How to use directories system calls in C

directory.c

#include <stdio.h>
#include <dirent.h>
#include <unistd.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <string.h>

int OUTPUT_FILE;

void write_to_output(char * path, char * type) {
    struct stat *buf = (struct stat *) malloc(sizeof(struct stat));
    size_t written = 0;

    char * str_buf = (char *) malloc(100);

    if ( -1 == stat(path, buf) ) {
        fprintf(stderr, "Error opening stat for file: %s\n", path);
        perror("err");
        return;
    }

    write(OUTPUT_FILE, path, strlen(path));
    write(OUTPUT_FILE, "\n", 1);
    
    write(OUTPUT_FILE, type, strlen(type));
    write(OUTPUT_FILE, "\n", 1);

    written = sprintf(str_buf, "%lld\n", buf->st_size);
    write(OUTPUT_FILE, str_buf, written);

    write(OUTPUT_FILE, "\n", 1);

    free(str_buf);
    free(buf);
}

void scan_directory(char * directory_name) {
    DIR * directory = NULL;
    struct dirent * directory_entry = NULL;

    if (NULL == (directory = opendir(directory_name)) ) {
        fprintf(stderr, "Directory open failed %s\n", directory_name);
        return;
    }

    while ( (directory_entry = readdir(directory)) ) {
        if (NULL == directory_entry) {
            break;
        }

        char * new_dir_path = (char *) malloc(100);
        unsigned char directory_type = directory_entry->d_type; // DT_REG, DT_DIR

        if ( DT_REG == directory_type ) {
            printf("Regular file %s\n", directory_entry->d_name);

            // Write to output file
            sprintf(new_dir_path, "%s/%s", directory_name, directory_entry->d_name);
            write_to_output(new_dir_path, "Regular File");

        } else if ( DT_DIR == directory_type ) {
            if ( (0 == strcmp(directory_entry->d_name, ".")) || (0 == strcmp(directory_entry->d_name, ".."))  ) {
                continue;
            }
            printf("Directory file %s\n", directory_entry->d_name);

            // Write to output file.
            write_to_output(directory_name, "Directory");

            // Call recursively...
            sprintf(new_dir_path, "%s/%s", directory_name, directory_entry->d_name);
            scan_directory(new_dir_path);
        }

        free(new_dir_path);
    }

    if (closedir(directory)) {
        perror("Error closing directory");
        exit(EXIT_FAILURE);
    }
    free(directory_entry);
}

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

    if (argc < 3) {
        fprintf(stderr, "Invalid usage, 2 arguments needed: directory, outputfile");
        return -1;
    }

    if ( -1 == (OUTPUT_FILE = open(argv[2], O_RDWR | O_CREAT, S_IXUSR | S_IWUSR | S_IRUSR | S_IRWXU))) {
        perror("Error opening output file.");
        exit(EXIT_FAILURE);
    }

    printf("DEBUG: scan_directory(%s)\n\n", argv[1]);
    scan_directory(argv[1]);

    if ( -1 == close(OUTPUT_FILE)) {
        perror("Error closing output file.");
        exit(EXIT_FAILURE);
    }

    return EXIT_SUCCESS;
}