// see proc(5) manpage for more information
#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <ctype.h>
#include <string.h>
#include <dirent.h>
#include <unistd.h>
#include <sys/types.h> // pid_t
#include <linux/kdev_t.h> // MAJOR() and MINOR() tty macros

#define __STR(n) #n
#define STR(n) __STR(n)
// Filename length of the process executable, truncated to 16 + nil terminator
// However, kernel processes have longer names than that.
#define _TASK_COMM_LEN 255
#define PROC_FOLDER "/proc"
#define INIT_PID 1
#define IS_KERNEL_PROC(p) (((p)->pid != INIT_PID && (p)->session_id == 0))

struct proc_t
{
	pid_t pid;
	pid_t parent_pid;
	gid_t group_id;
	id_t session_id;
	id_t tty;
	char *exe_name;
	char state;
	unsigned int flags;
};

void *xcalloc(size_t num, size_t size)
{
	void *ptr = calloc(num, size);
	if (ptr == NULL) {
		perror("xcalloc");
		//fcloseall();
		abort();
	}
	return ptr;
}

char *build_proc_path(char *buffer, const char *proc_pid, const char *dir)
{
	char *path = buffer; //xcalloc(256, sizeof(char));
	strcpy(path, PROC_FOLDER"/");
	strncat(path, proc_pid, 255 - 5); // always leave space for /stat
	char proc_dir[255] = {0};
	snprintf(proc_dir, sizeof(proc_dir), "/%s", dir);
	strcat(path, proc_dir);
	return path;
}

FILE *process_open(char *path)
{
	FILE *proc_fp = fopen(path, "rb");
	if (proc_fp == NULL) {
		perror("fopen");
		exit(EXIT_FAILURE);
	}
	return proc_fp;
}

int parse_proc(FILE *proc_fp, struct proc_t *p)
{
#define NMATCHES 7 // <- increment each time a format directive (that is bound to a reference) is added
	p->exe_name = malloc(_TASK_COMM_LEN * sizeof(char));
	int ret = fscanf(
		proc_fp,
		"%d %*[(]%"STR(_TASK_COMM_LEN)"[^)]%*[)] %c %d %d %d %u",
		&p->pid, p->exe_name, &p->state, &p->parent_pid, &p->session_id, &p->tty,
		&p->flags
	);
	if (ret != NMATCHES || (ret == EOF && ferror(proc_fp))) {
		fprintf(stderr, "couldn't parse stat info\n");
		return 1;
	}
	return 0;
}

int is_numeric_str(char *str)
{
	while (*str)
		if (!isdigit(*str++))
			return 0;
	return 1;
}

void list_procs_init(struct proc_t **proc_list, int proc_list_cnt, int offset)
{
	for (int i = offset; i < proc_list_cnt; ++i) {
		proc_list[i] = xcalloc(1, sizeof(struct proc_t));
	}
}

struct proc_t **list_procs()
{
	int proc_list_it = 0;
	int proc_list_cnt = 255;
	struct proc_t **proc_list = xcalloc(proc_list_cnt + 1, sizeof(**proc_list));
	list_procs_init(proc_list, proc_list_cnt, 0);

	char path_buf[255] = {0};
	DIR *proc_dir = opendir(PROC_FOLDER);
	if (proc_dir == NULL) {
		perror("opendir");
		exit(EXIT_FAILURE);
	}
	struct dirent *dir;
	while ((dir = readdir(proc_dir)) != NULL) {
		if (dir->d_type == DT_DIR && is_numeric_str(dir->d_name)) {
			if ((proc_list_it + 1) > proc_list_cnt) {
				proc_list_cnt += proc_list_cnt;
				proc_list = realloc(proc_list, (proc_list_cnt + 1) * sizeof(**proc_list));
				list_procs_init(proc_list, proc_list_cnt, proc_list_it);
			}
			FILE *pf = process_open(build_proc_path(path_buf, dir->d_name, "stat"));
			parse_proc(pf, proc_list[proc_list_it++]);
			fclose(pf);
		}
	}
	closedir(proc_dir);
	return proc_list;
}

void print_proc_info(struct proc_t *proc)
{
	char *is_kernel_proc = IS_KERNEL_PROC(proc) ? "yes" : "no";
	if (IS_KERNEL_PROC(proc)) // not init neither a child of it
		printf("process %s (PID: %d) is a kernel process\n", proc->exe_name, proc->pid);
	printf("PID: %d (parent: %d, session id: %d) Name: %s Status: %c TTY: %d"
		" flags: 0x%08x\n",
		proc->pid, proc->parent_pid, proc->session_id, proc->exe_name, proc->state, MAJOR(proc->tty), proc->flags);
}

void backtrace_to_init(struct proc_t *proc)
{
	if (proc->parent_pid == 0)
		return;
	// pid == group pid for parent processes
	while (proc->pid != INIT_PID && proc->parent_pid != 0) {
		char pid_str[32];
		char proc_path[255] = {0};
		snprintf(pid_str, sizeof(pid_str), "%d", proc->parent_pid);
		FILE *pf = process_open(build_proc_path(proc_path, pid_str, "stat"));
		if (parse_proc(pf, proc))
			break;
		print_proc_info(proc);
		fclose(pf);
		free(proc->exe_name);
		proc->exe_name = NULL;
	}
}

int main(int argc, char **argv)
{
	int c;
	int list_procs_flag = 0, kernel_procs_flag = 0;
	while ((c = getopt(argc, argv, "lkp:")) != -1) {
		switch (c) {
			case 'l': list_procs_flag = 1;
				break;
			case 'k': kernel_procs_flag = 1;
				break;
			case '?':
				fprintf(stderr, "usage: ./procread [-lk][-p PID]\n");
				return EXIT_FAILURE;
		}
	}

	if (list_procs_flag) {
		struct proc_t **proc, **proc_o;
		proc = proc_o = list_procs();
		while (*proc != NULL) {
			if ((*proc)->pid && (!IS_KERNEL_PROC(*proc) || kernel_procs_flag)) {
				print_proc_info(*proc);
			}
			free((*proc)->exe_name);
			free(*proc++);
		}
		free(proc_o);
	} else {
		FILE *proc_fp;
		char pid_str[24];
		char proc_path[255] = {0};
		if (optind >= argc) {
			snprintf(pid_str, sizeof(pid_str), "%d", getpid());
			proc_fp = process_open(build_proc_path(proc_path, pid_str, "stat"));
		} else {
			proc_fp = process_open(build_proc_path(proc_path, argv[optind], "stat"));
		}
		struct proc_t p = {};
		if (parse_proc(proc_fp, &p))
			return 1;
		print_proc_info(&p);
		free(p.exe_name);
		fclose(proc_fp);
		backtrace_to_init(&p);
	}

	return EXIT_SUCCESS;
}