Listing filenames of huge number of file available in the Directory using C language

listdir.c

#define _GNU_SOURCE
#include <dirent.h>     /* Defines DT_* constants */
#include <fcntl.h>
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <string.h>
#include <sys/stat.h>
#include <sys/syscall.h>

#define handle_error(msg) \
        do { perror(msg); exit(EXIT_FAILURE); } while (0)

struct linux_dirent {
long           d_ino;
off_t          d_off;
unsigned short d_reclen;
char           d_name[];
};

#define BUF_SIZE 1024*1024*5

void listdir(char * dname,char *otype,int inward) {
    
    int fd, nread;
    //char buf[BUF_SIZE];
    char * buf = malloc(BUF_SIZE);
    struct linux_dirent *d;
    int bpos;
    char d_type;
    
    fd = open(dname != NULL ? dname : ".", O_RDONLY | O_DIRECTORY);
    
    if (fd == -1)
        handle_error("open");

    for ( ; ; ) {
    
        nread = syscall(SYS_getdents, fd, buf, BUF_SIZE);
        if (nread == -1)
            handle_error("getdents");
        if (nread == 0)
            break;
        
        for (bpos = 0; bpos < nread;) {
            
            d = (struct linux_dirent *) (buf + bpos);
            d_type = *(buf + bpos + d->d_reclen - 1);                   
            bpos += d->d_reclen;
            
            if(d->d_ino && d->d_ino > 0 && strcmp(".",d->d_name)!=0 && strcmp("..",d->d_name)!=0 ) {

                if( (d_type == DT_DIR || d_type == DT_LNK ) && ( strcmp("dirs",otype) ==0||strcmp("dirfiles",otype) ==0 ))

                    printf("%s/%s\n",dname,(char *) d->d_name);                                                   
                
                else if(d_type == DT_REG && ( strcmp("files",otype) ==0 || strcmp("dirfiles",otype) ==0) )

                    printf("%s/%s\n",dname,(char *) d->d_name);
                
                if( (d_type == DT_DIR || d_type == DT_LNK) && inward == 1 ) {

                    int dirname_len = strlen(dname);
                    char * subdir = calloc(1, PATH_MAX + 1);
                    strcat(subdir, dname);
                    strcat(subdir + dirname_len, "/");
                    strcat(subdir + dirname_len + 1, d->d_name);
                    listdir(subdir,otype,inward);
                    free(subdir);
                }
            }                           
        }
    }

    close(fd);
    free(buf);
}

int main (int argc, char *argv[]) {

    int opt = 0;
    char *dname = NULL;
    char *otype = NULL;
    int inward = 0;

    while ((opt = getopt(argc, argv, "d:t:i:")) != -1) {
        switch(opt) {
            case 'd':
                dname = optarg;
            break;
            case 't':
                otype = optarg;
            break;
            case 'i':                
                inward = atoi(optarg) == 1 ? atoi(optarg) : 0;
                //inward = atoi(optarg);
            break;            
            case '?':
                if (optopt == 'd')
                    dname =".";
                else if (optopt == 't') 
                    otype = "files";
                else {
                    printf("\nInvalid option received\n");
                    return 1;
                }
            break;
        }
    }
    listdir(dname,otype,inward);
    exit(EXIT_SUCCESS);
    return 0;
}