c语言输出保留12位有效数字？？？

#include int get_ncount(double num){ int ncount=0; while(num>=1) { num/=10; ncount++; } return ncount; }int main(){ double a=12.345678901234; double b=0.12345678901234; int n=get_ncount(a); printf("%*.*lf\n",n,12-n,