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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | NOTES | ENVIRONMENT | SEE ALSO | COLOPHON |
SD_LISTEN_FDS(3) sd_listen_fds SD_LISTEN_FDS(3)
sd_listen_fds, sd_listen_fds_with_names, SD_LISTEN_FDS_START - Check
for file descriptors passed by the system manager
#include <systemd/sd-daemon.h>
#define SD_LISTEN_FDS_START 3
int sd_listen_fds(int unset_environment);
int sd_listen_fds_with_names(int unset_environment, char*** names);
sd_listen_fds() may be invoked by a daemon to check for file
descriptors passed by the service manager as part of the socket-based
activation logic. It returns the number of received file descriptors.
If no file descriptors have been received, zero is returned. The
first file descriptor may be found at file descriptor number 3 (i.e.
SD_LISTEN_FDS_START), the remaining descriptors follow at 4, 5, 6,
..., if any.
If a daemon receives more than one file descriptor, they will be
passed in the same order as configured in the systemd socket unit
file (see systemd.socket(5) for details). Nonetheless, it is
recommended to verify the correct socket types before using them. To
simplify this checking, the functions sd_is_fifo(3), sd_is_socket(3),
sd_is_socket_inet(3), sd_is_socket_unix(3) are provided. In order to
maximize flexibility, it is recommended to make these checks as loose
as possible without allowing incorrect setups. i.e. often, the actual
port number a socket is bound to matters little for the service to
work, hence it should not be verified. On the other hand, whether a
socket is a datagram or stream socket matters a lot for the most
common program logics and should be checked.
This function call will set the FD_CLOEXEC flag for all passed file
descriptors to avoid further inheritance to children of the calling
process.
If multiple socket units activate the same service, the order of the
file descriptors passed to its main process is undefined. If
additional file descriptors have been passed to the service manager
using sd_pid_notify_with_fds(3)'s "FDSTORE=1" messages, these file
descriptors are passed last, in arbitrary order, and with duplicates
removed.
If the unset_environment parameter is non-zero, sd_listen_fds() will
unset the $LISTEN_FDS, $LISTEN_PID and $LISTEN_FDNAMES environment
variables before returning (regardless of whether the function call
itself succeeded or not). Further calls to sd_listen_fds() will then
return zero, but the variables are no longer inherited by child
processes.
sd_listen_fds_with_names() is like sd_listen_fds(), but optionally
also returns an array of strings with identification names for the
passed file descriptors, if that is available and the names parameter
is non-NULL. This information is read from the $LISTEN_FDNAMES
variable, which may contain a colon-separated list of names. For
socket-activated services, these names may be configured with the
FileDescriptorName= setting in socket unit files, see
systemd.socket(5) for details. For file descriptors pushed into the
file descriptor store (see above), the name is set via the FDNAME=
field transmitted via sd_pid_notify_with_fds(). The primary usecase
for these names are services which accept a variety of file
descriptors which are not recognizable with functions like
sd_is_socket() alone, and thus require identification via a name. It
is recommended to rely on named file descriptors only if
identification via sd_is_socket() and related calls is not
sufficient. Note that the names used are not unique in any way. The
returned array of strings has as many entries as file descriptors
have been received, plus a final NULL pointer terminating the array.
The caller needs to free the array itself and each of its elements
with libc's free() call after use. If the names parameter is NULL,
the call is entirely equivalent to sd_listen_fds().
Under specific conditions, the following automatic file descriptor
names are returned:
Table 1. Special names
┌─────────────┬───────────────────────────┐
│Name │ Description │
├─────────────┼───────────────────────────┤
│"unknown" │ The process received no │
│ │ name for the specific │
│ │ file descriptor from the │
│ │ service manager. │
├─────────────┼───────────────────────────┤
│"stored" │ The file descriptor │
│ │ originates in the service │
│ │ manager's per-service │
│ │ file descriptor store, │
│ │ and the FDNAME= field was │
│ │ absent when the file │
│ │ descriptor was submitted │
│ │ to the service manager. │
├─────────────┼───────────────────────────┤
│"connection" │ The service was activated │
│ │ in per-connection style │
│ │ using Accept=yes in the │
│ │ socket unit file, and the │
│ │ file descriptor is the │
│ │ connection socket. │
└─────────────┴───────────────────────────┘
On failure, these calls returns a negative errno-style error code. If
$LISTEN_FDS/$LISTEN_PID was not set or was not correctly set for this
daemon and hence no file descriptors were received, 0 is returned.
Otherwise, the number of file descriptors passed is returned. The
application may find them starting with file descriptor
SD_LISTEN_FDS_START, i.e. file descriptor 3.
These APIs are implemented as a shared library, which can be compiled
and linked to with the libsystemd pkg-config(1) file.
Internally, sd_listen_fds() checks whether the $LISTEN_PID
environment variable equals the daemon PID. If not, it returns
immediately. Otherwise, it parses the number passed in the
$LISTEN_FDS environment variable, then sets the FD_CLOEXEC flag for
the parsed number of file descriptors starting from
SD_LISTEN_FDS_START. Finally, it returns the parsed number.
sd_listen_fds_with_names() does the same but also parses
$LISTEN_FDNAMES if set.
$LISTEN_PID, $LISTEN_FDS, $LISTEN_FDNAMES
Set by the service manager for supervised processes that use
socket-based activation. This environment variable specifies the
data sd_listen_fds() and sd_listen_fds_with_names() parses. See
above for details.
systemd(1), sd-daemon(3), sd_is_fifo(3), sd_is_socket(3),
sd_is_socket_inet(3), sd_is_socket_unix(3),
sd_pid_notify_with_fds(3), daemon(7), systemd.service(5),
systemd.socket(5)
This page is part of the systemd (systemd system and service manager)
project. Information about the project can be found at
⟨http://www.freedesktop.org/wiki/Software/systemd⟩. If you have a bug
report for this manual page, see
⟨http://www.freedesktop.org/wiki/Software/systemd/#bugreports⟩. This
page was obtained from the project's upstream Git repository
⟨https://github.com/systemd/systemd.git⟩ on 2017-07-05. If you dis‐
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you have corrections or improvements to the information in this
COLOPHON (which is not part of the original manual page), send a mail
to man-pages@man7.org
systemd 234 SD_LISTEN_FDS(3)
Pages that refer to this page: systemd(1), systemd-socket-activate(1), sd-daemon(3), sd_is_fifo(3), sd_notify(3), systemd.exec(5), systemd.socket(5), daemon(7), systemd.directives(7), systemd.index(7), systemd-activate(8)