Process’s father after setpgid(0,0);

The new process will be created within the fork() call, and will start by returning from it just like the parent. The return value (which you stored in retval) from fork() will be:

• 0 in the child process
• The PID of the child in the parent process
• -1 in the parent if there was a failure (there is no child, naturally)

Your testing code works correctly; it stores the return value from fork() in child_pid and uses if to check if it's 0 or not (although it doesn't check for an error)

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.