#include <sys/socket.h>

       int getpeername(int sockfd, struct sockaddr *addr, socklen_t *addrlen);

       getpeername()  returns  the address of the peer connected to the socket
       sockfd, in the buffer pointed to by addr.  The addrlen argument  should
       be  initialized to indicate the amount of space pointed to by addr.  On
       return it contains the actual size of the  name  returned  (in  bytes).
       The name is truncated if the buffer provided is too small.

       The  returned address is truncated if the buffer provided is too small;
       in this case, addrlen will return a value greater than was supplied  to
       the call.

       On  success,  zero is returned.  On error, -1 is returned, and errno is
       set appropriately.

       EBADF  The argument sockfd is not a valid descriptor.

       EFAULT The addr argument points to memory not in a valid  part  of  the
              process address space.

       EINVAL addrlen is invalid (e.g., is negative).

              Insufficient  resources  were available in the system to perform
              the operation.

              The socket is not connected.

              The file descriptor sockfd does not refer to a socket.

       POSIX.1-2001, POSIX.1-2008, SVr4, 4.4BSD (getpeername() first  appeared
       in 4.2BSD).

       The third argument of getpeername() is in reality an int * (and this is
       what 4.x BSD and libc4 and libc5 have).  Some POSIX confusion  resulted
       in the present socklen_t, also used by glibc.  See also accept(2).

       For stream sockets, once a connect(2) has been performed, either socket
       can call getpeername() to obtain the address of the  peer  socket.   On
       the  other  hand,  datagram  sockets  are connectionless.  Calling con-
       nect(2) on a datagram socket merely sets the peer address for  outgoing
       datagrams  sent with write(2) or recv(2).  The caller of connect(2) can
       use getpeername() to obtain the peer address that it  earlier  set  for
       the  socket.   However, the peer socket is unaware of this information,
       and calling getpeername() on the peer  socket  will  return  no  useful

Linux                             2015-12-28                    GETPEERNAME(2)
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