getpeereid man page
getpeereid — get the effective credentials of a UNIX-domain peer
getpeereid(int s, uid_t *euid, gid_t *egid);
The getpeereid() function returns the effective user and group IDs of the peer connected to a UNIX-domain socket. The argument s must be a UNIX-domain socket (unix(4)) of type SOCK_STREAM on which either connect(2) or listen(2) have been called. The effective used ID is placed in euid, and the effective group ID in egid.
The credentials returned to the listen(2) caller are those of its peer at the time it called connect(2); the credentials returned to the connect(2) caller are those of its peer at the time it called listen(2). This mechanism is reliable; there is no way for either side to influence the credentials returned to its peer except by calling the appropriate system call (i.e., either connect(2) or listen(2)) under different effective credentials.
One common use of this routine is for a UNIX-domain server to verify the credentials of its client. Likewise, the client can verify the credentials of the server.
On FreeBSD, getpeereid() is implemented in terms of the LOCAL_PEERCRED unix(4) socket option.
The getpeereid() function returns the value 0 if successful; otherwise the value -1 is returned and the global variable errno is set to indicate the error.
The getpeereid() function fails if:
The argument s is not a valid descriptor.
The argument s is a file, not a socket.
The argument s does not refer to a socket on which connect(2) or listen(2) have been called.
The argument s does not refer to a socket of type SOCK_STREAM, or the kernel returned invalid data.
connect(2), getpeername(2), getsockname(2), getsockopt(2), listen(2), unix(4)
The getpeereid() function appeared in FreeBSD 4.6.