Print a double as a decimal with a specified precision
Well, there's the obvious stuff, like whitespace and declaring variables at their first point of use. For example:

static void     print_decimal(double decimal, int precision)
{
    size_t  integer;

    while (precision--)
    {
        decimal *= 100.;
        integer = (int)decimal;


could usefully be rewritten as

static void print_decimal(double decimal, int precision)
{
    while (precision--) {
        decimal *= 100.;
        size_t integer = (int)decimal;


This spots a bug: you fail to handle negative inputs. If you want integer to hold a signed int, you should define it as int (not size_t); and vice versa, if you want it to hold an unsigned size_t, you should be casting (size_t)decimal (not (int)decimal).



    if (!precision  &amp;&amp; (integer % 10) &gt;= 5 &amp;&amp; decimal != (double)integer)
        integer += 10;


This line could use some comments. I imagine it has something to do with rounding?



    printf("%i", integer);


It's more idiomatic to use printf("%d", integer) to print a decimal integer in C (and C++). True, %i exists, but it's deservedly obscure.

(Also, because of the above bug where you made integer a size_t, down here you should be using %zu to print it. But once you make it an int, you can go back to using %d.)

Every compiler will warn you about the format-string mismatch between %i and size_t. Are you compiling with -Wall? (No.) Why not? (There's no reason not to.) So, start compiling with -Wall today! It'll find your bugs so that you don't have to.

Performance-wise... I conclude from context that you expect this line to only ever print one digit, is that right? That is, your code could benefit from some "design by contract":

    assert(0 &lt;= integer &amp;&amp; integer &lt;= 9);
    printf("%i", integer);


If the assertion is correct, then we can speed up the code by writing simply

    assert(0 &lt;= integer &amp;&amp; integer &lt;= 9);
    putchar('0' + integer);


However, by adding the assertion, we have made the code a little bit testable. We can write some simple tests — just loop over all the floating-point numbers in the world and see if any of them fail the assertion — and indeed it doesn't take long to find one that fails.

print_decimal(0.097, 2);


This prints

010


when it should actually have printed

10


according to my understanding of your intent.



Which reminds me: print_decimal is a weird name for this function, since it literally does not have the ability to print a decimal point! Even your example print_decimal(65.226, 4) actually prints 652260, not 65.2260. This seems problematic for your prospective users.



So in conclusion:


Make your code testable.
Test your code.
Compile your code with -Wall.