Testing directory suing S_ISDIR,But it doesn't work

I'm studying c programming in linux,and I wrote this to output information about the files and directories like the standard tool "ls" with "-l",everything works fine except the macro S_ISDIR,Here is my code. Additionally,my os is mint 14 x86_64.

#include<sys/types.h>
#include<time.h>
#include<string.h>
#include<dirent.h>
#include<stdio.h>
#include<sys/stat.h>
#include<unistd.h>
void do_ls(char []);
void show_file_info(struct stat *t){
    printf("mode:    %o\n",t->st_mode);
    if(S_ISDIR(t->st_mode)==0)
        printf("Is a dir\n");
    else
        printf("Is not a dir\n");
    printf("links:   %d\n",t->st_nlink);
    printf("group:   %d\n",t->st_gid);
    printf("user:    %d\n",t->st_uid);
    printf("size:    %d\n",t->st_size);
    printf("modtime: %s\n",ctime(&t->st_mtime));
}
int main(int num,char *a[]){
    if(num==1){
        do_ls(".");
    }
    else{
        while(--num){
            printf("%s :\n",*++a);
            do_ls(*a);
        }
    }
}
void do_ls(char dirname[]){
    DIR *tem=opendir(dirname);
    struct dirent *direntp;
    struct stat *buf;
    char t[256];
    if(tem==NULL){
        fprintf(stderr,"ls: cannot open %s\n",dirname);
    }
    else{
        while((direntp=readdir(tem))!=NULL){
            strcpy(t,dirname);
            printf("%s\n",direntp->d_name);
            strcat(t,"/");
            if(stat(t,buf)==-1){
                perror("");
                break;
            }
            else{
                show_file_info(buf);
            }
        }
        closedir(tem);
    }
}

The corrections of user1198331 about stat was right. It is a good practice to generally check all syscalls return values in order to prevent errors.

Still, in your original code, I think this part was wrong:

if(S_ISDIR(t->st_mode)==0)
    printf("Is a dir\n");
else
    printf("Is not a dir\n");

You consider that if S_ISDIR(t->st_mode) returns 0, it is a directory, but actually, S_ISDIR(t->st_mode) returns 0 if the file pointed to by t is not a directory. You, thus, have to do the reverse check.