%{fd: "rtd", name: "/", type: "DIR"} ->
	true

	# filter libc and friends
	%{fd: "mem", type: "REG", name: "/lib/x86_64-linux-gnu/" <> _} ->
	true

	%{fd: "mem", type: "REG", name: "/usr/lib/locale/C.UTF-8/" <> _} ->
	true

	%{fd: "mem", type: "REG", name: "/usr/lib/locale/locale-archive" <> _} ->
	true

	%{fd: "mem", type: "REG", name: "/usr/lib/x86_64-linux-gnu/gconv" <> _} ->
	true

	_ ->
	false
	end)
	end

	defp generate_binary(size) do
	Stream.repeatedly(fn -> "A" end) \|> Enum.take(size) \|> IO.iodata_to_binary()
	end
	end
	diff --git a/test/exile/sync_process_test.exs b/test/exile/sync_process_test.exs
	index cc53806..51a74e8 100644
	--- a/test/exile/sync_process_test.exs
	+++ b/test/exile/sync_process_test.exs
	@@ -1,26 +1,54 @@
	defmodule Exile.SyncProcessTest do
	use ExUnit.Case, async: false
	alias Exile.Process

	@bin Stream.repeatedly(fn -> "A" end) \|> Enum.take(65535) \|> IO.iodata_to_binary()

	test "memory leak" do
	:timer.sleep(1000)
	before_exec = :erlang.memory(:total)

	{:ok, s} = Process.start_link(~w(cat))

	Enum.each(1..500, fn _ ->
	:ok = Process.write(s, @bin)
	{:ok, _} = Process.read(s, 65535)
	end)

	:timer.sleep(1000)
	after_exec = :erlang.memory(:total)

	assert_in_delta before_exec, after_exec, 1024 * 1024

	assert :ok == Process.close_stdin(s)
	assert {:ok, {:exit, 0}} == Process.await_exit(s, 500)
	+ Process.stop(s)
	+ end
	+
	+ test "if watcher process exits on command exit" do
	+ stop_all_children(Exile.WatcherSupervisor)
	+
	+ assert %{active: 0, workers: 0} = DynamicSupervisor.count_children(Exile.WatcherSupervisor)
	+ assert {:ok, s} = Process.start_link(~w(cat))
	+
	+ # we spawn in background
	+ :timer.sleep(200)
	+
	+ assert %{active: 1, workers: 1} = DynamicSupervisor.count_children(Exile.WatcherSupervisor)
	+
	+ Process.close_stdin(s)
	+ assert {:ok, {:exit, 0}} = Process.await_exit(s, 500)
	+ Process.stop(s)
	+
	+ # wait for watcher to terminate
	+ :timer.sleep(200)
	+ assert %{active: 0, workers: 0} = DynamicSupervisor.count_children(Exile.WatcherSupervisor)
	+ end
	+
	+ defp stop_all_children(sup) do
	+ DynamicSupervisor.which_children(sup)
	+ \|> Enum.each(fn {_, pid, _, _} ->
	+ DynamicSupervisor.terminate_child(sup, pid)
	+ end)
	end
	end
	diff --git a/test/scripts/env.sh b/test/scripts/env.sh
	deleted file mode 100755
	index e684d9e..0000000
	--- a/test/scripts/env.sh
	+++ /dev/null
	@@ -1,3 +0,0 @@
	-#!/bin/bash
	-
	-printf ${TEST_ENV}

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