#define IADD   453806245
#define IMUL   314159269
#define MASK   2147483647
#define SCALE  0.4656612873e-9
int randSeedP = 17;
void InitRand (int randSeedI)
{
  struct timeval tv;
  if (randSeedI != 0) randSeedP = randSeedI;
  else {
    gettimeofday (&tv, 0);
    randSeedP = tv.tv_usec;
  }
}
real RandR ()
{
  randSeedP = (randSeedP * IMUL + IADD) & MASK;
  return (randSeedP * SCALE);
}
#if NDIM == 2                  //变量NDIM表示二维情形下
void VRand (VecR *p)
{
  real s;                        //自定义数据类型typedef double real
  s = 2. * M_PI * RandR ();
  p->x = cos (s);
  p->y = sin (s);
}
#elif NDIM == 3
void VRand (VecR *p)
{
  real s, x, y;
  s = 2.;
  while (s > 1.) {
    x = 2. * RandR () - 1.;
    y = 2. * RandR () - 1.;
    s = Sqr (x) + Sqr (y);
  }
  p->z = 1. - 2. * s;
  s = 2. * sqrt (1. - s);
  p->x = s * x;
  p->y = s * y;
}
#endif

函数可以放在main函数的外部，你那里面的处理函数就放在main函数的内部吧！你的程序不完整吧！struct timeval tv;这个编译不错去，你没有timeval的结构体就来定义变量，粗略给你改下#elif NDIM == 3这个表达不对吧！条件编译
#if表达式
程序段1
#else
程序段2
#endif
如果表达式为真就执行程序段1，否则执行程序段2，粗略给你改下：#include<stdio.h>
 #define IADD   453806245
 #define IMUL   314159269
 #define MASK   2147483647
 #define SCALE  0.4656612873e-9
 int randSeedP = 17;
 void InitRand (int randSeedI)
 {
   struct timeval tv;
   if (randSeedI != 0) randSeedP = randSeedI;
   else
   {
     gettimeofday (&tv, 0);
     randSeedP = tv.tv_usec;
   }
 }


 real RandR ()
 {
   randSeedP = (randSeedP * IMUL + IADD) & MASK;
   return (randSeedP * SCALE);
 }

 int main()
 {
 #if NDIM == 2                  //变量NDIM表示二维情形下
 void VRand (VecR *p)
 {
   real s;                        //自定义数据类型typedef double real
   s = 2. * M_PI * RandR ();
   p->x = cos (s);
   p->y = sin (s);
 }
 #else NDIM == 3
 void VRand (VecR *p)
 {
   real s, x, y;
   s = 2.;
   while (s > 1.) {
     x = 2. * RandR () - 1.;
     y = 2. * RandR () - 1.;
     s = Sqr (x) + Sqr (y);
   }
   p->z = 1. - 2. * s;
   s = 2. * sqrt (1. - s);
   p->x = s * x;
   p->y = s * y;
 }
 #endif
 return 0;
 }

嗯，不好意思，忘说清楚了：这个只是main函数中的一部分，目的应该是产生（0,1）之间的随机数。然后由VRand函数产生任意方向的单位向量。
这个是Unix平台上的代码，如果用<time.h>，那部分产生随机数的代码怎么改？

参考
https://bbs.bccn.net/thread-394709-1-1.html

我如果这样改，这部分源程序和原来的功能一样么？假定头文件都已包含在main主程序内
#define M_PI 3.1415926
real RandR ()
{
  int seed;
  double d;
  seed=time(0);
  srand((unsigned int)seed);
  d=rand()/(double)RAND_MAX;
  return d;
}
#if NDIM == 2
void VRand (VecR *p)
{
  real s;
  s = 2. * M_PI * RandR ();
  p->x = cos (s);
  p->y = sin (s);
}
#elif NDIM == 3
void VRand (VecR *p)
{
  real s, x, y;
  s = 2.;
  while (s > 1.) {
    x = 2. * RandR () - 1.;
    y = 2. * RandR () - 1.;
    s = Sqr (x) + Sqr (y);
  }
  p->z = 1. - 2. * s;
  s = 2. * sqrt (1. - s);
  p->x = s * x;
  p->y = s * y;
}
#endif