sprintf或printf的最小实现

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

我正在研究一种速度至关重要的嵌入式DSP，而且内存很短。

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.

目前，sprintf使用我代码中任何函数的大部分资源。我只用它来格式化一些简单的文本：％d，％e，％f，％s，没有精确或奇异的操作。

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

如何实现更适合我的使用的基本sprintf或printf函数？

1 个解决方案

#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.

这个假定存在一个itoa将int转换为字符表示，并且fputs将字符串写出到你希望它去的地方。

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.

浮点输出至少在一个方面是不符合的：它没有像标准要求那样正确地进行舍入，所以如果你有（例如）内部存储为1.2399999774的值1.234，它将会打印为1.2399而不是1.2340。这节省了相当多的工作量，并且对于大多数典型用途而言仍然足够。

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).

除了你询问的转换之外，这还支持％c和％x，但是如果你想要删除它们，那么删除它们是非常简单的（这样做显然可以节省一些内存）。

#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] = '\0';
        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 = '\0';
}

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] = '\0';
        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\n", "Some string", 1, 0x1234);

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

    return 0;
}

#endif