diff options
| author | Michael Kerrisk <mtk.manpages@gmail.com> | 2008-06-11 22:11:06 +0000 |
|---|---|---|
| committer | Michael Kerrisk <mtk.manpages@gmail.com> | 2008-06-11 22:11:06 +0000 |
| commit | 94b40171e1ed194976a1b1c0fc23e615c802161b (patch) | |
| tree | 21b945dbb998e47527f443a30dbd8b4fb801bbfa /man7/unix.7 | |
| parent | 063212eb6382f8ee390b3a7efc4e88111f39a8c4 (diff) | |
| download | man-pages-94b40171e1ed194976a1b1c0fc23e615c802161b.tar.gz | |
Provide a clear description of the three types of address that can
appear in the sockaddr_un structure: pathname, unnamed, and abstract.
Diffstat (limited to 'man7/unix.7')
| -rw-r--r-- | man7/unix.7 | 93 |
1 files changed, 67 insertions, 26 deletions
diff --git a/man7/unix.7 b/man7/unix.7 index a5299390ce..7d3c89de8b 100644 --- a/man7/unix.7 +++ b/man7/unix.7 @@ -8,8 +8,11 @@ .\" Modified, 2003-09-23, Adam Langley .\" Modified, 2004-05-27, Michael Kerrisk, <mtk.manpages@gmail.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. .\" -.TH UNIX 7 2004-05-27 "Linux" "Linux Programmer's Manual" +.TH UNIX 7 2008-06-17 "Linux" "Linux Programmer's Manual" .SH NAME unix, PF_UNIX, AF_UNIX, PF_LOCAL, AF_LOCAL \- Sockets for local interprocess communication @@ -28,9 +31,8 @@ The .BR PF_LOCAL ) socket family is used to communicate between processes on the same machine efficiently. -Unix sockets can be either anonymous (created by -.BR socketpair (2)) -or associated with a file of type socket. +Traditionally, Unix 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. @@ -49,44 +51,83 @@ and delivers messages in the order that they were sent. Unix sockets support passing file descriptors or process credentials to other processes using ancillary data. .SS Address Format -A Unix address is defined as a filename in the file system or -as a unique string in the abstract namespace. -Sockets created by -.BR socketpair (2) -are anonymous. -For non-anonymous sockets the target address can be set -using -.BR connect (2). -The local address can be set using -.BR bind (2). -When a socket is connected and it doesn't already have a local address a -unique address in the abstract namespace will be generated automatically. - +A Unix domain socket address is represented in the following structure: .in +4n .nf + #define UNIX_PATH_MAX 108 struct sockaddr_un { - sa_family_t sun_family; /* AF_UNIX */ - char sun_path[UNIX_PATH_MAX]; /* pathname */ + sa_family_t sun_family; /* AF_UNIX */ + char sun_path[UNIX_PATH_MAX]; /* pathname */ }; .fi .in - +.PP .I sun_family always contains .BR AF_UNIX . + +Three types of address are distinguished in this structure: +.IP * 3 +.IR pathname : +a Unix domain socket can be bound to a null-terminated file +system pathname using +.BR bind (2). +When the address of the socket is returned by +.BR getsockname (2), +.BR getpeername (2), +and +.BR accept (2), +its length is +.IR "sizeof(sa_family_t) + strlen(sun_path) + 1" , +and +.I sun_path +contains the null-terminated pathname. +.IP * +.IR unnamed : +A stream socket that has not been bound to a pathname using +.BR bind (2) +has no name. +Likewise, the two sockets created by +.BR socketpair (2) +are unnamed. +When the address of an unnamed socket is returned by +.BR getsockname (2), +.BR getpeername (2), +and +.BR accept (2), +its length is +.IR "sizeof(sa_family_t)" , +and .I sun_path -contains the zero-terminated pathname of the socket in the file system. -If +should not be inspected. +.\" There is quite some variation across implementations: FreeBSD +.\" says the length is 16 bytes, HP-UX 11 says it's zero bytes. +.IP * +.IR abstract : +an abstract socket address is distinguished by the fact that +.IR sun_path[0] +is a null byte (`\\0'). +All of the remaining bytes in .I sun_path -starts with a null byte (\(aq\\0\(aq), -then it refers to the abstract namespace maintained by -the Unix protocol module. +define the "name" of the socket. +(Null bytes in the name have no special significance.) +The name has no connection with file system pathnames. The socket's address in this namespace is given by the rest of the bytes in .IR sun_path . -Note that names in the abstract namespace are not zero-terminated. +When the address of an abstract socket is returned by +.BR getsockname (2), +.BR getpeername (2), +and +.BR accept (2), +its length is +.IR "sizeof(struct sockaddr_un)" , +and +.I sun_path +contains the abstract name. +The abstract socket namespace is a non-portable Linux extension. .SS Socket Options For historical reasons these socket options are specified with a .B SOL_SOCKET |