4.1.1. 为什么printf无法输出浮点数
为了缩减代码体积。目前大部分SDK中，并没有支持%f格式化的输出。也就是并不会打印浮点数。为了处理这个问题，可有两个办法：
打印浮点数的内存。例如，
float a; printf("%x\n", (uint32_t *)&a);或者使用
put_float函数单独打印浮点数。参考实现如下：
#include <math.h>
#include <string.h>
#include <stdio.h>
#define PAD_RIGHT 1
#define PAD_ZERO 2
#define SIGN 4 // Unsigned/signed long
#define SPACE 8 // Space if plus
#define LEFT 16 // Left justified
#define SPECIAL 32 // 0x
#define LARGE 64 // Use 'ABCDEF' instead of 'abcdef'
#define PLUS 128 // Show plus
#define CVTBUFSIZE 128
static char *flt(char **str, double num, int size, int precision, char fmt, int flags);
static char *ecvtbuf(double arg, int ndigits, int *decpt, int *sign, char *buf);
static char *fcvtbuf(double arg, int ndigits, int *decpt, int *sign, char *buf);
void put_float(double fv)
{
if (__builtin_isnan(fv)) {
puts("nan");
} else if (__builtin_isinf(fv)) {
puts("inf");
} else {
flt((void *)0, fv, 10, 3, 'f', ' ' | SIGN);
}
}
extern void putbyte(char a);
static void printchar(char **str, int c)
{
if (str) {
**str = c;
++(*str);
} else {
putbyte(c);
}
}
static void cfltcvt(double value, char *buffer, char fmt, int precision)
{
int decpt, sign, exp, pos;
char *digits = (char *)0;
char cvtbuf[CVTBUFSIZE + 1];
int capexp = 0;
int magnitude;
if (fmt == 'G' || fmt == 'E') {
capexp = 1;
fmt += 'a' - 'A';
}
if (fmt == 'g') {
digits = ecvtbuf(value, precision, & decpt, & sign, cvtbuf);
magnitude = decpt - 1;
if (magnitude < - 4 || magnitude > precision - 1) {
fmt = 'e';
precision -= 1;
} else {
fmt = 'f';
precision -= decpt;
}
}
if (fmt == 'e') {
digits = ecvtbuf(value, precision + 1, & decpt, & sign, cvtbuf);
if (sign) {
* buffer ++ = '-';
}
* buffer ++ = * digits;
if (precision > 0) {
* buffer ++ = '.';
}
memcpy(buffer, digits + 1, precision);
buffer += precision;
* buffer ++ = capexp ? 'E' : 'e';
if (decpt == 0) {
if (value == 0.0) {
exp = 0;
} else {
exp = - 1;
}
} else {
exp = decpt - 1;
}
if (exp < 0) {
* buffer ++ = '-';
exp = - exp;
} else {
* buffer ++ = '+';
}
buffer[2] = (exp % 10) + '0';
exp = exp / 10;
buffer[1] = (exp % 10) + '0';
exp = exp / 10;
buffer[0] = (exp % 10) + '0';
buffer += 3;
} else if (fmt == 'f') {
digits = fcvtbuf(value, precision, & decpt, & sign, cvtbuf);
if (sign) {
* buffer ++ = '-';
}
if (* digits) {
if (decpt <= 0) {
* buffer ++ = '0';
* buffer ++ = '.';
for (pos = 0; pos < - decpt; pos ++) {
* buffer ++ = '0';
}
while (* digits) {
* buffer ++ = * digits ++;
}
} else {
pos = 0;
while (* digits) {
if (pos ++ == decpt) {
* buffer ++ = '.';
}
* buffer ++ = * digits ++;
}
}
} else {
* buffer ++ = '0';
if (precision > 0) {
* buffer ++ = '.';
for (pos = 0; pos < precision; pos ++) {
* buffer ++ = '0';
}
}
}
}
* buffer = '\0';
}
static void forcdecpt(char *buffer)
{
while (* buffer) {
if (* buffer == '.') {
return;
}
if (* buffer == 'e' || * buffer == 'E') {
break;
}
buffer ++;
}
if (* buffer) {
int n = strlen(buffer);
while (n > 0) {
buffer[n + 1] = buffer[n];
n --;
}
* buffer = '.';
} else {
* buffer ++ = '.';
* buffer = '\0';
}
}
static void cropzeros(char *buffer)
{
char *stop;
while (* buffer && * buffer != '.') {
buffer ++;
}
if (* buffer ++) {
while (* buffer && * buffer != 'e' && * buffer != 'E') {
buffer ++;
}
stop = buffer --;
while (* buffer == '0') {
buffer --;
}
if (* buffer == '.') {
buffer --;
}
while ((*++ buffer = * stop ++));
}
}
static char *flt(char **str, double num, int size, int precision, char fmt, int flags)
{
char tmp[80];
char c, sign;
int n, i;
if (flags & LEFT) {
flags &= ~ PAD_ZERO;
}
// Determine padding and sign char
c = (flags & PAD_ZERO) ? '0' : ' ';
sign = 0;
if (flags & SIGN) {
if (num < 0.0) {
sign = '-';
num = - num;
size --;
} else if (flags & PLUS) {
sign = '+';
size --;
} else if (flags & SPACE) {
sign = ' ';
size --;
}
}
// Compute the precision value
if (precision < 0) {
precision = 6; // Default precision: 6
} else if (precision == 0 && fmt == 'g') {
precision = 1; // ANSI specified
}
// Convert floating point number to text
cfltcvt(num, tmp, fmt, precision);
// '#' and precision == 0 means force a decimal point
if ((flags & SPECIAL) && precision == 0) {
forcdecpt(tmp);
}
// 'g' format means crop zero unless '#' given
if (fmt == 'g' && !(flags & SPECIAL)) {
cropzeros(tmp);
}
n = strlen(tmp);
// Output number with alignment and padding
size -= n;
if (!(flags & (PAD_ZERO | LEFT))) while (size -- > 0) {
printchar(str, ' '); //* str ++ = ' ';
}
if (sign) {
printchar(str, sign); //* str ++ = sign;
}
if (!(flags & LEFT)) while (size -- > 0) {
printchar(str, c); //* str ++ = c;
}
for (i = 0; i < n; i ++) {
printchar(str, tmp[i]); //* str ++ = tmp[i];
}
while (size -- > 0) {
printchar(str, ' '); //* str ++ = ' ';
}
return (char *)str;
}
static char *cvt(double arg, int ndigits, int *decpt, int *sign, char *buf, int eflag)
{
int r2;
double fi, fj;
char *p, * p1;
if (ndigits < 0) {
ndigits = 0;
}
if (ndigits >= CVTBUFSIZE - 1) {
ndigits = CVTBUFSIZE - 2;
}
r2 = 0;
* sign = 0;
p = & buf[0];
if (arg < 0) {
* sign = 1;
arg = - arg;
}
arg = modf(arg, & fi);
p1 = & buf[CVTBUFSIZE];
if (fi != 0) {
p1 = & buf[CVTBUFSIZE];
while (fi != 0) {
fj = modf(fi / 10, & fi);
*-- p1 = (int)((fj + .03) * 10) + '0';
r2 ++;
}
while (p1 < & buf[CVTBUFSIZE]) {
* p ++ = * p1 ++;
}
} else if (arg > 0) {
while ((fj = arg * 10) < 1) {
arg = fj;
r2 --;
}
}
p1 = & buf[ndigits];
if (eflag == 0) {
p1 += r2;
}
* decpt = r2;
if (p1 < & buf[0]) {
buf[0] = '\0';
return buf;
}
while (p <= p1 && p < & buf[CVTBUFSIZE]) {
arg *= 10;
arg = modf(arg, & fj);
* p ++ = (int) fj + '0';
}
if (p1 >= & buf[CVTBUFSIZE]) {
buf[CVTBUFSIZE - 1] = '\0';
return buf;
}
p = p1;
* p1 += 5;
while (* p1 > '9') {
* p1 = '0';
if (p1 > buf) {
++*-- p1;
} else {
* p1 = '1';
(* decpt)++;
if (eflag == 0) {
if (p > buf) {
* p = '0';
}
p ++;
}
}
}
* p = '\0';
return buf;
}
static char *ecvtbuf(double arg, int ndigits, int *decpt, int *sign, char *buf)
{
return cvt(arg, ndigits, decpt, sign, buf, 1);
}
static char *fcvtbuf(double arg, int ndigits, int *decpt, int *sign, char *buf)
{
return cvt(arg, ndigits, decpt, sign, buf, 0);
}