Question

I’m trying to use UNIX sockets for inter-thread communication. The program is only intended to run on Linux. To avoid creating the socket files, I wanted to use “abstract” sockets, as documented in unix(7).

However, I don’t seem to be able to connect to these sockets. Everything works if I’m using “pathname” sockets, though.

Here is the code (I haven’t quoted any error handling, but it’s done):
thread#1:

int log_socket = socket(AF_LOCAL, SOCK_STREAM, 0);
struct sockaddr_un logaddr;
socklen_t sun_len = sizeof(struct sockaddr_un);
logaddr.sun_family = AF_UNIX;
logaddr.sun_path[0] = 0;
strcpy(logaddr.sun_path+1, "futurama");
bind(log_socket, &logaddr, sun_len);
listen(log_socket, 5);
accept(log_socket, &logaddr, &sun_len);
... // send - receive

thread#2:

struct sockaddr_un tolog;
int sock = socket(AF_LOCAL, SOCK_STREAM, 0);
tolog.sun_family = AF_UNIX;
tolog.sun_path[0] = 0;
strcpy(tolog.sun_path+1, "futurama");
connect(sock, (struct sockaddr*)&tolog, sizeof(struct sockaddr_un));

If all I do in the above code, is change the sun_path to not have leading \0, things work perfect.

strace output:

t1: socket(PF_FILE, SOCK_STREAM, 0)         = 0
t1: bind(0, {sa_family=AF_FILE, path=@"futurama"}, 110)
t1: listen(0, 5)
t2: socket(PF_FILE, SOCK_STREAM, 0) = 1
t2: connect(1, {sa_family=AF_FILE, path=@"futurama"}, 110 <unfinished ...>
t2: <... connect resumed> )     = -1 ECONNREFUSED (Connection refused)
t1: accept(0,  <unfinished ...>

I know that the connect comes before accept, that’s not an issue (I tried making sure that accept() is called before connect(), same result. Also, things are fine if the socket is “pathname” anyway).

Answer 1

While I was posting this question, and re-reading unix(7) man page, this wording caught my attention:

an abstract socket address is distinguished by the fact
that sun_path[0] is a null byte (’\0’). All of the remaining bytes
in sun_path define the “name” of the socket

So, if I bzero’ed the sun_path before filling in my name into it, things started to work. I figured that’s not necessarily straight-forward. Additionally, as rightfully pointed out by @davmac and @StoneThrow, the number of those “remaining bytes” can be reduced by specifying only enough length of the socket address structure to cover the bytes you want to consider as your address. One way to do that is to use SUN_LEN macro, however, the first byte of the sun_path will have to be set to !0, as SUN_LEN uses strlen.

elaboration

If sun_path[0] is \0, The kernel uses the entirety of the remainder of sun_path as the name of the socket, whether it’s \0-terminated or not, so all of that remainder counts. In my original code I would zero the first byte, and then strcpy() the socket name into the sun_path at position 1. Whatever gibberish that was in sun_path when the structure was allocated (especially likely to contain gibberish since it’s allocated on the stack), and was included in the length of the socket structure (as passed to the syscalls), counted as the name of the socket, and was different in bind() and connect().

IMHO, strace should fix the way it displays abstract socket names, and display all the sun_path bytes from 1 to whatever the structure length that was supplied, if sun_path[0] is 0