How to make two-directional unix domain sockets with SOCK_DGRAM?

Question

I am trying to write a simple Unix datagram server/client, and am having some problems. What I want is a server that listens on a datagram socket and sends a reply to every message received, to the original sender. I decided to try first using socat to be the "server" and writing the client in C. I am running socat like this:

socat UNIX-DGRAM:/tmp/test.socket,fork EXEC:echo

To the best of my understanding this should listen on /tmp/test.socket and reply to everything that is received with the same string? Then I have a client program that looks like this (error checking removed for clarity):

int s = socket(AF_UNIX, SOCK_DGRAM, 0);
struct sockaddr_un sa;
sa.sun_family = AF_UNIX;
strcpy(sa.sun_path, "/tmp/test.socket");

const char *data = "Testing data";
int err = sendto(s, data, strlen(data), 0, (struct sockaddr *)(&sa), sizeof(struct sockaddr_un));

printf("Sent!\n");

unsigned char *buffer = malloc(BUFFER_LENGTH);
struct sockaddr_storage recv_sa;
int recv_sa_len = 0;
int recv_len = recvfrom(s, buffer, BUFFER_LENGTH, 0, (struct sockaddr *)&recv_sa, &recv_sa_len);

for (int i = 0; i < recv_len; i++) {
    putc(buffer[i], stdout);
}
printf("\n");

It should send the packet (that works), receive a packet, and then print it out, but the program doesn't seem to be capable of receiving the packet. What am I doing wrong here, or do I have a fundamental misunderstanding about Unix sockets? Thanks!

Answer 1

看看Michael Kerrisk的AF_UNIX SOCK_DGRAM客户端/服务器程序（ 客户端 ， 服务器 ）的示例，该示例发表在他的书“Linux编程接口”第57章中。

Answer 2

Socket need to have address to be addressed to receive packets. You can call bind() with unique socket filename based on the PID or in linux use autobound abstract address if to call before send :

bind(s, (const struct sockaddr *)&sa, sizeof(sa_family_t));

Answer 3

You should call connect before of trying send nothing to the echo server.

Take a look at: http://beej.us/guide/bgipc/output/html/multipage/unixsock.html