path: root/slowbaud.c

/* Simulate low bitrate serial connection, like a 1980s modem.
   Author: B. Watson. License: WTFPL. */

#define VERSION "0.1.0"

/* configurables: */
#define FRACTIONAL_USEC
#define SHOW_CPU_TIME


#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/time.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <errno.h>
#include <sys/select.h>
#include <sys/wait.h>
#include <termios.h>
#include <sys/ioctl.h>
#include <signal.h>
#include <time.h>
#include <string.h>
#include <math.h>

/* portability stuff only tested on Linux and OSX. Hope it's OK. */
#if defined(__FreeBSD__) || defined(HAVE_LIBUTIL_H)
	#include <libutil.h>
#elif defined (__OpenBSD__) || defined (__NetBSD__) || defined (__APPLE__) || defined(HAVE_UTIL_H)
	#include <util.h>
#else
	#include <pty.h>
#endif

#define DEFAULT_BPS 2400
#define MIN_BPS 1
#define MAX_BPS 500000

const char *self;
unsigned int bps;
unsigned long interval, starttime, outbytes = 0;

int debug = 0;

int timer;
struct timeval tv;
struct itimerval itv, itv_disarm;
sigset_t sigmask;

#ifdef FRACTIONAL_USEC
#define FRAC_US_DENOM 100
int frac_us_num;
#endif

#define NOW_USEC() ( (gettimeofday(&tv, NULL)), tv.tv_sec * 1000000L + tv.tv_usec )

void die(const char *msg) {
	if(errno) {
		fprintf(stderr, "%s: %s: %s\n", self, strerror(errno), msg);
	} else {
		fprintf(stderr, "%s: %s\n", self, msg);
	}
	exit(1);
}

void setup_timer(void) {
	itv_disarm.it_interval.tv_sec =
	itv_disarm.it_interval.tv_usec =
	itv_disarm.it_value.tv_sec =
	itv_disarm.it_value.tv_usec = 0;

	itv = itv_disarm;
	itv.it_interval.tv_usec = interval;
	itv.it_value.tv_usec = interval;

	sigemptyset(&sigmask);
	sigaddset(&sigmask, SIGALRM);
	sigprocmask(SIG_BLOCK, &sigmask, NULL);
}

/* on both Linux and OSX, using setitimer() and sigwait() gives better
   timing accuracy than usleep() or nanosleep(). */
void slow_write(int outfd, char c) {
	int j;
	unsigned long now, target, overslept;

	setitimer(ITIMER_REAL, &itv, NULL);

	target = NOW_USEC() + interval;

#ifdef FRACTIONAL_USEC
	if((random() % FRAC_US_DENOM) < frac_us_num) target++;
#endif

	write(outfd, &c, 1);
	outbytes++;
	sigwait(&sigmask, &j);
	setitimer(ITIMER_REAL, &itv_disarm, NULL);

	now = NOW_USEC();
	if(now > target) {
		overslept = now - target;
		if(overslept < itv.it_interval.tv_usec) {
			// fprintf(stderr, "overslept %luus\n", overslept);
			itv.it_interval.tv_usec -= overslept;
			itv.it_value.tv_usec = itv.it_interval.tv_usec;
		}
	} else {
		while(NOW_USEC() < target)
			/* NOP */ ;
	}
}

void debug_stats(void) {
	if(outbytes) {
		unsigned long elapsed_us = NOW_USEC() - starttime;
		double elapsed_sec = ((double)elapsed_us/1000000.0L);
		double finterval = (1000000.0L / ((double)bps / 10.0L));
		double actual = ((double)outbytes * 10.0L) / elapsed_sec;
		double offby = 100.0L * (((double)bps / actual) - 1.0L);
		fprintf(stderr,
				"outbytes %lu, elapsed_us %lu, tv_usec %lu\nrequested bps %d (%.2fms), "
				"actual %.2f, accuracy %.2f%%\n",
				outbytes, elapsed_us, itv.it_value.tv_usec, bps, finterval, actual, 100.0 - offby);
#ifdef SHOW_CPU_TIME
		{
			struct timespec ts;
			long end_cpu_us;
			double cpu_pct;

			clock_gettime(CLOCK_PROCESS_CPUTIME_ID, &ts);
			end_cpu_us = ts.tv_sec * 1000000L + ts.tv_nsec / 1000L;
			cpu_pct = (double)(end_cpu_us) / (double)(elapsed_us) * 100.0L;
			fprintf(stderr, "CPU usage %luus, %.2f%%\n", end_cpu_us, cpu_pct);
		}
#endif

	}
}

void slowcat(int infd) {
	int c;

	outbytes = 0;
	starttime = NOW_USEC();

	while(read(infd, &c, 1) == 1) {
		slow_write(1, c);
	}

	if(debug) debug_stats();
}

void slowtty(char **args) {
	char *dflt_args[] = { NULL, NULL };
	pid_t kidpid;
	int master, status;
	struct termios term, oldterm;
	struct winsize winsz;
	char c;

	if(!*args) {
		args = dflt_args;
		args[0] = getenv("SHELL");
		if(!*args) args[0] = "/bin/sh";
	}
	if(debug) {
		char **a = args;
		fprintf(stderr, "executing cmd '%s', argv[]: ", a[0]);
		while(*a) {
			fprintf(stderr, "%s ", *a++);
		}
		fprintf(stderr, "\n");
	}

	if( (tcgetattr(0, &term)) ) die("tcgetattr");

	if( (ioctl(0, TIOCGWINSZ, &winsz)) ) die("ioctl(TIOCGWINSZ)");

	kidpid = forkpty(&master, NULL, &term, &winsz);
	if(kidpid < 0) die("forkpty");

	if(kidpid == 0) {
		/* we are the child, do childish things */
		execvp(args[0], args);
		perror("execvp"); /* don't use die() */
		exit(127);
	}

	/* we are the parent, watch the child */
	oldterm = term; /* so we can restore it later */
	cfmakeraw(&term);
	tcsetattr(0, TCSANOW, &term);

	while(1) {
		ssize_t nbytes;
		fd_set readfds;

		FD_ZERO(&readfds);
		FD_SET(0, &readfds);
		FD_SET(master, &readfds);

		if(select(master + 1, &readfds, NULL, NULL, NULL) < 0) {
			if(errno == EINTR) continue;
			break;
		}

      if(FD_ISSET(0, &readfds)) {
			nbytes = read(0, &c, 1);
			if(nbytes < 1) break;
			write(master, &c, 1);
		}

      if(FD_ISSET(master, &readfds)) {
			nbytes = read(master, &c, 1);
			if(nbytes < 1) break;
			slow_write(1, c);
		}
	}

	close(master);
	while (waitpid(kidpid, &status, 0) < 0 && errno == EINTR) /* NOP */ ;

	tcsetattr(STDIN_FILENO, TCSANOW, &oldterm);
	tcsetattr(STDOUT_FILENO, TCSANOW, &oldterm);
	tcsetattr(STDERR_FILENO, TCSANOW, &oldterm);

	exit(WIFEXITED(status) ? WEXITSTATUS(status) : 127);
}

void echo_args(char **args) {
	starttime = NOW_USEC();
	while(*args) {
		char *a = *args;
		while(*a) slow_write(1, *a++);
		args++;
		if(*args) slow_write(1, ' ');
	}

	slow_write(1, '\n');

	if(debug) debug_stats();
	exit(0);
}

void benchmark(char **args) {
	int bytes = 0, devnull;

	if(*args) bytes = atoi(*args);
	if(bytes < 1) bytes = 4096;
	fprintf(stderr, "benchmarking with %d bytes\n", bytes);

	devnull = open("/dev/null", O_RDONLY);
	if(devnull < 1) die("/dev/null");

	starttime = NOW_USEC();
	while(bytes--)
		slow_write(devnull, 0);
	close(devnull);

	debug_stats();
	exit(0);
}

void usage(int exitstat) {
	printf("slowbaud " VERSION ", (c) 2021 B. Watson, WTFPL.\n");
	printf("Usage: %s [<bits-per-sec>] [<file> [<file> ...]]\n", self);
	printf("       With no filenames, reads stdin.\n");
	printf("or:    %s [<bits-per-sec>] -c [<command> [<arg> ...]]\n", self);
	printf("       With no command, spawns a shell.\n");
	printf("or:    %s [<bits-per-sec>] -e <string> [<string> ...]\n", self);
	printf("or:    %s [<bits-per-sec>] -b [<bytes>]\n", self);
	printf("With no <bits-per-sec>, default rate is %d\n", DEFAULT_BPS);
	exit(exitstat);
}

int main(int argc, char **argv) {
	int infd;
	const char *p, *env_bps;

	if(getenv("SLOWBAUD_DEBUG")) debug = 1;

	/* for usage message, we want "slowbaud", not "/path/to/slowbaud" */
	self = argv[0];
	for(p = argv[0]; *p; p++) {
		if(*p == '/') self = p + 1;
	}
	argv++, argc--;

	/* set bps. command line option overrides env, env overrides builtin */
	bps = DEFAULT_BPS;

	if( (env_bps = getenv("SLOWBAUD_BPS")) )
		bps = atoi(env_bps);

	if(*argv && (**argv > '0' && **argv <= '9')) {
		char *e;
		long b = strtol(*argv, &e, 0);
		if(!*e) {
			bps = (int)b;
			argv++, argc--;
		}
	}

	if(bps < MIN_BPS || bps > MAX_BPS) {
		fprintf(stderr, "%s: invalid bps, range %d - %d\n", self, MIN_BPS, MAX_BPS);
		exit(1);
	}

	/* if we used only integer math here, we couldn't support bps not
      a multiple of 10 (e.g. 75 would be taken as 70). */
	interval = (unsigned long)(1000000.0L / ((double)bps / 10.0L));

#ifdef FRACTIONAL_USEC
	frac_us_num = roundl((double)FRAC_US_DENOM * ((1000000.0L / ((double)bps / 10.0L) - interval)));
	if(debug) fprintf(stderr, "interval %ld + %d/%d us\n", interval, frac_us_num, FRAC_US_DENOM);
	srandom(NOW_USEC());
#else
	if(debug) fprintf(stderr, "interval %ld us\n", interval);
#endif

	if(*argv && argv[0][0] == '-' && argv[0][1] == '-' && !argv[0][2]) {
		if(debug) fprintf(stderr, "skipping -- argument\n");
		argv++, argc--;
	}

	setup_timer();

	if(!argc) {
		if(debug) fprintf(stderr, "no args, reading stdin\n");
		slowcat(0);
	} else if(**argv == '-') {
		switch(argv[0][1]) {
			case 'c':
				if(debug) fprintf(stderr, "-c given, creating a pty\n");
				slowtty(++argv);
				break;
			case 'e':
				if(argc < 2) die("-e requires at least one argument");
				if(debug) fprintf(stderr, "-e given, echoing args\n");
				echo_args(++argv);
				break;
			case 'b':
				benchmark(++argv);
				break;
			case 'h':
			case '?':
				usage(0);
				break;
			default:
				die("invalid option, try -h for help");
				break;
		}
	} else {
		while(*argv) {
			if(debug) fprintf(stderr, "opening file %s for reading\n", *argv);
			if( (infd = open(*argv, O_RDONLY)) < 0) {
				die(*argv);
			}
			slowcat(infd);
			close(infd);
			argv++;
		}
	}
	exit(0);
}