小白跪求大神解答，，，运行出现str!=NULL

#include<stdio.h>
#include<math.h>
#define pi 3.14159
#define l 0.8
void main()
{FILE *fp;
if(fp=fopen("D:DAT","w")==NULL)
{ printf("error");
exit (0);
}
{double T,P,Q,D4,Ta,Pa,Pcr,Tcr,Rt,T9,Ma9,R9,x[800],r[800],A,z,i,Rg=287,CP=1005,k=1.4,R=287;
int y=1;
T=1300;
P=506625;
Q=100;
D4=1000;
Pcr=pow(2/(k+1),k/(k-1))*P;
Tcr=(1-(k-1)/(k+1))*T;
A=(Q*Rg*Tcr)*(1/Pcr)*(1/pow(k*Rg*Tcr,0.5));
Rt=pow((A/pi),0.5);
for(i=0;i<=800;i++)
{z=i/1000;
x[y]=z;
r[y]=Rt/pow(1-(1-4*Rt*Rt/(D4*D4))*(1-x[y]*x[y]/(l*l))*(1-x[y]*x[y]/(l*l))/pow(1+x[y]*x[y]*x[y]/(3*l*l),3),0.5);
fprintf(fp,"%3.3f,%8.5f\n",x[y],r[y]);
y=y+1;}
}
fclose(fp);}