CODEWITHSUNDEEP
csocketsunixprocess
Levenlol
Levenlol

Reputation: 415

How to create a socket between 2 processes after fork-exec

i'm learning C and this is my problem: i need to connect 2 processes with a socket AF_UNIX. My question is: how can i achieve that ?

I've already tried something with poor results:

Can i just open a socket in parent and then pass the file descriptor with execl ? or should i just try something more "complicated" ?

EDIT: code this is P1.c

int main (){

  printf("Hello this is process 1\n");
int fd=open("./foo",O_RDWR);
int h=fork();
if(h==0)
{
    sleep(2);
    dup2(fd,0);//note we will be loosing standard input in p2
    execvp("./client",NULL);
}
else
{
    printf("This is from p1 process\n");
    write(fd,"buf",4);
                //do some process with p1
    printf("This is end of p1 process\n");
}

return 0;

}

this is P2.c

int main (int argc, char * argv[]){
int fd=atoi(argv[1]);      
char buf[1024];
int n=read(fd,buf,1024);
buf[n]='\0';
printf("This is from p2\n");
write(1,buf,strlen(buf));
    exit(EXIT_SUCCESS);

}

note: i wasn't trying to use a socket.

Upvotes: 2

Views: 2824

Answers (3)

jcoppens
jcoppens

Reputation: 5440

Here is a description of unix sockets and an example in code.

The above link seems dead, but the Wayback Machine remembers

You need to designate one of the forked processes as server, and the other as client. In the server, you have to wait for connections. In the client, you have to establish the communication.

In the link are examples of each service. Don't be alarmed by the length of the code - most of it is comment.

Note that if you just want 'local' communication between processes, you might want to look into IPC: fifos, shared memory, message passing. They are much easier to implement.

Upvotes: 4

Scott Milano
Scott Milano

Reputation: 51

So an important question is does a socket file descriptor live past the execvp? Googling that question got me here... According to what I googled the answer is yes.

https://linux.die.net/man/3/execve

So you could create a pair of sockets using socketpair, then ensure the child process knows the new socket exists via argc,argv.

int fdl[2];
socketpair(AF_UNIX,SOCK_STREAM,0,fdl);

If I'm wrong, simply create a pair of AF_UNIX sockets using the child's pid as part of the sun_path, since both child and parent know this number.

Upvotes: 0

David VomLehn
David VomLehn

Reputation: 1

Sockets are great for interprocess communication and I use them all the time, but assigning ports can be an issue because, well, an integer value is hardly a rich namespace. Named pipes/FIFOs are a useful option if the two processes communicating are running on the same node. They they live in a normal Linux filesystem with names that look just like any other file. Also, you can use the simpler open/read/write/close system calls. You can even use things like the echo and cat commands for writing to them and the cat commands to read from them. Use the mkfifo command to create a FIFO.

Upvotes: 0

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