sprintf(zz, "%x", 1234);

#define NUMBER_OF_DIGITS 32

void _ultoa(unsigned long value, char* string, unsigned char radix)
{
unsigned char index;
char buffer[NUMBER_OF_DIGITS];  /* space for NUMBER_OF_DIGITS + '\0' */

  index = NUMBER_OF_DIGITS;

  do {
    buffer[--index] = '0' + (value % radix);
    if ( buffer[index] > '9') buffer[index] += 'A' - '9' - 1;
    value /= radix;
  } while (value != 0);

  do {
    *string++ = buffer[index++];
  } while ( index < NUMBER_OF_DIGITS );

  *string = 0;  /* string terminator */
}

void _ltoa(long value, char* string, unsigned char radix)
{
  if (value < 0 && radix == 10) {
    *string++ = '-';
    _ultoa(-value, string, radix);
  }
  else {
    _ultoa(value, string, radix);
  }
}

// ./sources/utoa.c из picc18
#include    <stdlib.h>
    
char *
ltoa(char * buf, long val, int base)
{
    char *    cp = buf;

    if(val < 0) {
        *buf++ = '-';
        val = -val;
    }
    ultoa(buf, val, base);
    return cp;
}
char *
ultoa(char * buf, unsigned long val, int base)
{
    unsigned    long    v;
    char        c;

    v = val;
    do {
        v /= base;
        buf++;
    } while(v != 0);
    *buf-- = 0;
    do {
        c = val % base;
        val /= base;
        if(c >= 10)
            c += 'A'-'0'-10;
        c += '0';
        *buf-- = c;
    } while(val != 0);
    return buf;
}

   c = val % base;
   val /= base;

C:\picc\std\9.60\lib>..\bin\libr s pic40a-c.lib *
sqrt.obj                        U ?_ldexp
                                U ?a_sqrt
                                U ?_frexp
                                U awdiv
                                D _sqrt
                                U ftadd
                                U ftcmp
                                U ftdiv
                                D __end_of_sqrt
                                U ?_sqrt
                                U _ldexp
                                U _frexp
                                U _errno
                                U used_btemp0
                                U used_btemp1
                                U used_btemp2
                                U used_btemp3
                                U used_btemp4
                                U used_btemp5
strtok.obj                      U ?_strtok
                                U ?_strspn
                                U ?a_strtok
                                U ?_strcspn
                                D _strtok
                                U _strspn
                                U _strcspn
                                D __end_of_strtok
                                U used_btemp0
picrt65x.obj                    U __Bdata
                                U __Hdata
                                U __Ldata
                                U _main
                                D _exit
                                D start
                                U powerup
                                U __Hbss
                                U __Lbss
picrt66x.obj                    U _main
                                D _exit
                                D start
                                U powerup
                                D intlevel0
                                D intlevel1

c:\picc\std\9.60\lib>..\bin\libr s pic40a-c.lib *