My implementation of echo(1)

echo.c

/*-
 * Copyright (c) 2014 Robert Clausecker <[email protected]>
 */

#define _POSIX_C_SOURCE 200809L
#include <stdio.h>
#include <stdlib.h>

#include "err.h"

extern int		 main(int, char*[]);
static const char	*escape(const char*);
static void		 put(int);

/*
 * Implementation of echo(1) according to IEEE Std 1003.1, 2013 edition.
 * This implementation implements the behavior for -n an \ as specified for
 * XSI-conformant systems and outputs unrecognized escape sequences unchanged.
 */
extern int
main(int argc, char *argv[])
{
	int i;
	const char *arg;

	setprogname(argv[0]);

	for (i = 1; i < argc; i++) {
		if (i > 1)
			put(' ');

		for (arg = argv[i]; *arg != '\0'; arg++)
			if (*arg == '\\') {
				if ((arg = escape(arg)) == NULL)
					goto printing_done;
			} else
				put(*arg);
	}

	put('\n');

printing_done:
	if (fflush(stdout) == EOF)
		/* LOCALIZE */
		err(1, "Error writing to stdout");

	return (0);
}

/*
 * expand the first escape sequence that makes up the beginning of arg and
 * print the expansion onto stdout. Return a pointer to the last character in
 * the escape sequence, or NULL if the escape sequence is \c.
 */
static const char*
escape(const char *arg)
{
	/* lookup table for escape sequences */
	static const char esctab[] = {
		['\\' - '\\'] = '\\',
		['a' - '\\'] = '\a',
		['b' - '\\'] = '\b',
		['f' - '\\'] = '\f',
		['n' - '\\'] = '\n',
		['r' - '\\'] = '\r',
		['v' - '\\'] = '\v',
	};

	int i, codepoint = 0;

	++arg;	/* assume *arg == '\\' */

	/* *arg in esctab? */
	if (*arg >= '\\' && *arg <= 'v' && esctab[*arg - '\\'] != 0) {
		put(esctab[*arg - '\\']);
		return (arg);
	}

	/* octal escape? */
	if (*arg == '0') {
		for (i = 0; i < 3; i++) {
			++arg;
			if ('0' <= *arg && *arg <= '7')
				codepoint = codepoint * 8 + (*arg - '0');
			else
				break;
		}

		put(codepoint);
		return (arg);
	}

	/* end of input? */
	if (*arg == 'c')
		return (NULL);

	/* unknown escape */
	put('\\');
	put(*arg);

	return (arg);
}

/*
 * putchar() wrapped to exit the program on IO-error.
 */
static void
put(int c)
{
	if (putchar(c) == EOF)
		/* LOCALIZE */
		err(1, "Error writing to stdout");
}