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###Use variable width format specifier###

When using sprintf, you can use a variable width specifier, and also a "pad with zero" specifier, so that you don't need to do these things manually:

void make_hex_string_easy(unsigned int invokeid, char** xref)
{
 int len = num_hex_digits(invokeid);
 /* if odd number, add 1 - zero pad number */
 if(len % 2 != 0)
 len++;
 sprintf(*xref, "%0*X", len, invokeid);
}

Here, after the % character, the 0 tells sprintf to pad the hex value with zeros. For example:

printf("%08x", 0x1234); // Will print "00001234" due to the %08x specifier

After the 0 there is a * character. This tells sprintf that the width will come from a variable instead of being hardcoded. For example:

printf("%0*x", 6, 0x1234); // Will print "001234" with a width of 6

So putting it all together, "%0*X" prints a zero padded hex string whose width comes from the next argument. In the modified code above, that argument is len.

Use variable width format specifier

When using sprintf, you can use a variable width specifier, and also a "pad with zero" specifier, so that you don't need to do these things manually:

void make_hex_string_easy(unsigned int invokeid, char** xref)
{
 int len = num_hex_digits(invokeid);
 /* if odd number, add 1 - zero pad number */
 if(len % 2 != 0)
 len++;
 sprintf(*xref, "%0*X", len, invokeid);
}

Here, after the % character, the 0 tells sprintf to pad the hex value with zeros. For example:

printf("%08x", 0x1234); // Will print "00001234" due to the %08x specifier

After the 0 there is a * character. This tells sprintf that the width will come from a variable instead of being hardcoded. For example:

printf("%0*x", 6, 0x1234); // Will print "001234" with a width of 6

So putting it all together, "%0*X" prints a zero padded hex string whose width comes from the next argument. In the modified code above, that argument is len.