Small printf source code

This small printf function is a tiny implementation of the standard C library function, it does not include all the capabilities of the standard one. It has been designed to have a small footprint and to be easy to include in an embedded software. The only external dependency is the putchar() functions which output a single character to the output device. Once you provide this putchar function, you may link your code with this printf and use it. This code is released under the LGPL license.

This printf function will accept integer formats (%d, %x, %X, %u), string format (%s) and character format (%c); left and right alignement, padding with space or 'O'. See the comments and the test code at the end of the source code. You might have to modify it to work with your compiler with regards to the variable parameter passing.

On my DragonEngine (68VZ328) board, I am using the following code to replace putchar :

void _outbyte(int c)
{
        while ((readb(UTX1) & 0x20) == 0)
                ;

        writeb ((char)c,UTX1+1);
}

void outbyte(int c)
{
        static char prev = 0;
        if (c < ' ' && c != '\r' && c != '\n' && c != '\t' && c != '\b')
                return;
        if (c == '\n' && prev != '\r') _outbyte('\r');
        _outbyte(c);
        prev = c;
}

_outbyte does the raw output of any char, while outbyte call it for printable chars only and add a CR character when needed. This is a very simple scheme, kind of raw versus cooked mode.

Source code

/*
    Copyright 2001 Georges Menie
    https://www.menie.org/georges/embedded/small_printf_source_code.html

    This program is free software; you can redistribute it and/or modify
    it under the terms of the GNU Lesser General Public License as published by
    the Free Software Foundation; either version 2 of the License, or
    (at your option) any later version.

    This program is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    GNU Lesser General Public License for more details.

    You should have received a copy of the GNU Lesser General Public License
    along with this program; if not, write to the Free Software
    Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
*/

/*
	putchar is the only external dependency for this file,
	if you have a working putchar, just remove the following
	define. If the function should be called something else,
	replace outbyte(c) by your own function call.
*/
#define putchar(c) outbyte(c)

static void printchar(char **str, int c)
{
	extern int putchar(int c);
	if (str) {
		**str = c;
		++(*str);
	}
	else (void)putchar(c);
}

#define PAD_RIGHT 1
#define PAD_ZERO 2

static int prints(char **out, const char *string, int width, int pad)
{
	register int pc = 0, padchar = ' ';

	if (width > 0) {
		register int len = 0;
		register const char *ptr;
		for (ptr = string; *ptr; ++ptr) ++len;
		if (len >= width) width = 0;
		else width -= len;
		if (pad & PAD_ZERO) padchar = '0';
	}
	if (!(pad & PAD_RIGHT)) {
		for ( ; width > 0; --width) {
			printchar (out, padchar);
			++pc;
		}
	}
	for ( ; *string ; ++string) {
		printchar (out, *string);
		++pc;
	}
	for ( ; width > 0; --width) {
		printchar (out, padchar);
		++pc;
	}

	return pc;
}

/* the following should be enough for 32 bit int */
#define PRINT_BUF_LEN 12

static int printi(char **out, int i, int b, int sg, int width, int pad, int letbase)
{
	char print_buf[PRINT_BUF_LEN];
	register char *s;
	register int t, neg = 0, pc = 0;
	register unsigned int u = i;

	if (i == 0) {
		print_buf[0] = '0';
		print_buf[1] = '\0';
		return prints (out, print_buf, width, pad);
	}

	if (sg && b == 10 && i < 0) {
		neg = 1;
		u = -i;
	}

	s = print_buf + PRINT_BUF_LEN-1;
	*s = '\0';

	while (u) {
		t = u % b;
		if( t >= 10 )
			t += letbase - '0' - 10;
		*--s = t + '0';
		u /= b;
	}

	if (neg) {
		if( width && (pad & PAD_ZERO) ) {
			printchar (out, '-');
			++pc;
			--width;
		}
		else {
			*--s = '-';
		}
	}

	return pc + prints (out, s, width, pad);
}

static int print(char **out, int *varg)
{
	register int width, pad;
	register int pc = 0;
	register char *format = (char *)(*varg++);
	char scr[2];

	for (; *format != 0; ++format) {
		if (*format == '%') {
			++format;
			width = pad = 0;
			if (*format == '\0') break;
			if (*format == '%') goto out;
			if (*format == '-') {
				++format;
				pad = PAD_RIGHT;
			}
			while (*format == '0') {
				++format;
				pad |= PAD_ZERO;
			}
			for ( ; *format >= '0' && *format <= '9'; ++format) {
				width *= 10;
				width += *format - '0';
			}
			if( *format == 's' ) {
				register char *s = *((char **)varg++);
				pc += prints (out, s?s:"(null)", width, pad);
				continue;
			}
			if( *format == 'd' ) {
				pc += printi (out, *varg++, 10, 1, width, pad, 'a');
				continue;
			}
			if( *format == 'x' ) {
				pc += printi (out, *varg++, 16, 0, width, pad, 'a');
				continue;
			}
			if( *format == 'X' ) {
				pc += printi (out, *varg++, 16, 0, width, pad, 'A');
				continue;
			}
			if( *format == 'u' ) {
				pc += printi (out, *varg++, 10, 0, width, pad, 'a');
				continue;
			}
			if( *format == 'c' ) {
				/* char are converted to int then pushed on the stack */
				scr[0] = *varg++;
				scr[1] = '\0';
				pc += prints (out, scr, width, pad);
				continue;
			}
		}
		else {
		out:
			printchar (out, *format);
			++pc;
		}
	}
	if (out) **out = '\0';
	return pc;
}

/* assuming sizeof(void *) == sizeof(int) */

int printf(const char *format, ...)
{
	register int *varg = (int *)(&format);
	return print(0, varg);
}

int sprintf(char *out, const char *format, ...)
{
	register int *varg = (int *)(&format);
	return print(&out, varg);
}

#ifdef TEST_PRINTF
int main(void)
{
	char *ptr = "Hello world!";
	char *np = 0;
	int i = 5;
	unsigned int bs = sizeof(int)*8;
	int mi;
	char buf[80];

	mi = (1 << (bs-1)) + 1;
	printf("%s\n", ptr);
	printf("printf test\n");
	printf("%s is null pointer\n", np);
	printf("%d = 5\n", i);
	printf("%d = - max int\n", mi);
	printf("char %c = 'a'\n", 'a');
	printf("hex %x = ff\n", 0xff);
	printf("hex %02x = 00\n", 0);
	printf("signed %d = unsigned %u = hex %x\n", -3, -3, -3);
	printf("%d %s(s)%", 0, "message");
	printf("\n");
	printf("%d %s(s) with %%\n", 0, "message");
	sprintf(buf, "justif: \"%-10s\"\n", "left"); printf("%s", buf);
	sprintf(buf, "justif: \"%10s\"\n", "right"); printf("%s", buf);
	sprintf(buf, " 3: %04d zero padded\n", 3); printf("%s", buf);
	sprintf(buf, " 3: %-4d left justif.\n", 3); printf("%s", buf);
	sprintf(buf, " 3: %4d right justif.\n", 3); printf("%s", buf);
	sprintf(buf, "-3: %04d zero padded\n", -3); printf("%s", buf);
	sprintf(buf, "-3: %-4d left justif.\n", -3); printf("%s", buf);
	sprintf(buf, "-3: %4d right justif.\n", -3); printf("%s", buf);

	return 0;
}

/*
 * if you compile this file with
 *   gcc -Wall $(YOUR_C_OPTIONS) -DTEST_PRINTF -c printf.c
 * you will get a normal warning:
 *   printf.c:214: warning: spurious trailing `%' in format
 * this line is testing an invalid % at the end of the format string.
 *
 * this should display (on 32bit int machine) :
 *
 * Hello world!
 * printf test
 * (null) is null pointer
 * 5 = 5
 * -2147483647 = - max int
 * char a = 'a'
 * hex ff = ff
 * hex 00 = 00
 * signed -3 = unsigned 4294967293 = hex fffffffd
 * 0 message(s)
 * 0 message(s) with %
 * justif: "left      "
 * justif: "     right"
 *  3: 0003 zero padded
 *  3: 3    left justif.
 *  3:    3 right justif.
 * -3: -003 zero padded
 * -3: -3   left justif.
 * -3:   -3 right justif.
 */

#endif