myfuser.c

#include <stdio.h>
#include <unistd.h>
#include <fcntl.h>
#include <dirent.h>
#include <string.h>
#include <stdbool.h>
#include <limits.h>
#include <stdlib.h>
#include <errno.h>

#define TASK_COMM_LEN 16

static bool is_numeric(const char *str)
{
	while (1) {
		char c = *str++;

		if (!c)
			return true;
		if (!('0' <= c && c <= '9'))
			return false;
	}
}

static bool pid_has_open_file(const char *pid, const char *file)
{
	bool has_the_file = false;
	char link[PATH_MAX];
	char buf[PATH_MAX];
	ssize_t ret;
	DIR *dir;

	snprintf(buf, sizeof(buf), "/proc/%s/fd", pid);
	dir = opendir(buf);
	if (!dir)
		return false;

	while (1) {
		struct dirent *d;
		const char *name;

		d = readdir(dir);
		if (!d)
			break;

		name = d->d_name;
		snprintf(buf, sizeof(buf), "/proc/%s/fd/%s", pid, name);
		ret = readlink(buf, link, sizeof(link));
		if (ret < 0)
			continue;

		link[ret] = '\0';
		if (!strcmp(file, link)) {
			has_the_file = true;
			break;
		}
	}

	closedir(dir);
	return has_the_file;
}

static int get_task_comm(const char *pid, char *comm)
{
	char buf[PATH_MAX];
	int ret;
	int fd;

	comm[0] = '\0';
	snprintf(buf, sizeof(buf), "/proc/%s/comm", pid);
	fd = open(buf, O_RDONLY);
	if (fd < 0)
		return -1;

	ret = read(fd, comm, TASK_COMM_LEN);
	if (ret < 0) {
		close(fd);
		return -1;
	}

	close(fd);
	comm[ret - 1] = '\0';
	return 0;
}

static int find_pid_open_file(const char *file)
{
	char comm[TASK_COMM_LEN + 1];
	char real_path[PATH_MAX];
	DIR *dir;

	if (access(file, F_OK)) {
		printf("File %s does not exist: %s\n", file, strerror(errno));
		return 1;
	}

	if (!realpath(file, real_path)) {
		printf("Cannot resolve the realpath %s: %s\n", file, strerror(errno));
		return 1;
	}

	dir = opendir("/proc");
	if (!dir) {
		printf("Cannot open /proc directory: %s\n", strerror(errno));
		return 1;
	}

	while (1) {
		struct dirent *d;
		const char *name;

		d = readdir(dir);
		if (!d)
			break;

		name = d->d_name;
		if (!is_numeric(name))
			continue;

		if (!pid_has_open_file(name, file))
			continue;

		get_task_comm(name, comm);
		printf("PID: %s; Comm: %s\n", name, comm);
	}

	closedir(dir);
	return 0;
}

static int do_fuser(int argc, char * const argv[])
{
	int i;

	for (i = 0; i < argc; i++)
		find_pid_open_file(argv[i]);

	return 0;
}

int main(int argc, char *argv[])
{
	if (argc < 2) {
		printf("Usage: %s [file1] [file2] ... [fileN]\n", argv[0]);
		return 1;
	}

	return do_fuser(argc - 1, argv + 1);
}