如何使用多个字符输出到文件

#define numeric_b '0'
#define numeric_e '9'
/** init string intervals ---*/
static char c0=numeric_b;
static char c1=numeric_b;
static char c2=numeric_b;
static char c3=numeric_b;
static char c4=numeric_b;
static char c5=numeric_b;
static char c6=numeric_b;
static char c7=numeric_b;
/** init start & end ----------------*/
static const char en = numeric_e +1;
static const char st = numeric_b +1;


void str_in(int length){
    FILE * fp = fopen("list.txt","w");

    switch(length){
        case 0:
            printf("%cn",c0);break;
        case 1:
            printf("%c%cn",c0,c1);break;
        case 2:
            printf("%c%c%cn",c0,c1,c2);break;
        case 3:
            printf("%c%c%c%cn",c0,c1,c2,c3);break;
        case 4:
            printf("%c%c%c%c%cn",c0,c1,c2,c3,c4);break;
        case 5:
            printf("%c%c%c%c%c%cn",c0,c1,c2,c3,c4,c5);break;
        case 6:
            printf("%c%c%c%c%c%c%cn",c0,c1,c2,c3,c4,c5,c6);break;
        case 7:
            printf("%c%c%c%c%c%c%c%cn",c0,c1,c2,c3,c4,c5,c6,c7);break;
    }

    fclose(fp);
}
void permute(int length){

    while(c0<=en){
        str_in(length);
        c0++;
        if(c0==en && length==0){break;}
        if(c0==en){
            c0=st;
            c1++;
            if(c1==en && length==1){break;}
            if(c1==en){
                c1=st;
                c2++;
                if(c2==en && length==2){break;}
                if(c2==en){
                    c2=st;
                    c3++;
                    if(c3==en && length==3){break;}
                    if(c3==en){
                        c3=st;
                        c4++;
                        if(c4==en && length==4){break;}
                        if(c4==en){
                            c4=st;
                            c5++;
                            if(c5==en && length==5){break;}
                            if(c5==en){
                                c5=st;
                                c6++;
                                if(c6==en && length==6){break;}
                                if(c6==en){
                                    c6=st;
                                    c7++;
                                    if(c7==en && length==7){break;}
                                }
                            }
                        }
                    }
                }
            }
        }
    }
}