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