What is the Flush Input Buffer setting used for in the Input

逻辑覆盖法是设计白盒测试用例的主要方法之一，通过对程序逻辑结构的遍历实现程序的覆盖。针对以下由C语言编写的程序，按要求回答问题。
int XOR(char * filename,unsigned long key){ FILE * input = NULL , *output =NULL; //i char * outfilename = NULL; int len = strlen(filename); unsigned char buffer; if( (filename[len-2] == '.')&& (filename[len-1] == 'c') ) { //2,3 outfilename = new char[len+1]; //4 strcpy(outfilename, filename); outfilename[len-2] = '\0'; } else{ //5 outfilename = new char[len+5]; strcpy(outfilename, filename); strncat(outfilename,".c",2); } input =fopen(filename,"rb"); if( input == NULL) { //6 cout ＜＜ "Error opening file " ＜＜ filename ＜＜ endl; //7 delete [] outfilename; outfilename = NULL; return 1; } output =fopen(outfilename,"wb"); if( output == NULL ) { //8 cout ＜＜ "Error creating output file " ＜＜ outfilename ＜＜ endl; //9 delete [] outfilename; outfilename = NULL; return 1; } while( ! feof(input) ) { //10 if(fread(&buffer,sizeof(unsigned char),1,input) != 1 ) { //11 if( ! feof(input) ) { //12 delete [] outfilename; //13 outfilename = NULL; fclose(input); fclose(output); return 1; } } else{ //14 buffer ^= key; fwrite(&buffer,sizeof(unsigned char),1,output); } } fclose(input); //15 fclose(output); delete [] outfilename; return 0; }
请给出满足100%DC(判定覆盖)所需的逻辑条件。