程序代码：

#include<stdio.h>
#include<stdlib.h>
#define MAX 40
int main()
{
    FILE *fp;
    char words[MAX];
    if((fp=fopen("55.txt","a+"))==NULL)
    {
        fprintf(stdout,"can't open \"xx.txt\"file.\n");
        exit(1);
    }
    puts("Enter words to add to the file:press the Enter");
    puts("Key at the beginning of a line to terminate.");
    while(gets(words)!=NULL&&words[0]!='\0')
        fprintf(fp,"s",words);
    puts("file contents: ");
    rewind(fp);
    while(fscanf(fp,"%s",words)==1)
        puts(words);
    if(fclose(fp)!=0)
        fprintf(stderr,"Don't close\n");
    return 0;
}