The C floating types match the IEC 60559 formats as follows:
— The float type matches the IEC 60559 single format.
— The double type matches the IEC 60559 double format.
— The long double type matches an IEC 60559 extended format,307) else a
non-IEC 60559 extended format, else the IEC 60559 double format.
Any non-IEC 60559 extended format used for the long double type shall have more
precision than IEC 60559 double and at least the range of IEC 60559 double.308)

float 32位 double 64位。对应IEEE标准的单精度和双精度浮点数。

long double按标准规定是80位，对应IEEE标准的扩展精度浮点数，但是不是所有编译器都符合标准。比如VC上long double就是double。有些编译器long double甚至是128位的。

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请帖主亲自打开include\float.h文件,然后看其具体内容。

你的意思是让我把所有的实现的这个文件都看一遍么？

既然你懒得看或者害怕看：
float.h

...
#define DBL_DIG         15                      /* # of decimal digits of precision */
#define DBL_EPSILON     2.2204460492503131e-016 /* smallest such that 1.0+DBL_EPSILON != 1.0 */
#define DBL_MANT_DIG    53                      /* # of bits in mantissa */
#define DBL_MAX         1.7976931348623158e+308 /* max value */
#define DBL_MAX_10_EXP  308                     /* max decimal exponent */
#define DBL_MAX_EXP     1024                    /* max binary exponent */
#define DBL_MIN         2.2250738585072014e-308 /* min positive value */
#define DBL_MIN_10_EXP  (-307)                  /* min decimal exponent */
#define DBL_MIN_EXP     (-1021)                 /* min binary exponent */
#define _DBL_RADIX      2                       /* exponent radix */
#define _DBL_ROUNDS     1                       /* addition rounding: near */

#define FLT_DIG         6                       /* # of decimal digits of precision */
#define FLT_EPSILON     1.192092896e-07F        /* smallest such that 1.0+FLT_EPSILON != 1.0 */
#define FLT_GUARD       0
#define FLT_MANT_DIG    24                      /* # of bits in mantissa */ 
 
#define FLT_MAX         3.402823466e+38F        /* max value */
#define FLT_MAX_10_EXP  38                      /* max decimal exponent */
#define FLT_MAX_EXP     128                     /* max binary exponent */
#define FLT_MIN         1.175494351e-38F        /* min positive value */
#define FLT_MIN_10_EXP  (-37)                   /* min decimal exponent */
#define FLT_MIN_EXP     (-125)                  /* min binary exponent */
#define FLT_NORMALIZE   0
#define FLT_RADIX       2                       /* exponent radix */
#define FLT_ROUNDS      1                       /* addition rounding: near */

#define LDBL_DIG        DBL_DIG                 /* # of decimal digits of precision */
#define LDBL_EPSILON    DBL_EPSILON             /* smallest such that 1.0+LDBL_EPSILON != 1.0 */
#define LDBL_MANT_DIG   DBL_MANT_DIG            /* # of bits in mantissa */
#define LDBL_MAX        DBL_MAX                 /* max value */
#define LDBL_MAX_10_EXP DBL_MAX_10_EXP          /* max decimal exponent */
#define LDBL_MAX_EXP    DBL_MAX_EXP             /* max binary exponent */
#define LDBL_MIN        DBL_MIN                 /* min positive value */ 
 
#define LDBL_MIN_10_EXP DBL_MIN_10_EXP          /* min decimal exponent */
#define LDBL_MIN_EXP    DBL_MIN_EXP             /* min binary exponent */
#define _LDBL_RADIX     DBL_RADIX               /* exponent radix */
#define _LDBL_ROUNDS    DBL_ROUNDS              /* addition rounding: near */
...

一个特定编译器的头文件有JB用？我说的是C++，不是某个编译器。