service to port number like getaddrinfo

Question

In C you can use a service name to get the port number when using getaddrinfo, like this:

getaddrinfo("stackoverflow.com", "http", &hints, &results);

This will assign 80 (big endian) to the first two bytes (uint16_t) of results->ai_addr.

Is there some easy way to get the port number from ?only? giving the service name? I need this for the server side, where I want to allow the user to specify either port number or service name for the port to bind and listen to.

@alk that is a good idea, but is there some standard C function for this? — Horse SMith, Nov 15 '14 at 17:15
How you want make a server listen on a "*service name*", btw? — alk, Nov 15 '14 at 17:19
@alk You are right, but it's from a rather common linux library to use when working with sockets. Is there something comparable? — Horse SMith, Nov 15 '14 at 17:19
@alk Bind a sockaddr, containing the port, to a socket, then listen. — Horse SMith, Nov 15 '14 at 17:21
`getaddrinfo()` is POSIX: http://pubs.opengroup.org/onlinepubs/9699919799/functions/freeaddrinfo.html — alk, Nov 15 '14 at 17:21
So what's the difference between "*port number*" and "*service name*" though? — alk, Nov 15 '14 at 17:23
@alk Service name is a char string and port number is big endian uint16_t. — Horse SMith, Nov 15 '14 at 17:28
I know, that's been the reason for the question in my 3rd comment. I get the impression there is a missunderstanding. — alk, Nov 15 '14 at 17:29
I got you now: "*... be specified ...*" is meant to be "*... be specified by the user ...*" and not "*... be specified to bind() ...*" — alk, Nov 15 '14 at 17:32
@alk I don't where my head was when I wrote that last sentence. I think I've fixed it now. lol — Horse SMith, Nov 15 '14 at 17:35
"*Is there something comparable?*" Where? In the C Standard? Comparable to what? — alk, Nov 15 '14 at 17:35
:-) The more precise the question, the more precices the anwers. — alk, Nov 15 '14 at 17:36
@alk A function in a library comparable, comparable as in as commonly used and as related as possible, to the one getaddrinfo is in. — Horse SMith, Nov 15 '14 at 17:37
@alk I said a standard c function, not the C standard library. Obviously bad wording on my part, but I meant a commonly used function/library that is pretty much the standard to use. — Horse SMith, Nov 15 '14 at 17:44

score 5 · Accepted Answer · answered Nov 15 '14 at 17:47

5

Using getaddrinfo with a null pointer for the hostname and AI_PASSIVE in the flags will give you a sockaddr you can bind on for running a server. See the documentation for getaddrinfo:

http://pubs.opengroup.org/onlinepubs/9699919799/functions/getaddrinfo.html

answered Nov 15 '14 at 17:47

R.. GitHub STOP HELPING ICE

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score 0 · Answer 2 · answered Nov 15 '14 at 17:36

0

man getservbyname

  #include \netdb.h>
   struct servent *getservent(void);
   struct servent *getservbyname(const char *name, const char *proto);
   struct servent *getservbyport(int port, const char *proto);

answered Nov 15 '14 at 17:36

helloV

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Still POSIX, 1+ however. – alk Nov 15 '14 at 17:40
@alk Do notice I never say in the question it needs to be from the standard C library. In fact, I mention getaddrinfo, which isn't in the standard C library. When I in the comment said some standard C function for this, I meant more as in a commonly used C function/library that is pretty much the standard to use, but I get the confusion. – Horse SMith Nov 15 '14 at 17:41
3

This is outdated/deprecated and non-thread-safe. The correct way is `getaddrinfo`. – R.. GitHub STOP HELPING ICE Nov 15 '14 at 17:45

service to port number like getaddrinfo

2 Answers2