#include <ucred.h> int getpeerucred(int fd, ucred_t **ucred);
The getpeerucred() function returns the credentials of the peer endpoint of a connection-oriented socket (SOCK_STREAM) or stream fd at the time the endpoint was created or the connection was established. A process that initiates a connection retrieves the credentials of its peer at the time the peer's endpoint was created. A process that listens for connections retrieves the credentials of the peer at the time the peer initiated the connection.
When successful, getpeerucred() stores the pointer to a freshly allocated ucred_t in the memory location pointed to by the ucred argument if that memory location contains the null pointer. If the memory location is non-null, it will reuse the existing ucred_t.
When ucred is no longer needed, a credential allocated by getpeerucred() should be freed with ucred_free(3C).
It is possible that all fields of the ucred_t are not available to all peer endpoints and all callers.
Upon successful completion, getpeerucred() returns 0. Otherwise, it returns −1 and errno is set to indicate the error.
The getpeerucred() function will fail if:
There is not enough memory available to allocate sufficient memory to hold the user credential. The application can try again later.
The fd argument is not a valid file descriptor.
The pointer location pointed to by the ucred_t ** argument points to an invalid, non-null address.
The socket is connected but the peer credentials are unknown.
The physical limits of the system are exceeded by the memory allocation needed to hold the user credential.
The socket or stream is not connected or the stream's peer is unknown.
This operation is not supported on this file descriptor.
See attributes(5) for descriptions of the following attributes:
The system currently supports both sides of connection endpoints for local AF_UNIX, AF_INET, and AF_INET6 sockets, /dev/tcp, /dev/ticots, and /dev/ticotsord XTI/TLI connections, and pipe file descriptors sent using I_SENDFD as a result of the open of a named pipe with the "connld" module pushed.