sd_is_socket() may be called to check whether the specified file descriptor refers to a socket. If the family parameter is not AF_UNSPEC, it is checked whether the socket is of the specified family (AF_UNIX, AF_INET, ...). If the type parameter is not 0, it is checked whether the socket is of the specified type (SOCK_STREAM, SOCK_DGRAM, ...). If the listening parameter is positive, it is checked whether the socket is in accepting mode, i.e. listen() has been called for it. If listening is 0, it is checked whether the socket is not in this mode. If the parameter is negative, no such check is made. The listening parameter should only be used for stream sockets and should be set to a negative value otherwise.
sd_is_socket_inet() is similar to sd_is_socket(), but optionally checks the IPv4 or IPv6 port number the socket is bound to, unless port is zero. For this call family must be passed as either AF_UNSPEC, AF_INET, or AF_INET6.
sd_is_socket_sockaddr() is similar to sd_is_socket_inet(), but checks if the socket is bound to the address specified by addr. The family specified by addr must be either AF_INET or AF_INET6 and addr_len must be large enough for that family. If addr specifies a non-zero port, it is also checked if the socket is bound to this port. In addition, for IPv6, if addr specifies non-zero sin6_flowinfo or sin6_scope_id, it is checked if the socket has the same values.
sd_is_socket_unix() is similar to sd_is_socket() but optionally checks the AF_UNIX path the socket is bound to, unless the path parameter is NULL. For normal file system AF_UNIX sockets, set the length parameter to 0. For Linux abstract namespace sockets, set the length to the size of the address, including the initial 0 byte, and set the path to the initial 0 byte of the socket address.
sd_is_mq() may be called to check whether the specified file descriptor refers to a POSIX message queue. If the path parameter is not NULL, it is checked whether the message queue is bound to the specified name.
sd_is_special() may be called to check whether the specified file descriptor refers to a special file. If the path parameter is not NULL, it is checked whether the file descriptor is bound to the specified filename. Special files in this context are character device nodes and files in /proc/ or /sys/.
On failure, these calls return a negative errno-style error code. If the file descriptor is of the specified type and bound to the specified address, a positive return value is returned, otherwise zero.
These APIs are implemented as a shared library, which can be compiled and linked to with the libsystemd pkg-config(1) file.