Generate random number for atribute in struct

I have a struct called drug, and i want to generate random numbers for the atributes minimal_quantity and quantity. But every time i run the code, i get the same numbers on these two atributes, for all drugs that are generated.

My struct is defined this way:

struct drug{
    int code;
    char name[25];
    int minimal_quantity;
    int quantity;};

And the method for generating drugs is this:

load_drugs_file(){
     int i;

     for(i=0;i<=50;i++){

       if ((fp=fopen("drugs.dat","a+"))==NULL){
          printf("Error: impossible to open the file. \n");
          exit(1);
       }

    struct drug m;
    srand(time(NULL));
    int r1 = rand() % 500; /* random int between 0 and 499 */
    int r2 = rand() % 1000; /* random int between 0 and 999 */

    m.code=i;
    strcpy(m.name,"A");
    m.minimal_quantity=r1;
    m.quantity=r2;

    fwrite(&m,sizeof(m),1,fp);

    fclose(fp); 
    }}

Any suggestions?