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.br socketpair (2).
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.\"
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.ss autobind feature
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if a
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.br bind (2)
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call specifies
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.i addrlen
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as
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.ir sizeof(sa_family_t) ,
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.\" i.e., sizeof(short)
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or the
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.br so_passcred
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socket option was specified for a socket that was
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not explicitly bound to an address,
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then the socket is autobound to an abstract address.
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the address consists of a null byte
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followed by 5 bytes in the character set
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.ir [0\-9a\-f] .
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thus, there is a limit of 2^20 autobind addresses.
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(from linux 2.1.15, when the autobind feature was added,
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8 bytes were used, and the limit was thus 2^32 autobind addresses.
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the change to 5 bytes came in linux 2.3.15.)
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.ss sockets api
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the following paragraphs describe domain-specific details and
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unsupported features of the sockets api for unix domain sockets on linux.
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.pp
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unix domain sockets do not support the transmission of
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out-of-band data (the
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.b msg_oob
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flag for
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.br send (2)
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and
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.br recv (2)).
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.pp
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the
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.br send (2)
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.b msg_more
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flag is not supported by unix domain sockets.
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.pp
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before linux 3.4,
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.\" commit 9f6f9af7694ede6314bed281eec74d588ba9474f
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the use of
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.b msg_trunc
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in the
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.i flags
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argument of
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.br recv (2)
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was not supported by unix domain sockets.
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.pp
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the
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.b so_sndbuf
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socket option does have an effect for unix domain sockets, but the
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.b so_rcvbuf
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option does not.
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for datagram sockets, the
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.b so_sndbuf
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value imposes an upper limit on the size of outgoing datagrams.
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this limit is calculated as the doubled (see
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.br socket (7))
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option value less 32 bytes used for overhead.
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.ss ancillary messages
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ancillary data is sent and received using
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.br sendmsg (2)
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and
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.br recvmsg (2).
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for historical reasons, the ancillary message types listed below
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are specified with a
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.b sol_socket
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type even though they are
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.b af_unix
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specific.
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to send them, set the
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.i cmsg_level
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field of the struct
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.i cmsghdr
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to
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.b sol_socket
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and the
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.i cmsg_type
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field to the type.
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for more information, see
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.br cmsg (3).
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.tp
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.b scm_rights
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send or receive a set of open file descriptors from another process.
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the data portion contains an integer array of the file descriptors.
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.ip
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commonly, this operation is referred to as "passing a file descriptor"
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to another process.
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however, more accurately,
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what is being passed is a reference to an open file description (see
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.br open (2)),
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and in the receiving process it is likely that a different
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file descriptor number will be used.
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semantically, this operation is equivalent to duplicating
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.rb ( dup (2))
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a file descriptor into the file descriptor table of another process.
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.ip
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if the buffer used to receive the ancillary data containing
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file descriptors is too small (or is absent),
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