getpeername man page
getpeername — get name of connected peer socket
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.
The argument sockfd is not a valid file descriptor.
The addr argument points to memory not in a valid part of the process address space.
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).
For background on the socklen_t type, see 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 connect(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 information (unless a connect(2) call was also executed on the peer). Note also that the receiver of a datagram can obtain the address of the sender when using recvfrom(2).
accept(2), bind(2), getsockname(2), ip(7), socket(7), unix(7)
This page is part of release 4.13 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at https://www.kernel.org/doc/man-pages/.
explain(1), explain(3), explain_getpeername(3), explain_getpeername_or_die(3), getnameinfo(3), getpeereid(3), signal-safety(7), socket(2), socket(7), socketcall(2), syscalls(2), unix(7).