查查msdn：
函式名称：fabs
说明：
Calculates the absolute value of the floating-point argument.

Function prototype:
double fabs( double x );

回传值：
fabs returns the absolute value of its argument. There is no error return.

Example:

/* ABS.C: This program computes and displays
* the absolute values of several numbers.
*/

#include <stdio.h>
#include <math.h>
#include <stdlib.h>

void main( void )
{
  int   ix = -4, iy;
  long   lx = -41567L, ly;
  double dx = -3.141593, dy;

  iy = abs( ix );
  printf( "The absolute value of %d is %d\n", ix, iy);

  ly = labs( lx );
  printf( "The absolute value of %ld is %ld\n", lx, ly);

  dy = fabs( dx );
  printf( "The absolute value of %f is %f\n", dx, dy );
}

输出结果：

The absolute value of -4 is 4
The absolute value of -41567 is 41567
The absolute value of -3.141593 is 3.141593

是"绝对值"的意思
例如:fabs(-3.6) = 3.6

绝队值不是abs而已吗= =??

iy = abs( ix );
printf( "The absolute value of %d is %d\n", ix, iy);

ly = labs( lx );
printf( "The absolute value of %ld is %ld\n", lx, ly);

dy = fabs( dx );
printf( "The absolute value of %f is %f\n", dx, dy );

abs是int数字的绝对值
labs是long数字的绝对值
fabs是float数字的绝对值