Issue
I am trying to use the BlueZ HCI function:
int hci_open_dev(int dev_id) {...}
which internally tries to create a socket like this:
socket(AF_BLUETOOTH, SOCK_RAW | SOCK_CLOEXEC, BTPROTO_HCI);
I tried to understand the linux kernel code for socket() but feel lost.
Id like to know what exactly does it mean to create a socket for the given domain (AF_BLUETOOTH), data transmission type (SOCK_RAW) and protocol (BTPROTO_HCI).
The man page just states that it takes these params, creates a socket and returns a device descriptor.
But id like to understand what exactly happens and the exact kernel steps involved in creating a socket.
Solution
Here is a very broad description (hope that helps understanding the main scheme).
Kernel developers will probably be horrified...
A socket is common abstract interface for many different communication means.
It provides many generic operations, such as closing, sending/receiving data, setting/retrieving options, which can be used on almost any kind of socket.
Creating a socket implies specifying the exact properties of this communication means.
It's a bit like the instantiation of a concrete type implementing an interface.
These properties are first organised by protocol families; this is the first argument to the socket() call.
For example:
- PF_INET is used for communications relying on IPv4,
- PF_INET6 is used for communications relying on IPv6,
- PF_LOCAL is used for inter-process communication inside the system (kind of pipe),
- PF_NETLINK is used for communication with the OS kernel,
- PF_PACKET is used for direct communication with network interfaces,
- ... (there exist many of them)
Once a protocol family is chosen, you have to specify, which protocol you want to use amongst those which are provided by this family; this is the second argument to the socket() call.
For example:
- SOCK_DGRAM is used for UDP over IPv4 or IPv6, or distinct messages in PF_LOCAL,
- SOCK_STREAM is used for TCP over IPv4 or IPv6, or a continuous byte stream in PF_LOCAL,
- SOCK_RAW, accesses directly is the raw underlying protocol in the family if any (IPv4, or IPv6 for example),
- ... (each family can provide many on them)
Some protocols can accept some variants or some restrictions; this is the third argument to the socket() call.
Often 0 is sufficient, but for example we can find:
- PF_PACKET, SOCK_RAW, htons(ETH_P_ALL) to capture any kind of network packet received on a network interface,
- PF_PACKET, SOCK_RAW, htons(ETH_P_ARP) to capture only ARP frames,
When we ask for the creation of a socket with these three arguments, the operating system creates an internal resource associated with the socket handle which will be obtained.
Of course, the exact structure of this resource depends on the chosen family/protocol/variant, and it is associated to kernel callbacks which are specific to it.
Each time an operation in invoked on this socket (through a system call), the specific callback will be called.
Answered By - prog-fh Answer Checked By - Mildred Charles (WPSolving Admin)
