C / C++ Code:
// Converts a binary (internal) 32 bit value into a character string
// to output the binary representation
[src]
#include < stdio.h >
void Convert(int, char*);

int main()
{
int binary;
char string[33]; // assuming 32bit integers

printf("Enter a number: ");
scanf("%d", &binary);

Convert(binary, string);

printf("%s %d %Xh n", string, binary, binary);
return 0;
}

void Convert(int bin, char *str)
{
unsigned int mask; // used to check each individual bit, unsigned
// to alleviate sign extension problems

mask = 0x80000000; // Set only the high-end bit
while (mask) // Loop until MASK is empty
{
if (bin & mask) // test the masked bit
*str = '1'; // if true, value is 1
else
*str = '0'; // if false, value is 0
str++; // next character
mask >>= 1; // shift the mask 1 bit
}
*str = 0; // add the trailing null
}
[/src]
So conversions actually are necessary only when converting from character string to binary or vice versa. So to "convert from hex to decimal" implies you have a character string in hex and you want a character string in decimal -- 90% of the time not what is required. But if it is, first convert to internal binary then back to string.