'\" t
.TH "SD_VARLINK_PUSH_FD" "3" "" "systemd 258" "sd_varlink_push_fd"
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.SH "NAME"
sd_varlink_push_fd, sd_varlink_push_dup_fd \- Submit a file descriptor to send along with the next outgoing Varlink message
.SH "SYNOPSIS"
.sp
.ft B
.nf
#include <systemd/sd\-varlink\&.h>
.fi
.ft
.HP \w'int\ sd_varlink_push_fd('u
.BI "int sd_varlink_push_fd(sd_varlink\ *" "link" ", int\ " "fd" ");"
.HP \w'int\ sd_varlink_push_dup_fd('u
.BI "int sd_varlink_push_dup_fd(sd_varlink\ *" "link" ", int\ " "fd" ");"
.SH "DESCRIPTION"
.PP
\fBsd_varlink_push_fd()\fR
submits a file descriptor to send along with the next outgoing Varlink message\&. Takes a Varlink connection object and a file descriptor as parameter\&. The file descriptor is not duplicated, and hence ownership of the file descriptor is passed to the Varlink connection object (only on success; on failure the caller retains ownership)\&. Once the file descriptor has been written to the underlying transport socket it is automatically closed\&. The calling application code should not touch the file descriptor or close it on its own, otherwise it will interfere with the Varlink protocol implementation\&. This call is only supported if the backing transport supports file descriptor passing (effectively this means the functionality is supported on local
\fBAF_UNIX\fR
only), and the concept is not part of the Varlink protocol, but simply a feature of the underlying transport\&.
.PP
\fBsd_varlink_push_dup_fd()\fR
is identical to
\fBsd_varlink_push_fd()\fR, except that the file descriptor is duplicated automatically, and the calling application code hence retains ownership of the provided file descriptor, and must close it on its own\&.
.PP
Note that file descriptor passing is only permitted after a call to
\fBsd_varlink_set_allow_fd_passing_output()\fR
that enables it, otherwise these calls will fail with
\fB\-EPERM\fR\&.
.PP
Note that on Linux a maximum of 253 file descriptors may be enqueued on
\fBAF_UNIX\fR
sockets at once\&. Attempting to enqueue more on a single Varlink message will fail with
\fB\-ENOBUFS\fR\&.
.SH "RETURN VALUE"
.PP
On success,
\fBsd_varlink_push_fd()\fR
and
\fBsd_varlink_push_dup_fd()\fR
return a non\-negative integer\&. On failure, they return a negative errno\-style error code\&.
.SS "Errors"
.PP
Returned errors may indicate the following problems:
.PP
\fB\-EINVAL\fR
.RS 4
An argument is invalid\&.
.RE
.PP
\fB\-EBADF\fR
.RS 4
The provided file descriptor is not valid\&.
.RE
.PP
\fB\-EPERM\fR
.RS 4
File descriptor passing has not been enabled via
\fBsd_varlink_set_allow_fd_passing_output()\fR\&.
.RE
.PP
\fB\-ENOBUFS\fR
.RS 4
The maximum of 253 file descriptors have already been submitted for the next outgoing Varlink message, no further descriptors may be enqueued for this message\&.
.RE
.PP
\fB\-ENOMEM\fR
.RS 4
Memory allocation failed\&.
.RE
.SH "NOTES"
.PP
Functions described here are available as a shared library, which can be compiled against and linked to with the
\fBlibsystemd\fR\ \&\fBpkg-config\fR(1)
file\&.
.PP
The code described here uses
\fBgetenv\fR(3), which is declared to be not multi\-thread\-safe\&. This means that the code calling the functions described here must not call
\fBsetenv\fR(3)
from a parallel thread\&. It is recommended to only do calls to
\fBsetenv()\fR
from an early phase of the program when no other threads have been started\&.
.SH "HISTORY"
.PP
\fBsd_varlink_push_fd()\fR
and
\fBsd_varlink_push_dup_fd()\fR
were added in version 257\&.
.SH "SEE ALSO"
.PP
\fBsystemd\fR(1), \fBsd-varlink\fR(3)