According to this article poll vs select vs event-based:
select() only uses (at maximum) three bits of data per file descriptor, while poll() typically uses 64 bits per file descriptor. In each syscall invoke poll() thus needs to copy a lot more over to kernel space. A small win for select().
Here is the implementation of fd_set (found on Advisories : multiple applications fd_set structure bitmap array index overflow
#ifndef FD_SETSIZE
#define FD_SETSIZE 1024
#endif
#define NBBY 8 /* number of bits in a byte */
typedef long fd_mask;
#define NFDBITS (sizeof (fd_mask) * NBBY) /* bits per mask */
#define howmany(x,y) (((x)+((y)-1))/(y))
typedef struct _types_fd_set {
fd_mask fds_bits[howmany(FD_SETSIZE, NFDBITS)];
} _types_fd_set;
#define fd_set _types_fd_set
So, in the end, fd_set is just an array of long. It is also written:
A call to FD_SET sets a bit to 1 using socket number as an index:
which means, if I got a socket fd number 5, the element indexed 5 will be seleted, and its first bit will be flipped from 0 to 1. Since select() uses 3 bits, I guess the other two bits are for sending and receive. Is this correct? Why does select() use a long when it only needs 3 bits?
Also, stated above, poll() uses 64 bits for checking. Why does poll need to check every bit in pollfd struct? Here is the pollfd struct:
struct pollfd {
int fd; // the socket descriptor
short events; // bitmap of events we're interested in
short revents; // when poll() returns, bitmap of events that occurred
};
The total bits in the struct is 64 bits, for a 32-bit int and two 16-bit short. I know the usual way of checking a bit flag is using AND (&) operator to filter out other irrelevant bits. Does that apply to this case?
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