Looking at examples about socket programming, we can see that some people use
AF_INET while others use
PF_INET. In addition, sometimes both of them are used at the same example. The question is: Is there any difference between them? Which one should we use?
If you can answer that, another question would be... Why there are these two similar (but equal) constants?
What I've discovered, so far:
In (Unix) socket programming, we have the
socket() function that receives the following parameters:
int socket(int domain, int type, int protocol);
The manpage says:
domainargument specifies a communication domain; this selects the protocol family which will be used for communication. These families are defined in <sys/socket.h>.
And the manpage cites
AF_INET as well as some other
AF_ constants for the
domain parameter. Also, at the
NOTES section of the same manpage, we can read:
The manifest constants used under 4.x BSD for protocol families are PF_UNIX, PF_INET, etc., while AF_UNIX etc. are used for address families. However, already the BSD man page promises: "The protocol family generally is the same as the address family", and subsequent standards use AF_* everywhere.
The C headers
sys/socket.h does not actually define those constants, but instead includes
bits/socket.h. This file defines around 38
AF_ constants and 38
PF_ constants like this:
#define PF_INET 2 /* IP protocol family. */ #define AF_INET PF_INET
The Python socket module is very similar to the C API. However, there are many
AF_ constants but only one
PF_ constant (PF_PACKET). Thus, in Python we have no choice but use
I think this decision to include only the
AF_ constants follows one of the guiding principles: "There should be one-- and preferably only one --obvious way to do it." (The Zen of Python)