Checking if a port is listening

[Marc Schütz via Digitalmars-d-learn]

Looking at an strace of nmap, it seems it opens a bunch of 
sockets, puts them into non-blocking mode, calls connect on them 
(which will return EINPROGRESS), and then uses select(2) to wait 
for them (in a loop, until all have either been accepted or 
rejected). select(2) accepts a timeout value, so you can 
determine how long you want to wait.

Here's an excerpt:

...
socket(PF_INET, SOCK_STREAM, IPPROTO_TCP) = 50
fcntl(50, F_GETFL)                      = 0x2 (flags O_RDWR)
fcntl(50, F_SETFL, O_RDWR|O_NONBLOCK)   = 0
setsockopt(50, SOL_SOCKET, SO_LINGER, {onoff=1, linger=0}, 8) = 0
setsockopt(50, SOL_SOCKET, SO_BINDTODEVICE, [0], 4) = -1 EPERM 
(Operation not permitted)
setsockopt(50, SOL_IP, IP_TTL, [-1], 4) = 0
connect(50, {sa_family=AF_INET, sin_port=htons(32778), 
sin_addr=inet_addr("127.0.0.1")}, 16) = -1 EINPROGRESS (Operation 
now in progress)
socket(PF_INET, SOCK_STREAM, IPPROTO_TCP) = 51
fcntl(51, F_GETFL)                      = 0x2 (flags O_RDWR)
fcntl(51, F_SETFL, O_RDWR|O_NONBLOCK)   = 0
setsockopt(51, SOL_SOCKET, SO_LINGER, {onoff=1, linger=0}, 8) = 0
setsockopt(51, SOL_SOCKET, SO_BINDTODEVICE, [0], 4) = -1 EPERM 
(Operation not permitted)
setsockopt(51, SOL_IP, IP_TTL, [-1], 4) = 0
connect(51, {sa_family=AF_INET, sin_port=htons(1029), 
sin_addr=inet_addr("127.0.0.1")}, 16) = -1 EINPROGRESS (Operation 
now in progress)
socket(PF_INET, SOCK_STREAM, IPPROTO_TCP) = 52
fcntl(52, F_GETFL)                      = 0x2 (flags O_RDWR)
fcntl(52, F_SETFL, O_RDWR|O_NONBLOCK)   = 0
setsockopt(52, SOL_SOCKET, SO_LINGER, {onoff=1, linger=0}, 8) = 0
setsockopt(52, SOL_SOCKET, SO_BINDTODEVICE, [0], 4) = -1 EPERM 
(Operation not permitted)
setsockopt(52, SOL_IP, IP_TTL, [-1], 4) = 0
connect(52, {sa_family=AF_INET, sin_port=htons(2013), 
sin_addr=inet_addr("127.0.0.1")}, 16) = -1 EINPROGRESS (Operation 
now in progress)
select(53, [3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 
23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 
45 46 47 48 49 50 51 52], [3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 
18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 
40 41 42 43 44 45 46 47 48 49 50 51 52], [3 4 5 6 7 8 9 10 11 12 
13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 
35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52], {0, 0}) = 
100 (in [3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 
24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 
46 47 48 49 50 51 52], out [3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 
18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 
40 41 42 43 44 45 46 47 48 49 50 51 52], left {0, 0})
...

I'm pretty sure the setsockopt() calls aren't essential.