select函数源码阅读

fd_set结构体

#undef  __NFDBITS
/* It's easier to assume 8-bit bytes than to get CHAR_BIT.  */
#define __NFDBITS (8 * (int) sizeof (__fd_mask))
#define __FD_ELT(d) ((d) / __NFDBITS)
#define __FD_MASK(d)  ((__fd_mask) (1UL << ((d) % __NFDBITS)))

/* fd_set for select and pselect.  */
typedef struct
  {
   /* XPG4.2 requires this member name.  Otherwise avoid the name
       from the global namespace.  */
 #ifdef __USE_XOPEN
    __fd_mask fds_bits[__FD_SETSIZE / __NFDBITS];
# define __FDS_BITS(set) ((set)->fds_bits)
#else
    __fd_mask __fds_bits[__FD_SETSIZE / __NFDBITS];
# define __FDS_BITS(set) ((set)->__fds_bits)
#endif
  } fd_set;