Are Unix Internet sockets files

filessocketunix-philosophy

I understand that "Everything is a file" is one of the major concepts of Unix, but sockets use different APIs that are provided by the kernel (like socket, sendto, recv, etc.), not like normal file system interfaces.

How does this "Everything is a file" apply here?

Best Answer

sockets use different APIs

That's not entirely true. There are some additional functions for use with sockets, but you can use, e.g., normal read() and write() on a socket fd.

how does this "Everything is a file" apply here?

In the sense that a file descriptor is involved.

If your definition of "file" is a discrete sequence of bytes stored in a filesystem, then not everything is a file. However, if your definition of file is more handle like -- a conduit for information, i.e., an I/O connection -- then "everything is a file" starts to make more sense. These things inevitably do involve sequences of bytes, but where they come from or go to may differ contextually.

It's not really intended literally, however. A daemon is not a file, a daemon is a process; but if you are doing IPC your method of relating to another process might well be mitigated by file style entities.

Related Solutions

Unix Socket – Who’s Got the Other End of This Socketpair

Since kernel 3.3, it is possible using ss or lsof-4.89 or above — see Stéphane Chazelas's answer.

In older versions, according to the author of lsof, it was impossible to find this out: the Linux kernel does not expose this information. Source: 2003 thread on comp.unix.admin.

The number shown in /proc/$pid/fd/$fd is the socket's inode number in the virtual socket filesystem. When you create a pipe or socket pair, each end successively receives an inode number. The numbers are attributed sequentially, so there is a high probability that the numbers differ by 1, but this is not guaranteed (either because the first socket was N and N+1 was already in use due to wrapping, or because some other thread was scheduled between the two inode allocations and that thread created some inodes too).

I checked the definition of socketpair in kernel 2.6.39, and the two ends of the socket are not correlated except by the type-specific socketpair method. For unix sockets, that's unix_socketpair in net/unix/af_unix.c.

Linux – Size of data that can be written to / read from sockets

net.core.rmem_max and net.core.wmem_max are your thing. You can examine their values with

# sysctl net.core.rmem_max

and set them with

# sysctl -w net.core.rmem_max=8388608

These are the socket buffer sizes, when receiving and sending, respectively. They have default values, too, - rmem_default and wmem_default.

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Unix Socket – Who’s Got the Other End of This Socketpair

Linux – Size of data that can be written to / read from sockets

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