What data structure is behind FD_SET and FD_ISSET when working with sockets in c?

Question

What data structure is behind FD_SET and FD_ISSET macro when working with sockets?

Answer 1

Prototypes:

void FD_SET(int fd, fd_set* fdset);
int FD_ISSET(int fd, fd_set* fdset);

From sys/select.h

typedef struct fd_set {
  u_int  fd_count;
  SOCKET fd_array[FD_SETSIZE];
} fd_set;