c - Minimal implementation of sprintf or printf

Question

I'm working on an embedded DSP where speed is crucial, and memory is very short.

At the moment, sprintf uses the most resources of any function in my code. I only use it to format some simple text: %d, %e, %f, %s, nothing with precision or exotic manipulations.

How can I implement a basic sprintf or printf function that would be more suitable for my usage?

Answer 1

This one assumes the existence of an itoa to convert an int to character representation, and an fputs to write out a string to wherever you want it to go.

The floating point output is non-conforming in at least one respect: it makes no attempt at rounding correctly, as the standard requires, so if you have have (for example) a value of 1.234 that is internally stored as 1.2399999774, it'll be printed out as 1.2399 instead of 1.2340. This saves quite a bit of work, and remains sufficient for most typical purposes.

This also supports %c and %x in addition to the conversions you asked about, but they're pretty trivial to remove if you want to get rid of them (and doing so will obviously save a little memory).

#include <stdarg.h>
#include <stdio.h>
#include <string.h>
#include <windows.h>

static void ftoa_fixed(char *buffer, double value);
static void ftoa_sci(char *buffer, double value);

int my_vfprintf(FILE *file, char const *fmt, va_list arg) {

    int int_temp;
    char char_temp;
    char *string_temp;
    double double_temp;

    char ch;
    int length = 0;

    char buffer[512];

    while ( ch = *fmt++) {
        if ( '%' == ch ) {
            switch (ch = *fmt++) {
                /* %% - print out a single %    */
                case '%':
                    fputc('%', file);
                    length++;
                    break;

                /* %c: print out a character    */
                case 'c':
                    char_temp = va_arg(arg, int);
                    fputc(char_temp, file);
                    length++;
                    break;

                /* %s: print out a string       */
                case 's':
                    string_temp = va_arg(arg, char *);
                    fputs(string_temp, file);
                    length += strlen(string_temp);
                    break;

                /* %d: print out an int         */
                case 'd':
                    int_temp = va_arg(arg, int);
                    itoa(int_temp, buffer, 10);
                    fputs(buffer, file);
                    length += strlen(buffer);
                    break;

                /* %x: print out an int in hex  */
                case 'x':
                    int_temp = va_arg(arg, int);
                    itoa(int_temp, buffer, 16);
                    fputs(buffer, file);
                    length += strlen(buffer);
                    break;

                case 'f':
                    double_temp = va_arg(arg, double);
                    ftoa_fixed(buffer, double_temp);
                    fputs(buffer, file);
                    length += strlen(buffer);
                    break;

                case 'e':
                    double_temp = va_arg(arg, double);
                    ftoa_sci(buffer, double_temp);
                    fputs(buffer, file);
                    length += strlen(buffer);
                    break;
            }
        }
        else {
            putc(ch, file);
            length++;
        }
    }
    return length;
}

int normalize(double *val) {
    int exponent = 0;
    double value = *val;

    while (value >= 1.0) {
        value /= 10.0;
        ++exponent;
    }

    while (value < 0.1) {
        value *= 10.0;
        --exponent;
    }
    *val = value;
    return exponent;
}

static void ftoa_fixed(char *buffer, double value) {  
    /* carry out a fixed conversion of a double value to a string, with a precision of 5 decimal digits. 
     * Values with absolute values less than 0.000001 are rounded to 0.0
     * Note: this blindly assumes that the buffer will be large enough to hold the largest possible result.
     * The largest value we expect is an IEEE 754 double precision real, with maximum magnitude of approximately
     * e+308. The C standard requires an implementation to allow a single conversion to produce up to 512 
     * characters, so that's what we really expect as the buffer size.     
     */

    int exponent = 0;
    int places = 0;
    static const int width = 4;

    if (value == 0.0) {
        buffer[0] = '0';
        buffer[1] = '';
        return;
    }         

    if (value < 0.0) {
        *buffer++ = '-';
        value = -value;
    }

    exponent = normalize(&value);

    while (exponent > 0) {
        int digit = value * 10;
        *buffer++ = digit + '0';
        value = value * 10 - digit;
        ++places;
        --exponent;
    }

    if (places == 0)
        *buffer++ = '0';

    *buffer++ = '.';

    while (exponent < 0 && places < width) {
        *buffer++ = '0';
        --exponent;
        ++places;
    }

    while (places < width) {
        int digit = value * 10.0;
        *buffer++ = digit + '0';
        value = value * 10.0 - digit;
        ++places;
    }
    *buffer = '';
}

void ftoa_sci(char *buffer, double value) {
    int exponent = 0;
    int places = 0;
    static const int width = 4;

    if (value == 0.0) {
        buffer[0] = '0';
        buffer[1] = '';
        return;
    }

    if (value < 0.0) {
        *buffer++ = '-';
        value = -value;
    }

    exponent = normalize(&value);

    int digit = value * 10.0;
    *buffer++ = digit + '0';
    value = value * 10.0 - digit;
    --exponent;

    *buffer++ = '.';

    for (int i = 0; i < width; i++) {
        int digit = value * 10.0;
        *buffer++ = digit + '0';
        value = value * 10.0 - digit;
    }

    *buffer++ = 'e';
    itoa(exponent, buffer, 10);
}

int my_printf(char const *fmt, ...) {
    va_list arg;
    int length;

    va_start(arg, fmt);
    length = my_vfprintf(stdout, fmt, arg);
    va_end(arg);
    return length;
}

int my_fprintf(FILE *file, char const *fmt, ...) {
    va_list arg;
    int length;

    va_start(arg, fmt);
    length = my_vfprintf(file, fmt, arg);
    va_end(arg);
    return length;
}


#ifdef TEST 

int main() {

    float floats[] = { 0.0, 1.234e-10, 1.234e+10, -1.234e-10, -1.234e-10 };

    my_printf("%s, %d, %x
", "Some string", 1, 0x1234);

    for (int i = 0; i < sizeof(floats) / sizeof(floats[0]); i++)
        my_printf("%f, %e
", floats[i], floats[i]);

    return 0;
}

#endif