list_fds.c

// Source: https://oroboro.com/file-handle-leaks-server/

#define _POSIX_C_SOURCE 200112L
#include <unistd.h>
#include <fcntl.h>

int maxfd = sysconf(_SC_OPEN_MAX);
for (int fd = 3; fd < maxfd; fd++) {
	int fd_flags = fcntl(fd, F_GETFD);
	if (fd_flags < 0) {
		continue;
	}

	int fl_flags = fcntl(fd, F_GETFL);
	if (fl_flags < 0) {
		continue;
	}

	char buf[256];
	char path[256];
	sprintf(path, "/proc/self/fd/%d", fd);
	memset(&buf[0], 0, 256);
	ssize_t s = readlink(path, &buf[0], 256);
	if (s == -1)
	{
	printf("%d (%s): not available ", fd, path);
		continue;
	}
	printf("%d (%s): ", fd, buf);

	if (fd_flags & FD_CLOEXEC)  printf("cloexec ");

	// file status
	if (fl_flags & O_APPEND  )  printf("append ");
	if (fl_flags & O_NONBLOCK)  printf("nonblock ");

	// acc mode
	if (fl_flags & O_RDONLY  )  printf("read-only ");
	if (fl_flags & O_RDWR    )  printf("read-write ");
	if (fl_flags & O_WRONLY  )  printf("write-only ");

	if (fl_flags & O_DSYNC   )  printf("dsync ");
	if (fl_flags & O_RSYNC   )  printf("rsync ");
	if (fl_flags & O_SYNC    )  printf("sync ");

	struct flock fl;
	fl.l_type = F_WRLCK;
	fl.l_whence = 0;
	fl.l_start = 0;
	fl.l_len = 0;
	fcntl(fd, F_GETLK, &fl);
	if (fl.l_type != F_UNLCK)
	{
	if (fl.l_type == F_WRLCK)
		 printf("write-locked");
	else
		 printf("read-locked");
	printf("(pid: %d) ", fl.l_pid);
	}
	printf("\n");
}