Need help on padding hex and some conversions

Question

I will briefly explain what I want to do and help appreciated.

I have a hex number which is formatted as 16 byte number like this:

1: std::string myhex = "00000000000000000000000000000FFD";  

Then I want to convert it to int. Which I think I successfully do using this:

// convert hex to int
unsigned int x = strtoul(myhex.c_str(), NULL, 16);
printf("x = %d\n", x); // prints 4093 as needed

Now, I want to convert this integer back to hex. Which I think I also successfully do using this:

// Convert int back to hex
char buff[50];
string hexval;
sprintf(buff,"%x",x);
hexval = buff;
cout << hexval.c_str(); // prints "ffd".

But my problem is that now, I want to convert the "ffd" string as above back to the format it was before, eg, 16 byte number padded with zeros like this:

00000000000000000000000000000FFD

I want to convert the string not only print it.

Any help how to do this? Also any corrections if anything I was achieving above is wrong or not OK are welcome. Preferably I would like this to compile on Linux also.

Answer 1

Use the 0 flag (prefix) for zero-padding and field width specification in a printf :

printf("%032X", x);

Use snprintf to store it in your string:

snprintf(buff, sizeof(buff), "%032X", x);

Or use asprintf to store it in a newly-allocated string, to be certain that the memory available for the string is sufficient (since it's allocated by asprintf ):

char *as_string = NULL;
asprintf(&as_string, "%032X", x);