sd_is_fifo, sd_is_socket, sd_is_socket_inet, sd_is_socket_unix, sd_is_mq − Check the type of a file descriptor


#include "sd−daemon.h"

int sd_is_fifo(int fd, const char *path);

int sd_is_socket(int fd, int family, int type, int listening);

int sd_is_socket_inet(int fd, int family, int type, int listening, uint16_t port);

int sd_is_socket_unix(int fd, int type, int listening, const char* path, size_t length);

int sd_is_mq(int fd, const char *path);


sd_is_fifo() may be called to check whether the specified file descriptor refers to a FIFO or pipe. If the path parameter is not NULL, it is checked whether the FIFO is bound to the specified file system path.

sd_is_socket() may be called to check whether the specified file descriptor refers to a socket. It 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_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 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.


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 functions are provided by the reference implementation of APIs for new−style daemons and distributed with the systemd package. The algorithms they implement are simple, and can easily be reimplemented in daemons if it is important to support this interface without using the reference implementation.

Internally, these function use a combination of fstat() and getsockname() to check the file descriptor type and where it is bound to.

For details about the algorithms check the liberally licensed reference implementation sources: resp.

sd_is_fifo() and the related functions are implemented in the reference implementation's sd−daemon.c and sd−daemon.h files. These interfaces are available as shared library, which can be compiled and linked to with the libsystemd−daemon pkg-config(1) file. Alternatively, applications consuming these APIs may copy the implementation into their source tree. For more details about the reference implementation see sd_daemon(7).

These functions continue to work as described, even if −DDISABLE_SYSTEMD is set during compilation.


systemd(1), sd-daemon(7), sd_listen_fds(3), systemd.service(5), systemd.socket(5)


Lennart Poettering <>


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