# unix

## NAME



           unix - sockets for local interprocess communication
    


## SYNOPSIS



           **#include <sys/socket.h>**
           **#include <sys/un.h>**
    
           _unix_socket_ **= socket(AF_UNIX, type, 0);**
           _error_ **= socketpair(AF_UNIX, type, 0, int ***_sv_**);**
    


## DESCRIPTION



           The **AF_UNIX**(also known as **AF_LOCAL**) socket family is used to
           communicate between processes on the same machine efficiently.
           Traditionally, UNIX domain sockets can be either unnamed, or bound
           to a filesystem pathname (marked as being of type socket).  Linux
           also supports an abstract namespace which is independent of the
           filesystem.
    
           Valid socket types in the UNIX domain are: **SOCK_STREAM** , for a
           stream-oriented socket; **SOCK_DGRAM** , for a datagram-oriented socket
           that preserves message boundaries (as on most UNIX
           implementations, UNIX domain datagram sockets are always reliable
           and don't reorder datagrams); and (since Linux 2.6.4)
           **SOCK_SEQPACKET** , for a sequenced-packet socket that is connection-
           oriented, preserves message boundaries, and delivers messages in
           the order that they were sent.
    
           UNIX domain sockets support passing file descriptors or process
           credentials to other processes using ancillary data.
    
       **Address format**
           A UNIX domain socket address is represented in the following
           structure:
    
               struct sockaddr_un {
                   sa_family_t sun_family;               /* AF_UNIX */
                   char        sun_path[108];            /* Pathname */
               };
    
           The _sun_family_ field always contains **AF_UNIX**.  On Linux, _sun_path_
           is 108 bytes in size; see also BUGS, below.
    
           Various system calls (for example, bind(2), connect(2), and
           sendto(2)) take a _sockaddr_un_ argument as input.  Some other
           system calls (for example, getsockname(2), getpeername(2),
           recvfrom(2), and accept(2)) return an argument of this type.
    
           Three types of address are distinguished in the _sockaddr_un_
           structure:
    
           pathname
                  a UNIX domain socket can be bound to a null-terminated
                  filesystem pathname using bind(2).  When the address of a
                  pathname socket is returned (by one of the system calls
                  noted above), its length is
    
                      offsetof(struct sockaddr_un, sun_path) + strlen(sun_path) + 1
    
                  and _sun_path_ contains the null-terminated pathname.  (On
                  Linux, the above **offsetof**() expression equates to the same
                  value as _sizeof(sa_family_t)_ , but some other
                  implementations include other fields before _sun_path_ , so
                  the **offsetof**() expression more portably describes the size
                  of the address structure.)
    
                  For further details of pathname sockets, see below.
    
           unnamed
                  A stream socket that has not been bound to a pathname using
                  bind(2) has no name.  Likewise, the two sockets created by
                  socketpair(2) are unnamed.  When the address of an unnamed
                  socket is returned, its length is _sizeof(sa_family_t)_ , and
                  _sun_path_ should not be inspected.
    
           abstract
                  an abstract socket address is distinguished (from a
                  pathname socket) by the fact that _sun_path[0]_ is a null
                  byte ('\0').  The socket's address in this namespace is
                  given by the additional bytes in _sun_path_ that are covered
                  by the specified length of the address structure.  (Null
                  bytes in the name have no special significance.)  The name
                  has no connection with filesystem pathnames.  When the
                  address of an abstract socket is returned, the returned
                  _addrlen_ is greater than _sizeof(sa_family_t)_ (i.e., greater
                  than 2), and the name of the socket is contained in the
                  first _(addrlen - sizeof(sa_family_t))_ bytes of _sun_path_.
    
       **Pathname sockets**
           When binding a socket to a pathname, a few rules should be
           observed for maximum portability and ease of coding:
    
           •  The pathname in _sun_path_ should be null-terminated.
    
           •  The length of the pathname, including the terminating null
              byte, should not exceed the size of _sun_path_.
    
           •  The _addrlen_ argument that describes the enclosing _sockaddr_un_
              structure should have a value of at least:
    
                  offsetof(struct sockaddr_un, sun_path)+strlen(addr.sun_path)+1
    
              or, more simply, _addrlen_ can be specified as _sizeof(struct_
              _sockaddr_un)_.
    
           There is some variation in how implementations handle UNIX domain
           socket addresses that do not follow the above rules.  For example,
           some (but not all) implementations append a null terminator if
           none is present in the supplied _sun_path_.
    
           When coding portable applications, keep in mind that some
           implementations have _sun_path_ as short as 92 bytes.
    
           Various system calls (accept(2), recvfrom(2), getsockname(2),
           getpeername(2)) return socket address structures.  When applied to
           UNIX domain sockets, the value-result _addrlen_ argument supplied to
           the call should be initialized as above.  Upon return, the
           argument is set to indicate the _actual_ size of the address
           structure.  The caller should check the value returned in this
           argument: if the output value exceeds the input value, then there
           is no guarantee that a null terminator is present in _sun_path_.
           (See BUGS.)
    
       **Pathname socket ownership and permissions**
           In the Linux implementation, pathname sockets honor the
           permissions of the directory they are in.  Creation of a new
           socket fails if the process does not have write and search
           (execute) permission on the directory in which the socket is
           created.
    
           On Linux, connecting to a stream socket object requires write
           permission on that socket; sending a datagram to a datagram socket
           likewise requires write permission on that socket.  POSIX does not
           make any statement about the effect of the permissions on a socket
           file, and on some systems (e.g., older BSDs), the socket
           permissions are ignored.  Portable programs should not rely on
           this feature for security.
    
           When creating a new socket, the owner and group of the socket file
           are set according to the usual rules.  The socket file has all
           permissions enabled, other than those that are turned off by the
           process umask(2).
    
           The owner, group, and permissions of a pathname socket can be
           changed (using chown(2) and chmod(2)).
    
       **Abstract sockets**
           Socket permissions have no meaning for abstract sockets: the
           process umask(2) has no effect when binding an abstract socket,
           and changing the ownership and permissions of the object (via
           fchown(2) and fchmod(2)) has no effect on the accessibility of the
           socket.
    
           Abstract sockets automatically disappear when all open references
           to the socket are closed.
    
           The abstract socket namespace is a nonportable Linux extension.
    
       **Socket options**
           For historical reasons, these socket options are specified with a
           **SOL_SOCKET** type even though they are **AF_UNIX** specific.  They can
           be set with setsockopt(2) and read with getsockopt(2) by
           specifying **SOL_SOCKET** as the socket family.
    
           **SO_PASSCRED**
                  Enabling this socket option causes receipt of the
                  credentials of the sending process in an **SCM_CREDENTIALS**
                  **ancillary** message in each subsequently received message.
                  The returned credentials are those specified by the sender
                  using **SCM_CREDENTIALS** , or a default that includes the
                  sender's PID, real user ID, and real group ID, if the
                  sender did not specify **SCM_CREDENTIALS** ancillary data.
    
                  When this option is set and the socket is not yet
                  connected, a unique name in the abstract namespace will be
                  generated automatically.
    
                  The value given as an argument to setsockopt(2) and
                  returned as the result of getsockopt(2) is an integer
                  boolean flag.
    
           **SO_PASSSEC**
                  Enables receiving of the SELinux security label of the peer
                  socket in an ancillary message of type **SCM_SECURITY**(see
                  below).
    
                  The value given as an argument to setsockopt(2) and
                  returned as the result of getsockopt(2) is an integer
                  boolean flag.
    
                  The **SO_PASSSEC** option is supported for UNIX domain datagram
                  sockets since Linux 2.6.18; support for UNIX domain stream
                  sockets was added in Linux 4.2.
    
           **SO_PEEK_OFF**
                  See socket(7).
    
           **SO_PEERCRED**
                  This read-only socket option returns the credentials of the
                  peer process connected to this socket.  The returned
                  credentials are those that were in effect at the time of
                  the call to connect(2), listen(2), or socketpair(2).
    
                  The argument to getsockopt(2) is a pointer to a _ucred_
                  structure; define the **_GNU_SOURCE** feature test macro to
                  obtain the definition of that structure from
                  _< sys/socket.h>_.
    
                  The use of this option is possible only for connected
                  **AF_UNIX** stream sockets and for **AF_UNIX** stream and datagram
                  socket pairs created using socketpair(2).
    
           **SO_PEERSEC**
                  This read-only socket option returns the security context
                  of the peer socket connected to this socket.  By default,
                  this will be the same as the security context of the
                  process that created the peer socket unless overridden by
                  the policy or by a process with the required permissions.
    
                  The argument to getsockopt(2) is a pointer to a buffer of
                  the specified length in bytes into which the security
                  context string will be copied.  If the buffer length is
                  less than the length of the security context string, then
                  getsockopt(2) returns -1, sets _errno_ to **ERANGE** , and returns
                  the required length via _optlen_.  The caller should allocate
                  at least **NAME_MAX** bytes for the buffer initially, although
                  this is not guaranteed to be sufficient.  Resizing the
                  buffer to the returned length and retrying may be
                  necessary.
    
                  The security context string may include a terminating null
                  character in the returned length, but is not guaranteed to
                  do so: a security context "foo" might be represented as
                  either {'f','o','o'} of length 3 or {'f','o','o','\0'} of
                  length 4, which are considered to be interchangeable.  The
                  string is printable, does not contain non-terminating null
                  characters, and is in an unspecified encoding (in
                  particular, it is not guaranteed to be ASCII or UTF-8).
    
                  The use of this option for sockets in the **AF_UNIX** address
                  family is supported since Linux 2.6.2 for connected stream
                  sockets, and since Linux 4.18 also for stream and datagram
                  socket pairs created using socketpair(2).
    
       **Autobind feature**
           If a bind(2) call specifies _addrlen_ as _sizeof(sa_family_t)_ , or the
           **SO_PASSCRED** socket option was specified for a socket that was not
           explicitly bound to an address, then the socket is autobound to an
           abstract address.  The address consists of a null byte followed by
           5 bytes in the character set _[0-9a-f]_.  Thus, there is a limit of
           2^20 autobind addresses.  (From Linux 2.1.15, when the autobind
           feature was added, 8 bytes were used, and the limit was thus 2^32
           autobind addresses.  The change to 5 bytes came in Linux 2.3.15.)
    
       **Sockets API**
           The following paragraphs describe domain-specific details and
           unsupported features of the sockets API for UNIX domain sockets on
           Linux.
    
           UNIX domain sockets do not support the transmission of out-of-band
           data (the **MSG_OOB** flag for send(2) and recv(2)).
    
           The send(2) **MSG_MORE** flag is not supported by UNIX domain sockets.
    
           Before Linux 3.4, the use of **MSG_TRUNC** in the _flags_ argument of
           recv(2) was not supported by UNIX domain sockets.
    
           The **SO_SNDBUF** socket option does have an effect for UNIX domain
           sockets, but the **SO_RCVBUF** option does not.  For datagram sockets,
           the **SO_SNDBUF** value imposes an upper limit on the size of outgoing
           datagrams.  This limit is calculated as the doubled (see
           socket(7)) option value less 32 bytes used for overhead.
    
       **Ancillary messages**
           Ancillary data is sent and received using sendmsg(2) and
           recvmsg(2).  For historical reasons, the ancillary message types
           listed below are specified with a **SOL_SOCKET** type even though they
           are **AF_UNIX** specific.  To send them, set the _cmsg_level_ field of
           the struct _cmsghdr_ to **SOL_SOCKET** and the _cmsg_type_ field to the
           type.  For more information, see cmsg(3).
    
           **SCM_RIGHTS**
                  Send or receive a set of open file descriptors from another
                  process.  The data portion contains an integer array of the
                  file descriptors.
    
                  Commonly, this operation is referred to as "passing a file
                  descriptor" to another process.  However, more accurately,
                  what is being passed is a reference to an open file
                  description (see open(2)), and in the receiving process it
                  is likely that a different file descriptor number will be
                  used.  Semantically, this operation is equivalent to
                  duplicating (dup(2)) a file descriptor into the file
                  descriptor table of another process.
    
                  If the buffer used to receive the ancillary data containing
                  file descriptors is too small (or is absent), then the
                  ancillary data is truncated (or discarded) and the excess
                  file descriptors are automatically closed in the receiving
                  process.
    
                  If the number of file descriptors received in the ancillary
                  data would cause the process to exceed its **RLIMIT_NOFILE**
                  resource limit (see getrlimit(2)), the excess file
                  descriptors are automatically closed in the receiving
                  process.
    
                  The kernel constant **SCM_MAX_FD** defines a limit on the
                  number of file descriptors in the array.  Attempting to
                  send an array larger than this limit causes sendmsg(2) to
                  fail with the error **EINVAL**.  **SCM_MAX_FD** has the value 253
                  (or 255 before Linux 2.6.38).
    
           **SCM_CREDENTIALS**
                  Send or receive UNIX credentials.  This can be used for
                  authentication.  The credentials are passed as a _struct_
                  _ucred_ ancillary message.  This structure is defined in
                  _< sys/socket.h>_ as follows:
    
                      struct ucred {
                          pid_t pid;    /* Process ID of the sending process */
                          uid_t uid;    /* User ID of the sending process */
                          gid_t gid;    /* Group ID of the sending process */
                      };
    
                  Since glibc 2.8, the **_GNU_SOURCE** feature test macro must be
                  defined (before including _any_ header files) in order to
                  obtain the definition of this structure.
    
                  The credentials which the sender specifies are checked by
                  the kernel.  A privileged process is allowed to specify
                  values that do not match its own.  The sender must specify
                  its own process ID (unless it has the capability
                  **CAP_SYS_ADMIN** , in which case the PID of any existing
                  process may be specified), its real user ID, effective user
                  ID, or saved set-user-ID (unless it has **CAP_SETUID**), and
                  its real group ID, effective group ID, or saved set-group-
                  ID (unless it has **CAP_SETGID**).
    
                  To receive a _struct ucred_ message, the **SO_PASSCRED** option
                  must be enabled on the socket.
    
           **SCM_SECURITY**
                  Receive the SELinux security context (the security label)
                  of the peer socket.  The received ancillary data is a null-
                  terminated string containing the security context.  The
                  receiver should allocate at least **NAME_MAX** bytes in the
                  data portion of the ancillary message for this data.
    
                  To receive the security context, the **SO_PASSSEC** option must
                  be enabled on the socket (see above).
    
           When sending ancillary data with sendmsg(2), only one item of each
           of the above types may be included in the sent message.
    
           At least one byte of real data should be sent when sending
           ancillary data.  On Linux, this is required to successfully send
           ancillary data over a UNIX domain stream socket.  When sending
           ancillary data over a UNIX domain datagram socket, it is not
           necessary on Linux to send any accompanying real data.  However,
           portable applications should also include at least one byte of
           real data when sending ancillary data over a datagram socket.
    
           When receiving from a stream socket, ancillary data forms a kind
           of barrier for the received data.  For example, suppose that the
           sender transmits as follows:
    
                  (1)  sendmsg(2) of four bytes, with no ancillary data.
                  (2)  sendmsg(2) of one byte, with ancillary data.
                  (3)  sendmsg(2) of four bytes, with no ancillary data.
    
           Suppose that the receiver now performs recvmsg(2) calls each with
           a buffer size of 20 bytes.  The first call will receive five bytes
           of data, along with the ancillary data sent by the second
           sendmsg(2) call.  The next call will receive the remaining four
           bytes of data.
    
           If the space allocated for receiving incoming ancillary data is
           too small then the ancillary data is truncated to the number of
           headers that will fit in the supplied buffer (or, in the case of
           an **SCM_RIGHTS** file descriptor list, the list of file descriptors
           may be truncated).  If no buffer is provided for incoming
           ancillary data (i.e., the _msg_control_ field of the _msghdr_
           structure supplied to recvmsg(2) is NULL), then the incoming
           ancillary data is discarded.  In both of these cases, the
           **MSG_CTRUNC** flag will be set in the _msg.msg_flags_ value returned by
           recvmsg(2).
    
       **Ioctls**
           The following ioctl(2) calls return information in _value_.  The
           correct syntax is:
    
                  **int** _value_**;**
                  _error_ **= ioctl(**_unix_socket_**,**_ioctl_type_**, &**_value_**);**
    
           _ioctl_type_ can be:
    
           **SIOCINQ**
                  For **SOCK_STREAM** sockets, this call returns the number of
                  unread bytes in the receive buffer.  The socket must not be
                  in LISTEN state, otherwise an error (**EINVAL**) is returned.
                  **SIOCINQ** is defined in _< linux/sockios.h>_.  Alternatively,
                  you can use the synonymous **FIONREAD** , defined in
                  _< sys/ioctl.h>_.  For **SOCK_DGRAM** sockets, the returned value
                  is the same as for Internet domain datagram sockets; see
                  udp(7).
    


## ERRORS



           **EADDRINUSE**
                  The specified local address is already in use or the
                  filesystem socket object already exists.
    
           **EBADF** This error can occur for sendmsg(2) when sending a file
                  descriptor as ancillary data over a UNIX domain socket (see
                  the description of **SCM_RIGHTS** , above), and indicates that
                  the file descriptor number that is being sent is not valid
                  (e.g., it is not an open file descriptor).
    
           **ECONNREFUSED**
                  The remote address specified by connect(2) was not a
                  listening socket.  This error can also occur if the target
                  pathname is not a socket.
    
           **ECONNRESET**
                  Remote socket was unexpectedly closed.
    
           **EFAULT** User memory address was not valid.
    
           **EINVAL** Invalid argument passed.  A common cause is that the value
                  **AF_UNIX** was not specified in the _sun_type_ field of passed
                  addresses, or the socket was in an invalid state for the
                  applied operation.
    
           **EISCONN**
                  connect(2) called on an already connected socket or a
                  target address was specified on a connected socket.
    
           **ENFILE** The system-wide limit on the total number of open files has
                  been reached.
    
           **ENOENT** The pathname in the remote address specified to connect(2)
                  did not exist.
    
           **ENOMEM** Out of memory.
    
           **ENOTCONN**
                  Socket operation needs a target address, but the socket is
                  not connected.
    
           **EOPNOTSUPP**
                  Stream operation called on non-stream oriented socket or
                  tried to use the out-of-band data option.
    
           **EPERM** The sender passed invalid credentials in the _struct ucred_.
    
           **EPIPE** Remote socket was closed on a stream socket.  If enabled, a
                  **SIGPIPE** is sent as well.  This can be avoided by passing
                  the **MSG_NOSIGNAL** flag to send(2) or sendmsg(2).
    
           **EPROTONOSUPPORT**
                  Passed protocol is not **AF_UNIX**.
    
           **EPROTOTYPE**
                  Remote socket does not match the local socket type
                  (**SOCK_DGRAM** versus **SOCK_STREAM**).
    
           **ESOCKTNOSUPPORT**
                  Unknown socket type.
    
           **ESRCH** While sending an ancillary message containing credentials
                  (**SCM_CREDENTIALS**), the caller specified a PID that does not
                  match any existing process.
    
           **ETOOMANYREFS**
                  This error can occur for sendmsg(2) when sending a file
                  descriptor as ancillary data over a UNIX domain socket (see
                  the description of **SCM_RIGHTS** , above).  It occurs if the
                  number of "in-flight" file descriptors exceeds the
                  **RLIMIT_NOFILE** resource limit and the caller does not have
                  the **CAP_SYS_RESOURCE** capability.  An in-flight file
                  descriptor is one that has been sent using sendmsg(2) but
                  has not yet been accepted in the recipient process using
                  recvmsg(2).
    
                  This error is diagnosed since mainline Linux 4.5 (and in
                  some earlier kernel versions where the fix has been
                  backported).  In earlier kernel versions, it was possible
                  to place an unlimited number of file descriptors in flight,
                  by sending each file descriptor with sendmsg(2) and then
                  closing the file descriptor so that it was not accounted
                  against the **RLIMIT_NOFILE** resource limit.
    
           Other errors can be generated by the generic socket layer or by
           the filesystem while generating a filesystem socket object.  See
           the appropriate manual pages for more information.
    


## VERSIONS



           **SCM_CREDENTIALS** and the abstract namespace were introduced with
           Linux 2.2 and should not be used in portable programs.  (Some BSD-
           derived systems also support credential passing, but the
           implementation details differ.)
    


## NOTES



           Binding to a socket with a filename creates a socket in the
           filesystem that must be deleted by the caller when it is no longer
           needed (using unlink(2)).  The usual UNIX close-behind semantics
           apply; the socket can be unlinked at any time and will be finally
           removed from the filesystem when the last reference to it is
           closed.
    
           To pass file descriptors or credentials over a **SOCK_STREAM** socket,
           you must send or receive at least one byte of nonancillary data in
           the same sendmsg(2) or recvmsg(2) call.
    
           UNIX domain stream sockets do not support the notion of out-of-
           band data.
    


## BUGS



           When binding a socket to an address, Linux is one of the
           implementations that append a null terminator if none is supplied
           in _sun_path_.  In most cases this is unproblematic: when the socket
           address is retrieved, it will be one byte longer than that
           supplied when the socket was bound.  However, there is one case
           where confusing behavior can result: if 108 non-null bytes are
           supplied when a socket is bound, then the addition of the null
           terminator takes the length of the pathname beyond
           _sizeof(sun_path)_.  Consequently, when retrieving the socket
           address (for example, via accept(2)), if the input _addrlen_
           argument for the retrieving call is specified as _sizeof(struct_
           _sockaddr_un)_ , then the returned address structure _won't_ have a
           null terminator in _sun_path_.
    
           In addition, some implementations don't require a null terminator
           when binding a socket (the _addrlen_ argument is used to determine
           the length of _sun_path_) and when the socket address is retrieved
           on these implementations, there is no null terminator in _sun_path_.
    
           Applications that retrieve socket addresses can (portably) code to
           handle the possibility that there is no null terminator in
           _sun_path_ by respecting the fact that the number of valid bytes in
           the pathname is:
    
               strnlen(addr.sun_path, addrlen - offsetof(sockaddr_un, sun_path))
    
           Alternatively, an application can retrieve the socket address by
           allocating a buffer of size _sizeof(struct sockaddr_un)+1_ that is
           zeroed out before the retrieval.  The retrieving call can specify
           _addrlen_ as _sizeof(struct sockaddr_un)_ , and the extra zero byte
           ensures that there will be a null terminator for the string
           returned in _sun_path_ :
    
               void *addrp;
    
               addrlen = sizeof(struct sockaddr_un);
               addrp = malloc(addrlen + 1);
               if (addrp == NULL)
                   /* Handle error */ ;
               memset(addrp, 0, addrlen + 1);
    
               if (getsockname(sfd, (struct sockaddr *) addrp, &addrlen)) == -1)
                   /* handle error */ ;
    
               printf("sun_path = %s\n", ((struct sockaddr_un *) addrp)->sun_path);
    
           This sort of messiness can be avoided if it is guaranteed that the
           applications that _create_ pathname sockets follow the rules
           outlined above under _Pathname sockets_.
    


## EXAMPLES



           The following code demonstrates the use of sequenced-packet
           sockets for local interprocess communication.  It consists of two
           programs.  The server program waits for a connection from the
           client program.  The client sends each of its command-line
           arguments in separate messages.  The server treats the incoming
           messages as integers and adds them up.  The client sends the
           command string "END".  The server sends back a message containing
           the sum of the client's integers.  The client prints the sum and
           exits.  The server waits for the next client to connect.  To stop
           the server, the client is called with the command-line argument
           "DOWN".
    
           The following output was recorded while running the server in the
           background and repeatedly executing the client.  Execution of the
           server program ends when it receives the "DOWN" command.
    
       **Example output**
               $ **./server &**
               [1] 25887
               $ **./client 3 4** ;
               Result = 7
               $ **./client 11 -5** ;
               Result = 6
               $ **./client DOWN** ;
               Result = 0
               [1]+  Done                    ./server
               $
    
       **Program source**
    
           /*
            * File connection.h
            */
           #ifndef CONNECTION_H
           #define CONNECTION_H
    
           #define SOCKET_NAME "/tmp/9Lq7BNBnBycd6nxy.socket"
           #define BUFFER_SIZE 12
    
           #endif  // include guard
    
           /*
            * File server.c
            */
    
           #include <stdio.h>
           #include <stdlib.h>
           #include <string.h>
           #include <sys/socket.h>
           #include <sys/types.h>
           #include <sys/un.h>
           #include <unistd.h>
    
           #include "connection.h"
    
           int
           main(void)
           {
               int                 down_flag = 0;
               int                 ret;
               int                 connection_socket;
               int                 data_socket;
               int                 result;
               ssize_t             r, w;
               struct sockaddr_un  name;
               char                buffer[BUFFER_SIZE];
    
               /* Create local socket. */
    
               connection_socket = socket(AF_UNIX, SOCK_SEQPACKET, 0);
               if (connection_socket == -1) {
                   perror("socket");
                   exit(EXIT_FAILURE);
               }
    
               /*
                * For portability clear the whole structure, since some
                * implementations have additional (nonstandard) fields in
                * the structure.
                */
    
               memset(&name, 0, sizeof(name));
    
               /* Bind socket to socket name. */
    
               name.sun_family = AF_UNIX;
               strncpy(name.sun_path, SOCKET_NAME, sizeof(name.sun_path) - 1);
    
               ret = bind(connection_socket, (const struct sockaddr *) &name,
                          sizeof(name));
               if (ret == -1) {
                   perror("bind");
                   exit(EXIT_FAILURE);
               }
    
               /*
                * Prepare for accepting connections. The backlog size is set
                * to 20. So while one request is being processed other requests
                * can be waiting.
                */
    
               ret = listen(connection_socket, 20);
               if (ret == -1) {
                   perror("listen");
                   exit(EXIT_FAILURE);
               }
    
               /* This is the main loop for handling connections. */
    
               for (;;) {
    
                   /* Wait for incoming connection. */
    
                   data_socket = accept(connection_socket, NULL, NULL);
                   if (data_socket == -1) {
                       perror("accept");
                       exit(EXIT_FAILURE);
                   }
    
                   result = 0;
                   for (;;) {
    
                       /* Wait for next data packet. */
    
                       r = read(data_socket, buffer, sizeof(buffer));
                       if (r == -1) {
                           perror("read");
                           exit(EXIT_FAILURE);
                       }
    
                       /* Ensure buffer is 0-terminated. */
    
                       buffer[sizeof(buffer) - 1] = 0;
    
                       /* Handle commands. */
    
                       if (!strncmp(buffer, "DOWN", sizeof(buffer))) {
                           down_flag = 1;
                           continue;
                       }
    
                       if (!strncmp(buffer, "END", sizeof(buffer))) {
                           break;
                       }
    
                       if (down_flag) {
                           continue;
                       }
    
                       /* Add received summand. */
    
                       result += atoi(buffer);
                   }
    
                   /* Send result. */
    
                   sprintf(buffer, "%d", result);
                   w = write(data_socket, buffer, sizeof(buffer));
                   if (w == -1) {
                       perror("write");
                       exit(EXIT_FAILURE);
                   }
    
                   /* Close socket. */
    
                   close(data_socket);
    
                   /* Quit on DOWN command. */
    
                   if (down_flag) {
                       break;
                   }
               }
    
               close(connection_socket);
    
               /* Unlink the socket. */
    
               unlink(SOCKET_NAME);
    
               exit(EXIT_SUCCESS);
           }
    
           /*
            * File client.c
            */
    
           #include <stdio.h>
           #include <stdlib.h>
           #include <string.h>
           #include <sys/socket.h>
           #include <sys/types.h>
           #include <sys/un.h>
           #include <unistd.h>
    
           #include "connection.h"
    
           int
           main(int argc, char *argv[])
           {
               int                 ret;
               int                 data_socket;
               ssize_t             r, w;
               struct sockaddr_un  addr;
               char                buffer[BUFFER_SIZE];
    
               /* Create local socket. */
    
               data_socket = socket(AF_UNIX, SOCK_SEQPACKET, 0);
               if (data_socket == -1) {
                   perror("socket");
                   exit(EXIT_FAILURE);
               }
    
               /*
                * For portability clear the whole structure, since some
                * implementations have additional (nonstandard) fields in
                * the structure.
                */
    
               memset(&addr, 0, sizeof(addr));
    
               /* Connect socket to socket address. */
    
               addr.sun_family = AF_UNIX;
               strncpy(addr.sun_path, SOCKET_NAME, sizeof(addr.sun_path) - 1);
    
               ret = connect(data_socket, (const struct sockaddr *) &addr,
                              sizeof(addr));
               if (ret == -1) {
                   fprintf(stderr, "The server is down.\n");
                   exit(EXIT_FAILURE);
               }
    
               /* Send arguments. */
    
               for (int i = 1; i < argc; ++i) {
                   w = write(data_socket, argv[i], strlen(argv[i]) + 1);
                   if (w == -1) {
                       perror("write");
                       break;
                   }
               }
    
               /* Request result. */
    
               strcpy(buffer, "END");
               w = write(data_socket, buffer, strlen(buffer) + 1);
               if (w == -1) {
                   perror("write");
                   exit(EXIT_FAILURE);
               }
    
               /* Receive result. */
    
               r = read(data_socket, buffer, sizeof(buffer));
               if (r == -1) {
                   perror("read");
                   exit(EXIT_FAILURE);
               }
    
               /* Ensure buffer is 0-terminated. */
    
               buffer[sizeof(buffer) - 1] = 0;
    
               printf("Result = %s\n", buffer);
    
               /* Close socket. */
    
               close(data_socket);
    
               exit(EXIT_SUCCESS);
           }
    
           For examples of the use of **SCM_RIGHTS** , see cmsg(3) and
           seccomp_unotify(2).
    


* * *