DaniWeb IT Discussion Community - explanation for source code of printf

/*

 Copyright 2001, 2002 Georges Menie (<URL snipped>)

    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