unix — sockets for local interprocess communication
#include <sys/socket.h> #include <sys/un.h>
unix_socket =
socket( |
AF_UNIX, |
type, | |
0) ; |
error =
socketpair( |
AF_UNIX, |
type, | |
0, | |
int *sv) ; |
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 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 credentials to other processes using ancillary data.
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
. The abstract
socket namespace is a nonportable Linux
extension.
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.)
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
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.
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.)
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.
The use of MSG_TRUNC
in
the flags
argument of recv(2) is 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 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. The passed file descriptors behave as though they have been created with dup(2).
SCM_CREDENTIALS
Send or receive UNIX credentials. This can be used
for authentication. The credentials are passed as a
struct ucred
ancillary message. Thus 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 process with effective user
ID 0 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
), its user ID,
effective user ID, or saved set-user-ID (unless it
has CAP_SETUID
), and
its 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.
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
socket the function returns the amount of queued
unread data 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
socket, the returned
value is the same as for Internet domain datagram
socket; see udp(7).
The specified local address is already in use or the filesystem socket object already exists.
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.
Remote socket was unexpectedly closed.
User memory address was not valid.
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.
connect(2) called on an already connected socket or a target address was specified on a connected socket.
The pathname in the remote address specified to connect(2) did not exist.
Out of memory.
Socket operation needs a target address, but the socket is not connected.
Stream operation called on non-stream oriented socket or tried to use the out-of-band data option.
The sender passed invalid credentials in the struct ucred.
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
sendmsg(2) or
recvmsg(2).
Passed protocol is not AF_UNIX
.
Remote socket does not match the local socket type
(SOCK_DGRAM
versus
SOCK_STREAM
)
Unknown socket type.
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.
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.)
In the Linux implementation, sockets which are visible in the filesystem honor the permissions of the directory they are in. Their owner, group, and permissions can be changed. Creation of a new socket will fail if the process does not have write and search (execute) permission 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 security.
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
, 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.
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.
recvmsg(2), sendmsg(2), socket(2), socketpair(2), cmsg(3), capabilities(7), credentials(7), socket(7), udp(7)
This page is part of release 4.00 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
http://www.kernel.org/doc/man−pages/.
This man page is Copyright (C) 1999 Andi Kleen <akmuc.de>. and Copyright (C) 2008-2014, Michael Kerrisk <mtk.manpagesgmail.com> %%%LICENSE_START(VERBATIM_ONE_PARA) and Copyright (C) 2008, 2012 Michael Kerrisk <mtk.manpagesgmail.com> Permission is granted to distribute possibly modified copies of this page provided the header is included verbatim, and in case of nontrivial modification author and date of the modification is added to the header. %%%LICENSE_END Modified, 2003-12-02, Michael Kerrisk, <mtk.manpagesgmail.com> Modified, 2003-09-23, Adam Langley Modified, 2004-05-27, Michael Kerrisk, <mtk.manpagesgmail.com> Added SOCK_SEQPACKET 2008-05-27, mtk, Provide a clear description of the three types of address that can appear in the sockaddr_un structure: pathname, unnamed, and abstract. |