Simple ls command implementation in C

ls.c

#include <stdio.h>
#include <dirent.h>
#include <string.h>

#define GREEN "\x1b[32m"
#define BLUE "\x1b[34m"
#define WHITE "\x1b[37m"

void Usage() {
    fprintf(stderr, "\nUsage: exec [OPTION]... [DIR]...\n");
    fprintf(stderr, "List DIR's (directory) contents\n");
    fprintf(stderr, "\nOptions\n-R\tlist subdirectories recursively\n");
    return;
}

void RecDir(char * path, int flag) {
    DIR * dp = opendir(path);
    if(!dp) {
        perror(path);
        return;
    }
    struct dirent * ep;
    char newdir[512];
    printf(BLUE "\n%s :\n" WHITE, path);
    while((ep = readdir(dp)))
        if(strncmp(ep->d_name, ".", 1))
            printf(GREEN "\t%s\n" WHITE, ep->d_name);
    closedir(dp);
    dp = opendir(path);
    while((ep = readdir(dp))) if(strncmp(ep->d_name, ".", 1)) {
        if(flag && ep->d_type == 4) {
            sprintf(newdir, "%s/%s", path, ep->d_name);
            RecDir(newdir, 1);
        }
    }
    closedir(dp);
}

int main(int argc, char ** argv) {
    switch(argc) {
    case 2:
        if(strcmp(argv[1], "-R") == 0) Usage();
        else RecDir(argv[1], 0);
        break;
    case 3:
        if(strcmp(argv[1], "-R") == 0) RecDir(argv[2], 1);
        else Usage();
        break;
    default: Usage();
    }
    return 0;
}

Usually on linux when I run ls, it prints the files in the “.” Directory. Is that not necessary for POSIX, or is there some other particular reason why this one prints out usage instructions instead?