NAME
unix - sockets for local interprocess communication SYNOPSIS
#include
#include
unixsocket = socket(AFUNIX, type, 0); error = socketpair(AFUNIX, type, 0, int *sv); DESCRIPTION The AFUNIX (also known as AFLOCAL) socket family is used to communi‐ cate between processes on the same machine efficiently. Traditionally, UNIX domain sockets can be either unnamed, or bound to a file system pathname (marked as being of type socket). Linux also supports an abstract namespace which is independent of the file system. Valid types are: SOCKSTREAM, for a stream-oriented socket and
SOCKDGRAM, for a datagram-oriented socket that preserves message boundaries (as on most UNIX implementations, UNIX domain datagram sock‐ ets are always reliable and don't reorder datagrams); and (since Linux
2.6.4) SOCKSEQPACKET, for a connection-oriented socket that preserves message boundaries and delivers messages in the order that they were sent. UNIX domain sockets support passing file descriptors or process creden‐ tials to other processes using ancillary data. Address format A UNIX domain socket address is represented in the following structure:
#define UNIXPATHMAX 108 struct sockaddrun { safamilyt sunfamily; /* AFUNIX */ char sunpath[UNIXPATHMAX]; /* pathname */ }; sunfamily always contains AFUNIX. Three types of address are distinguished in this structure:
* pathname: a UNIX domain socket can be bound to a null-terminated file system pathname using bind(2). When the address of the socket is returned by getsockname(2), getpeername(2), and accept(2), its length is offsetof(struct sockaddrun, sunpath) + strlen(sunpath)
+ 1, and sunpath contains the null-terminated pathname. * unnamed: A stream socket that has not been bound to a pathname using bind(2) has no name. Likewise, the two sockets created by socket‐ pair(2) are unnamed. When the address of an unnamed socket is returned by getsockname(2), getpeername(2), and accept(2), its length is sizeof(safamilyt), and sunpath should not be inspected. * abstract: an abstract socket address is distinguished by the fact that sunpath[0] is a null byte ('\0'). The socket's address in this namespace is given by the additional bytes in sunpath 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 file system pathnames. When the address of an abstract socket is returned by getsockname(2), getpeername(2), and accept(2), the returned addrlen is greater than sizeof(safamilyt) (i.e., greater than 2), and the name of the socket is contained in
the first (addrlen - sizeof(safamilyt)) bytes of sunpath. The abstract socket namespace is a nonportable Linux extension. Socket options For historical reasons these socket options are specified with a SOLSOCKET type even though they are AFUNIX specific. They can be set with setsockopt(2) and read with getsockopt(2) by specifying SOLSOCKET as the socket family. SOPASSCRED Enables the receiving of the credentials of the sending process in an ancillary message. When this option is set and the socket is not yet connected a unique name in the abstract namespace will be generated automatically. Expects an integer boolean flag. Autobind feature If a bind(2) call specifies addrlen as sizeof(safamilyt), or the SOPASSCRED 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 unsup‐ ported 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 MSGOOB flag for send(2) and recv(2)). The send(2) MSGMORE flag is not supported by UNIX domain sockets. The use of MSGTRUNC in the flags argument of recv(2) is not supported by UNIX domain sockets. The SOSNDBUF socket option does have an effect for UNIX domain sock‐ ets, but the SORCVBUF option does not. For datagram sockets, the SOSNDBUF value imposes an upper limit on the size of outgoing data‐ grams. 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 SOLSOCKET type even though they are AFUNIX specific. To send them set the cmsglevel field of the struct cmsghdr to SOLSOCKET and the cmsgtype field to the type. For more information see cmsg(3). SCMRIGHTS Send or receive a set of open file descriptors from another process. The data portion contains an integer array of the file descriptors. The passed file descriptors behave as though they have been created with dup(2). SCMCREDENTIALS Send or receive UNIX credentials. This can be used for authen‐ tication. The credentials are passed as a struct ucred ancil‐ lary message. Thus structure is defined in
as follows: struct ucred { pidt pid; /* process ID of the sending process */ uidt uid; /* user ID of the sending process */ gidt gid; /* group ID of the sending process */ }; Since glibc 2.8, the GNUSOURCE 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 process with effective user ID 0 is allowed to spec‐ ify values that do not match its own. The sender must specify its own process ID (unless it has the capability CAPSYSADMIN), its user ID, effective user ID, or saved set-user-ID (unless it has CAPSETUID), and its group ID, effective group ID, or saved
set-group-ID (unless it has CAPSETGID). To receive a struct ucred message the SOPASSCRED option must be enabled on the socket. Ioctls The following ioctl(2) calls return information in value. The correct syntax is: int value; error = ioctl(unixsocket, ioctltype, &value); ioctltype can be: SIOCINQ Returns the amount of queued unread data in the receive buffer. The socket must not be in LISTEN state, otherwise an error (EIN‐ VAL) is returned. SIOCINQ is defined in
. Alternatively, you can use the synonymous FIONREAD, defined in . ERRORS EADDRINUSE The specified local address is already in use or the file system socket object already exists. ECONNREFUSED The remote address specified by connect(2) was not a listening socket. This error can also occur if the target filename 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 AFUNIX was not specified in the suntype 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 SIG‐ PIPE is sent as well. This can be avoided by passing the MSGNOSIGNAL flag to sendmsg(2) or recvmsg(2). EPROTONOSUPPORT Passed protocol is not AFUNIX. EPROTOTYPE Remote socket does not match the local socket type (SOCKDGRAM versus SOCKSTREAM) ESOCKTNOSUPPORT Unknown socket type. Other errors can be generated by the generic socket layer or by the file system while generating a file system socket object. See the appropriate manual pages for more information. VERSIONS SCMCREDENTIALS 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 In the Linux implementation, sockets which are visible in the file sys‐ tem honor the permissions of the directory they are in. Their owner, group and their permissions can be changed. Creation of a new socket will fail if the process does not have write and search (execute) per‐ mission on the directory the socket is created in. Connecting to the socket object requires read/write permission. This behavior differs
from many BSD-derived systems which ignore permissions for UNIX domain sockets. Portable programs should not rely on this feature for secu‐ rity. Binding to a socket with a filename creates a socket in the file system 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 file system when the last reference to it is closed. To pass file descriptors or credentials over a SOCKSTREAM, you need 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. EXAMPLE See bind(2). For an example of the use of SCMRIGHTS see cmsg(3). SEE ALSO recvmsg(2), sendmsg(2), socket(2), socketpair(2), cmsg(3), capabili‐ ties(7), credentials(7), socket(7) COLOPHON
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Linux 2012-05-10 UNIX(7)