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.Dd February 16, 2024
.Dt MQ_GETFD_NP 2
.Os
.Sh NAME
.Nm mq_getfd_np
.Nd "get a file descriptor on an open message queue descriptor"
.Sh LIBRARY
.Lb librt
.Sh SYNOPSIS
.In mqueue.h
.Ft int
.Fn mq_getfd_np "mqd_t mqd"
.Sh DESCRIPTION
The
.Fn mq_getfd_np
retrieves the underlying file descriptor from the given message queue descriptor
returned from 
.Xr mq_open 2 .
The file descriptor is used by other functions to refer to that message queue.

The
.Fa mqd
argument must be an open message queue descriptor.
.Sh NOTES
.Fx
implements retrieval or alteration of message queue attributes based on file 
descriptor.
An open file descriptor is inherited by child after
.Xr fork 2 .
The file descriptor is closed in a new image after
.Xr exec 3 .
The
.Xr fstat 2 ,
.Xr fchmod 2 ,
and
.Xr fchown 2
system calls are supported for file descriptor.
.Pp
Do not call
.Xr close 2
on the file descriptor. Instead, call
.Xr mq_close 2
on the mqd_t descriptor returned by 
.Xr mq_open 2 .
Do not use the file descriptor after calling 
.Xr mq_close 2 .
At best, file operations on the file descriptor will fail with
.Xr errno 2
set to EBADF (Bad file descriptor) error. 
Worst case, the descriptor may be reused and now refers to a wholly different
file or device.
The file descriptor is safe to use so long as the message queue descriptor
remains open.
.Pp
Please see the
.Xr mqueuefs 5
man page for instructions on loading the module or compiling the service into
the kernel.
.Sh RETURN VALUES
Upon successful completion, the function returns a file descriptor; otherwise,
an invalid file descriptor may be returned.
.Sh ERRORS
The
.Fn mq_getfd_np
system call does not return any errors but may cause SIGSEGV on an invalid
message queue descriptor.
.Sh SEE ALSO
.Xr mq_close 2 ,
.Xr mq_getattr 2 ,
.Xr mq_open 2 ,
.Xr mq_receive 2 ,
.Xr mq_send 2 ,
.Xr mq_setattr 2 ,
.Xr mq_timedreceive 3 ,
.Xr mq_timedsend 3 ,
.Xr mq_unlink 2 ,
.Xr mqueuefs 5
.Sh HISTORY
Support for mq_getdf_np first appeared in
.Fx 11.0 .