sys_un.h - Man Page

definitions for UNIX domain sockets


This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux.


#include <sys/un.h>


The <sys/un.h> header shall define the sockaddr_un structure, which shall include at least the following members:

sa_family_t  sun_family  Address family.
char         sun_path[]  Socket pathname.

The sockaddr_un structure is used to store addresses for UNIX domain sockets. Pointers to this type shall be cast by applications to struct sockaddr * for use with socket functions.

The <sys/un.h> header shall define the sa_family_t type as described in <sys/socket.h>.

The following sections are informative.

Application Usage

The size of sun_path has intentionally been left undefined. This is because different implementations use different sizes. For example, 4.3 BSD uses a size of 108, and 4.4 BSD uses a size of 104. Since most implementations originate from BSD versions, the size is typically in the range 92 to 108.

Applications should not assume a particular length for sun_path or assume that it can hold {_POSIX_PATH_MAX} bytes (256).

Although applications are required to initialize all members (including any non-standard ones) of a sockaddr_in6 structure (see <netinet_in.h>), the same is not required for the sockaddr_un structure, since historically many applications only initialized the standard members. Despite this, applications are encouraged to initialize sockaddr_un structures in a manner similar to the required initialization of sockaddr_in6 structures.



Future Directions


See Also

<netinet_in.h>, <sys_socket.h>

The System Interfaces volume of POSIX.1-2017, bind(), socket(), socketpair()


2017 IEEE/The Open Group POSIX Programmer's Manual