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Poor Man's snprintf()

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Poor Man's snprintf()
PostPosted: Thu Mar 12, 2015 2:28 pm     Reply with quote

Below is a quick and dirty way to get a snprintf() method without having to write all the parsing code. It leverages a CCS extension to work. It makes use of macros, so always be aware of the dangers there.


* Name:  snprintf.h
* Auth:  Jeremiah
* Desc:  This file contains a poor man's implementation of snprintf() for the
*        CCS compiler.  It is not meant to be as efficient as a homemade
*        snprintf() method.  It leverages the putc() substitution in printf(),
*        which is a CCS extension.
*        FUNCTIONS:
*           snprintf()
* Compiled with PCWHD v5.025
* History:
* 03/12/2015 - JHB - Creation
#ifndef SNPRINTF_H
#define SNPRINTF_H

// NAME: snprintf()
// DESC: Composes a string with the same text that would be printed if fmt
//       was used on printf, but instead of being printed, the content is
//       stored as a string in the buffer pointed by buf (taking max as the
//       maximum buffer capacity to fill).
// IN:   buf = Pointer to a buffer where the resulting string is stored
//       max = Maximum number of bytes to be used in the buffer.  The
//             generated string has a length of max-1, leaving space for a
//             null character.  Expects an unsigned int16 > 0.
//       fmt = CString that contains a format that follows the same format
//             in printf (see printf for details).  This must be a constant
//             string (not a variable).
//       ... = List of parameters used as inputs to the format string.
// OUT:  The number of characters that would have been written if max was
//       large enough to hold them.
// NOTE: Does not return a negative value for format errors.  However the
//       CCS compiler reports format errors at compile time.
#define snprintf(buf,max,fmt,...) (       \
   snprintf_init(buf,max),                \
   printf(snprintf_putc,fmt,__VA_ARGS__), \
   snprintf_cleanup(),                    \
   g_snprintfNum                          )
//Macros that use a comma separated list of statements return the final
//statement as if it were a function return value.  Also note that
//semicolons are not needed using this method (they are implied)

//****************** Implementation (Do Not Use Directly) *******************//
static unsigned int16 g_snprintfMax;  //this will decrement with each character
static unsigned int16 g_snprintfNum;  //this will increment with each character
static char *         g_snprintfBuf;  //holds address of the output buffer

//function passed to printf() ... CCS option only
void snprintf_putc(char c){
   //only copy the maximum number of allowed characters
         g_snprintfBuf[g_snprintfNum] = c;   
      }else{ //last spot in the buffer must be a '\0'
         g_snprintfBuf[g_snprintfNum] = '\0';  //append a null
   //update this regardless of max so the snprintf() return value is correct.

//function used to prepare for a new snprintf() call
void snprintf_init(char *buf, unsigned int16 max){
   g_snprintfMax = max; 
   g_snprintfNum = 0;   
   g_snprintfBuf = buf;
   if(g_snprintfMax){  //only write to it if buffer size non zero
      g_snprintfBuf[0] = '\0';

//function used to finish up buffer with a null if needed. 
void snprintf_cleanup(){
   //if maximum buffer size was never reached, then null terminate
      g_snprintfBuf[g_snprintfNum] = '\0';


Test Code (supply your own FUSES, etc.):

#include "pic.h"  //has #device, #FUSES, #use delay(), #use rs232()

#include "snprintf.h"

void main(void){
   char buffer[15] = {
   unsigned int16 i;
   unsigned int16 val=255;
   printf("\r\n*********Program Start*********\r\n");
   //Test empty input string
   val = snprintf(buffer,sizeof(buffer),"");
   printf("BUFFER(%03u,%03u): %s\r\n",sizeof(buffer),val,buffer);
   //Test being under the max
   val = snprintf(buffer,sizeof(buffer),"Hello");
   //show remaining buffer just for this one to verify algorithm isn't
   //overwriting the entire buffer.
   printf("BUFFER(%03u,%03u): %s %s\r\n", sizeof(buffer),
   val = snprintf(buffer,sizeof(buffer),"Hello ");
   printf("BUFFER(%03u,%03u): %s\r\n",sizeof(buffer),val,buffer);
   val = snprintf(buffer,sizeof(buffer),"Hello 7");
   printf("BUFFER(%03u,%03u): %s\r\n",sizeof(buffer),val,buffer);
   val = snprintf(buffer,sizeof(buffer),"Hello 78");
   printf("BUFFER(%03u,%03u): %s\r\n",sizeof(buffer),val,buffer);
   val = snprintf(buffer,sizeof(buffer),"Hello 789");
   printf("BUFFER(%03u,%03u): %s\r\n",sizeof(buffer),val,buffer);
   val = snprintf(buffer,sizeof(buffer),"Hello 789A");
   printf("BUFFER(%03u,%03u): %s\r\n",sizeof(buffer),val,buffer);
   val = snprintf(buffer,sizeof(buffer),"Hello 789AB");
   printf("BUFFER(%03u,%03u): %s\r\n",sizeof(buffer),val,buffer);
   val = snprintf(buffer,sizeof(buffer),"Hello 789ABC");
   printf("BUFFER(%03u,%03u): %s\r\n",sizeof(buffer),val,buffer);
   val = snprintf(buffer,sizeof(buffer),"Hello 789ABCD");
   printf("BUFFER(%03u,%03u): %s\r\n",sizeof(buffer),val,buffer);
   val = snprintf(buffer,sizeof(buffer),"Hello 789ABCDE");
   printf("BUFFER(%03u,%03u): %s\r\n",sizeof(buffer),val,buffer);
   val = snprintf(buffer,sizeof(buffer),"Hello 789ABCDEF");
   printf("BUFFER(%03u,%03u): %s\r\n",sizeof(buffer),val,buffer);
   //Test exceeding the max.  First loop should print out results from
   //previous snprintf() call since buffer max is 0
   for(i=0; i<=sizeof(buffer); i++){
      val = snprintf(buffer,i,"Hello %d ABCDEFGHIJKLMNOP",78);
      printf("BUFFER(%03u,%03u): %s\r\n",i,val,buffer);

Expected Output:

*********Program Start*********
BUFFER(015,005): Hello zzzzzzzz
BUFFER(015,006): Hello
BUFFER(015,007): Hello 7
BUFFER(015,008): Hello 78
BUFFER(015,009): Hello 789
BUFFER(015,010): Hello 789A
BUFFER(015,011): Hello 789AB
BUFFER(015,012): Hello 789ABC
BUFFER(015,013): Hello 789ABCD
BUFFER(015,014): Hello 789ABCDE
BUFFER(015,015): Hello 789ABCDE
BUFFER(000,025): Hello 789ABCDE
BUFFER(002,025): H
BUFFER(003,025): He
BUFFER(004,025): Hel
BUFFER(005,025): Hell
BUFFER(006,025): Hello
BUFFER(007,025): Hello
BUFFER(008,025): Hello 7
BUFFER(009,025): Hello 78
BUFFER(010,025): Hello 78
BUFFER(011,025): Hello 78 A
BUFFER(012,025): Hello 78 AB
BUFFER(013,025): Hello 78 ABC
BUFFER(014,025): Hello 78 ABCD
BUFFER(015,025): Hello 78 ABCDE

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