#include <stdio.h>
#include <stdlib.h>  /* exit() 函数 */

int main()
{
   char sentence[1000];
   FILE *fptr;

   fptr = fopen("nowcoder.txt", "w");
   if(fptr == NULL)
   {
      printf("Error!");
      exit(1);
   }

   printf("输入字符串:\n");
   fgets(sentence, (sizeof sentence / sizeof sentence[0]), stdin);

   fprintf(fptr,"%s", sentence);
   fclose(fptr);

   return 0;
}

输入字符串:
gonshiku.com

$ cat nowcoder.txt 
gongshiku.com