On success, zero is returned. On error, -1 is returned, and errno is set appropriately.
The argument s is not a valid descriptor.
The name parameter yiibais to memory not in a valid part of the process address space.
namelen is invalid (e.g., is negative).
Insufficient resources were available in the system to perform the operation.
The socket is not connected.
The argument s is a file, not a socket.
SVr4, 4.4BSD (the getpeername() function call first appeared in 4.2BSD), POSIX.1-2001.
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).