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.Dd November 16, 2023
.Dt GETPEEREID 3
.Os
.Sh NAME
.Nm getpeereid
.Nd get the effective credentials of a UNIX-domain peer
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In unistd.h
.Ft int
.Fn getpeereid "int s" "uid_t *euid" "gid_t *egid"
.Sh DESCRIPTION
The
.Fn getpeereid
function returns the effective user and group IDs of the
peer connected to a
.Ux Ns -domain
socket.
The argument
.Fa s
must be a connected
.Ux Ns -domain
socket
.Pq Xr unix 4
of type
.Dv SOCK_STREAM
on which either
.Xr connect 2
or
.Xr listen 2
has been called.
The effective user ID is placed in
.Fa euid ,
and the effective group ID in
.Fa egid .
.Pp
The credentials returned to the
.Xr listen 2
caller are those of its peer at the time it called
.Xr connect 2 ;
the credentials returned to the
.Xr connect 2
caller are those of its peer at the time it called
.Xr listen 2 .
This mechanism is reliable; there is no way for either side to influence
the credentials returned to its peer except by calling the appropriate
system call (i.e., either
.Xr connect 2
or
.Xr listen 2 )
under different effective credentials.
.Pp
One common use of this routine is for a
.Ux Ns -domain
server
to verify the credentials of its client.
Likewise, the client can verify the credentials of the server.
.Sh IMPLEMENTATION NOTES
On
.Fx ,
.Fn getpeereid
is implemented in terms of the
.Dv LOCAL_PEERCRED
.Xr unix 4
socket option.
.Sh RETURN VALUES
.Rv -std getpeereid
.Sh ERRORS
The
.Fn getpeereid
function
fails if:
.Bl -tag -width Er
.It Bq Er EBADF
The argument
.Fa s
is not a valid descriptor.
.It Bq Er ENOTSOCK
The argument
.Fa s
is a file, not a socket.
.It Bq Er ENOTCONN
The argument
.Fa s
does not refer to a socket on which
.Xr connect 2
or
.Xr listen 2
have been called.
.It Bq Er EINVAL
The argument
.Fa s
does not refer to a socket of type
.Dv SOCK_STREAM ,
or the kernel returned invalid data.
.El
.Sh SEE ALSO
.Xr connect 2 ,
.Xr getpeername 2 ,
.Xr getsockname 2 ,
.Xr getsockopt 2 ,
.Xr listen 2 ,
.Xr unix 4
.Sh HISTORY
The
.Fn getpeereid
function appeared in
.Fx 4.6 .