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.Dd October 6, 2024
.Dt MEMBARRIER 2
.Os
.Sh NAME
.Nm membarrier
.Nd memory barrier
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In sys/membarrier.h
.Ft int
.Fn membarrier "int cmd" "unsigned flags" "int cpu_id"
.Sh DESCRIPTION
The
.Nm
system call provides a memory barrier, ensuring ordering between memory
accesses performed by different threads or processes within the same address
space.
.Pp
The following values for
.Fa cmd
may be specified:
.Bl -tag -width Dv
.It Dv MEMBARRIER_CMD_QUERY
Query supported commands.
A bitmask of commands supported by the kernel is returned.
.It Dv MEMBARRIER_CMD_GLOBAL
Issue a memory barrier for all threads from all processes.
.It Dv MEMBARRIER_CMD_SHARED
This is an alias for
.Dv MEMBARRIER_CMD_GLOBAL .
.It Dv MEMBARRIER_CMD_GLOBAL_EXPEDITED
Execute a memory barrier on all running threads of all processes registered
with
.Dv MEMBARRIER_CMD_REGISTER_GLOBAL_EXPEDITED .
.It Dv MEMBARRIER_CMD_REGISTER_GLOBAL_EXPEDITED
Register the process to receive
.Dv MEMBARRIER_CMD_GLOBAL_EXPEDITED
memory barriers.
.It Dv MEMBARRIER_CMD_PRIVATE_EXPEDITED
Execute a memory barrier on each running thread belonging to the same process
as the thread calling
.Nm .
.It Dv MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED
Register the process to receive
.Dv MEMBARRIER_CMD_PRIVATE_EXPEDITED
memory barriers.
.It Dv MEMBARRIER_CMD_PRIVATE_EXPEDITED_SYNC_CORE
In addition to the guarantees provided by
.Dv MEMBARRIER_CMD_PRIVATE_EXPEDITED
it also executes machine-specific serialization instructions, which
ensures that all possible speculative and out-of-order activities on the target
CPU are fenced.
.It Dv MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED_SYNC_CORE
Register the process to receive
.Dv MEMBARRIER_CMD_PRIVATE_EXPEDITED_SYNC_CORE
memory barriers.
.El
.Pp
The following
.Fa cmd
values are defined for source compatibility but are not yet supported:
.Pp
.Bl -tag -width Dv -compact
.It Dv MEMBARRIER_CMD_PRIVATE_EXPEDITED_RSEQ
.It Dv MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED_RSEQ
.El
.Pp
The
.Fa flags
argument must be 0.
The
.Fa cpu_id
argument is ignored.
.Sh RETURN VALUES
If the
.Fa cmd
is
.Dv MEMBARRIER_CMD_QUERY
a bitmask of supported commands is returned.
Otherwise, on success,
.Nm
returns 0.
On error, -1 is returned and
.Va errno
is set to indicate the error.
.Sh ERRORS
.Nm
may fail with the following errors:
.Bl -tag -width Er
.It Bq Er EINVAL
.Fa cmd
does not specify a valid command.
.It Bq Er EINVAL
.Fa flags
is not 0.
.It Bq Er EPERM
An attempt was made to use
.Dv MEMBARRIER_CMD_GLOBAL_EXPEDITED ,
.Dv MEMBARRIER_CMD_PRIVATE_EXPEDITED ,
or
.Dv MEMBARRIER_CMD_PRIVATE_EXPEDITED_SYNC_CORE
by a process that did not previously register use with the corresponding
.Dv MEMBARRIER_CMD_REGISTER_*
.Fa cmd .
.El
.\" .Sh SEE ALSO
.Sh STANDARDS
The
.Nm
system call originated in Linux.
This implementation aims to be source-compatible with the Linux implementation.
Certain
.Fa cmd
and
.Fa flags
values are currently not supported by
.Fx .
.Sh HISTORY
The
.Nm
function was introduced in
.Fx 14.1 .