unix(7) — Linux manual page

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UNIX(7)                 Linux Programmer's Manual                UNIX(7)

NAME         top

       unix - sockets for local interprocess communication

SYNOPSIS         top

       #include <sys/socket.h>
       #include <sys/un.h>

       unix_socket = socket(AF_UNIX, type, 0);
       error = socketpair(AF_UNIX, type, 0, int *sv);

DESCRIPTION         top

       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 NOTES, below.

       Various systems 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) 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 in kernels before 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         top

       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.

       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         top

       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         top

       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 to 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         top

       When binding a socket to an address, Linux is one of the
       implementations that appends 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         top

       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
        */

       #define SOCKET_NAME "/tmp/9Lq7BNBnBycd6nxy.socket"
       #define BUFFER_SIZE 12

       /*
        * File server.c
        */

       #include <stdio.h>
       #include <stdlib.h>
       #include <string.h>
       #include <sys/socket.h>
       #include <sys/un.h>
       #include <unistd.h>
       #include "connection.h"

       int
       main(int argc, char *argv[])
       {
           struct sockaddr_un name;
           int down_flag = 0;
           int ret;
           int connection_socket;
           int data_socket;
           int result;
           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. */

                   ret = read(data_socket, buffer, sizeof(buffer));
                   if (ret == -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;
                       break;
                   }

                   if (!strncmp(buffer, "END", sizeof(buffer))) {
                       break;
                   }

                   /* Add received summand. */

                   result += atoi(buffer);
               }

               /* Send result. */

               sprintf(buffer, "%d", result);
               ret = write(data_socket, buffer, sizeof(buffer));
               if (ret == -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 <errno.h>
       #include <stdio.h>
       #include <stdlib.h>
       #include <string.h>
       #include <sys/socket.h>
       #include <sys/un.h>
       #include <unistd.h>
       #include "connection.h"

       int
       main(int argc, char *argv[])
       {
           struct sockaddr_un addr;
           int ret;
           int data_socket;
           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) {
               ret = write(data_socket, argv[i], strlen(argv[i]) + 1);
               if (ret == -1) {
                   perror("write");
                   break;
               }
           }

           /* Request result. */

           strcpy(buffer, "END");
           ret = write(data_socket, buffer, strlen(buffer) + 1);
           if (ret == -1) {
               perror("write");
               exit(EXIT_FAILURE);
           }

           /* Receive result. */

           ret = read(data_socket, buffer, sizeof(buffer));
           if (ret == -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 an example of the use of SCM_RIGHTS see cmsg(3).

SEE ALSO         top

       recvmsg(2), sendmsg(2), socket(2), socketpair(2), cmsg(3),
       capabilities(7), credentials(7), socket(7), udp(7)

COLOPHON         top

       This page is part of release 5.11 of the Linux man-pages project.
       A description of the project, information about reporting bugs,
       and the latest version of this page, can be found at
       https://www.kernel.org/doc/man-pages/.

Linux                          2021-03-22                        UNIX(7)

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