Escrevendo no arquivo, em C

binwrite.c

#include <stdio.h>

int main(int argc, char **argv) {
   int i = 123;
   double d = 456789;
   FILE *file;
   file = fopen("teste.bin","wb");
   printf("gravando int com %d bytes\n",sizeof(int));
   fwrite(&i,sizeof(int),1,file);
   printf("gravando double com %d bytes\n",sizeof(double));
   fwrite(&d,sizeof(double),1,file);
   fclose(file);
   return 0;
}