uintx_t to const char* in freestanding c++ using GNU compiler

Question

so I am trying to convert some integers in to character arrays that my terminal can write. so I can see the value of my codes calculations for debugging purposes when its running. as in if the int_t count = 57 I want the terminal to write 57. so char* would be an array of character of 5 and 7

The kicker here though is that this is in an freestanding environment so that means no standard c++ library. EDIT: this means No std::string, no c_str, no _tostring, I cant just print integers.

The headers I have access to are iso646,stddef,float,limits,stdint,stdalign, stdarg, stdbool and stdnoreturn

Ive tried a few things from casting the int as an const char*, witch just led to random characters being displayed. To feeding my compiler different headers from the GCC collection but they just keeped needing other headers that I continued feeding it until I did not know what header the compiler wanted.

so here is where the code needs to be used to be printed.

uint8_t count = 0;
while (true)
{
    terminal_setcolor(3);
    terminal_writestring("hello\n");

    count++;

    terminal_writestring((const char*)count);
    terminal_writestring("\n");
}

any advice with this would be greatly appreciated.

I am using an gnu, g++ cross compiler targeted at 686-elf and I guess I am using C++11 since I have access to stdnoreturn.h but it could be C++14 since I only just built the compiler with the latest gnu software dependencies.

You say "to const char*" but you didn't actually say what you want the result to look like. Do you want space-delimited decimal representations? Hexadecimal representations? ASCII-encoded character equivalents? What? — Lightness Races in Orbit
– Lightness Races in Orbit, Commented Jan 23, 2017 at 12:10
ASCII-encoded character equivalents if count = 50 i want 50 printed by the terminal — skyline
– skyline, Commented Jan 23, 2017 at 12:12
Well, that's the space-delimited decimal representation, not the ASCII option (which would, in your example, be the character P). Please add specific requirements to the question itself. — Lightness Races in Orbit
– Lightness Races in Orbit, Commented Jan 23, 2017 at 12:16
@skyline Your code doesnt do justice to the question. If you only want to write to terminal why cant you just print integeres? — Rushikesh Deshpande
– Rushikesh Deshpande, Commented Jan 23, 2017 at 12:23

Rost · Accepted Answer · 2017-01-24 21:33:35Z

1

Without C/C++ Standard Library you have no options except writing conversion function manually, e.g.:

template <int N>
const char* uint_to_string(
    unsigned int val,
    char (&str)[N],
    unsigned int base = 10)
{
    static_assert(N > 1, "Buffer too small");
    static const char* const digits = "0123456789ABCDEF";

    if (base < 2 || base > 16) return nullptr;

    int i = N - 1;
    str[i] = 0;

    do
    {
        --i;
        str[i] = digits[val % base];
        val /= base;
    }
    while (val != 0 && i > 0);

    return val == 0 ? str + i : nullptr;
}

template <int N>
const char* int_to_string(
    int val,
    char (&str)[N],
    unsigned int base = 10)
{
    // Output as unsigned.
    if (val >= 0) return uint_to_string(val, str, base);

    // Output as binary representation if base is not decimal.
    if (base != 10) return uint_to_string(val, str, base);

    // Output signed decimal representation.
    const char* res = uint_to_string(-val, str, base);

    // Buffer has place for minus sign
    if (res > str) 
    {
        const auto i = res - str - 1;
        str[i] = '-';
        return str + i;
    }
    else return nullptr;
}

Usage:

char buf[100];
terminal_writestring(int_to_string(42, buf));      // Will print '42'
terminal_writestring(int_to_string(42, buf, 2));   // Will print '101010'
terminal_writestring(int_to_string(42, buf, 8));   // Will print '52'
terminal_writestring(int_to_string(42, buf, 16));  // Will print '2A'
terminal_writestring(int_to_string(-42, buf));     // Will print '-42'
terminal_writestring(int_to_string(-42, buf, 2));  // Will print '11111111111111111111111111010110'
terminal_writestring(int_to_string(-42, buf, 8));  // Will print '37777777726'
terminal_writestring(int_to_string(-42, buf, 16)); // Will print 'FFFFFFD6'

Live example: http://cpp.sh/5ras

edited Jan 24, 2017 at 21:33

answered Jan 23, 2017 at 12:40

Rost

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8 Comments

Lightness Races in Orbit Over a year ago

This is very nice!

skyline Over a year ago

This is Perfect. (except I have a feeling it might not work with negative numbers i am yet to test that though) at first I had to manually go through each line and comment in my own words what it dose before I finally worked it out what it did. however theres still one part I dont understand and that is int N and eavey thingthat goes with it (&str)[N], int i = N-1, static_assert(N > 1, "Buffer too small"); Im assuming that that is the part that splits 42 in to 4 and 2 but how I do not understand.

Rost Over a year ago

Yes, this function will not work with negatives, as the question mentioned uintX_t types as target. But it's easy to improve it so it will work with negatives as well.

Rost Over a year ago

char (&str)[N] means "param 'str' is reference to array of chars of length N". N is template parameter, so array of any length can be passed to function and N will be deducted automatically by compiler. Static assert checks that array is at least 2 chars long to keep valid zero-terminated string. int i = N-1 means we start filling that array from end,not from beginning (as we don't know final string length at start).

skyline Over a year ago

so dose that then mean that a buffer of 100 (witch btw i dont think ill have interges the size of an googol :p) would not be any slower then an buffer of 50 since the compiler automatically deducts the size of the array to N? also where dose N get its value? has to some how come from the length of val but val is an T how dose it get the length of an T value?

|

fzd · Accepted Answer · 2017-01-23 12:14:10Z

-2

You could declare a string and get the pointer to it :

std::string str = std::to_string(count);
str += "\n";
terminal_writestring(str.c_str());

answered Jan 23, 2017 at 12:14

fzd

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1 Comment

Lightness Races in Orbit Over a year ago

"The kicker here though is that this is in an freestanding environment so that means no standard c++ library. The headers I have access to are iso646,stddef,float,limits,stdint,stdalign, stdarg, stdbool and stdnoreturn"

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