diff --git a/include/pthread_np.h b/include/pthread_np.h
--- a/include/pthread_np.h
+++ b/include/pthread_np.h
@@ -59,6 +59,8 @@
 int pthread_peekjoin_np(pthread_t, void **);
 void pthread_set_name_np(pthread_t, const char *);
 int pthread_setaffinity_np(pthread_t, size_t, const cpuset_t *);
+void pthread_signals_block_np(void);
+void pthread_signals_unblock_np(void);
 int pthread_single_np(void);
 void pthread_suspend_all_np(void);
 int pthread_suspend_np(pthread_t);
diff --git a/lib/libthr/pthread.map b/lib/libthr/pthread.map
--- a/lib/libthr/pthread.map
+++ b/lib/libthr/pthread.map
@@ -339,5 +339,7 @@
 };
 
 FBSD_1.8 {
+	 pthread_signals_block_np;
+	 pthread_signals_unblock_np;
 	 pthread_sigqueue;
 };
diff --git a/lib/libthr/thread/thr_sig.c b/lib/libthr/thread/thr_sig.c
--- a/lib/libthr/thread/thr_sig.c
+++ b/lib/libthr/thread/thr_sig.c
@@ -37,6 +37,7 @@
 #include <stdlib.h>
 #include <string.h>
 #include <pthread.h>
+#include <pthread_np.h>
 #include "un-namespace.h"
 #include "libc_private.h"
 
@@ -163,6 +164,24 @@
 	__sys_sigfastblock(SIGFASTBLOCK_SETPTR, &curthread->fsigblock);
 }
 
+void
+pthread_signals_block_np(void)
+{
+	struct pthread *curthread;
+
+	curthread = _get_curthread();
+	_thr_signal_block(curthread);
+}
+
+void
+pthread_signals_unblock_np(void)
+{
+	struct pthread *curthread;
+
+	curthread = _get_curthread();
+	_thr_signal_unblock(curthread);
+}
+
 int
 _thr_send_sig(struct pthread *thread, int sig)
 {
diff --git a/share/man/man3/Makefile b/share/man/man3/Makefile
--- a/share/man/man3/Makefile
+++ b/share/man/man3/Makefile
@@ -458,6 +458,7 @@
 		pthread_setspecific.3 \
 		pthread_sigmask.3 \
 		pthread_sigqueue.3 \
+		pthread_signals_block_np.3 \
 		pthread_spin_init.3 \
 		pthread_spin_lock.3 \
 		pthread_suspend_all_np.3 \
@@ -523,6 +524,8 @@
 PTHREAD_MLINKS+=pthread_set_name_np.3 pthread_get_name_np.3 \
 		pthread_set_name_np.3 pthread_getname_np.3 \
 		pthread_set_name_np.3 pthread_setname_np.3
+PTHREAD_MLINKS+=pthread_signals_block_np.3 \
+		pthread_signals_unblock_np.3
 PTHREAD_MLINKS+=pthread_spin_init.3 pthread_spin_destroy.3 \
 		pthread_spin_lock.3 pthread_spin_trylock.3 \
 		pthread_spin_lock.3 pthread_spin_unlock.3
diff --git a/share/man/man3/pthread_np.3 b/share/man/man3/pthread_np.3
--- a/share/man/man3/pthread_np.3
+++ b/share/man/man3/pthread_np.3
@@ -116,6 +116,11 @@
 .Xc
 Sets the specified thread's name.
 .It Xo
+.Ft void
+.Fn pthread_signals_block_np void
+.Xc
+Blocks all asynchronous signals, quickly.
+.It Xo
 .Ft int
 .Fn pthread_single_np void
 .Xc
@@ -213,6 +218,7 @@
 .Xr pthread_resume_all_np 3 ,
 .Xr pthread_resume_np 3 ,
 .Xr pthread_set_name_np 3 ,
+.Xr pthread_signals_block_np 3 ,
 .Xr pthread_suspend_all_np 3 ,
 .Xr pthread_suspend_np 3 ,
 .Xr pthread_switch_add_np 3
diff --git a/share/man/man3/pthread_signals_block_np.3 b/share/man/man3/pthread_signals_block_np.3
new file mode 100644
--- /dev/null
+++ b/share/man/man3/pthread_signals_block_np.3
@@ -0,0 +1,81 @@
+.\" Copyright (c) 2025 The FreeBSD Foundation
+.\" All rights reserved.
+.\"
+.\" SPDX-License-Identifier: BSD-2-Clause
+.\"
+.\" This documentation was written by
+.\" Konstantin Belousov <kib@FreeBSD.org> under sponsorship
+.\" from the FreeBSD Foundation.
+.\"
+.Dd May 16, 2025
+.Dt PTHREAD_SIGNALS_BLOCK_NP 3
+.Os
+.Sh NAME
+.Nm pthread_signals_block_np ,
+.Nm pthread_signals_unblock_np
+.Nd fast asynchronous signals blocking and unblocking
+.Sh LIBRARY
+.Lb libpthread
+.Sh SYNOPSIS
+.In pthread_np.h
+.Ft void
+.Fn pthread_signals_block_np "void"
+.Ft void
+.Fn pthread_signals_unblock_np "void"
+.Sh DESCRIPTION
+The
+.Fn pthread_signals_block_np
+and
+.Fn pthread_signals_unblock_np
+functions provide user programs an interface to the fast asynchronous
+signals blocking facility
+.Xr sigfastblock 2 .
+.Pp
+Blocking signals with
+.Fn pthread_signals_block_np
+disables delivery of any asynchronous signal, until unblocked.
+Signal blocking establishes a critical section where the execution
+flow of the thread cannot be diverted into a signal handler.
+Blocking signals is fast, it is performed by a single memory write into
+a location established with the kernel.
+.Pp
+Synchronous signal delivery cannot be blocked in general, including with
+these functions.
+.Pp
+The blocked state established by the
+.Fn pthread_signals_block_np
+is not completely POSIX-compliant.
+Specifically, system calls executed while in a blocked section,
+might abort sleep and return
+.Er EINTR
+upon queuing of an asynchronous signal to the thread,
+but the signal handler is not called until the last unblock is done.
+.Pp
+Calls to
+.Nm pthread_signals_block_np
+can be nested, and must be complemented by the equal count of
+calls to
+.Nm pthread_signals_unblock_np
+to return the calling thread into the standard mode of signal receiving.
+.Pp
+An example of the function usage might be a construction of the CPU
+state that cannot be done atomically, and which includes stages where
+the state of the thread is not ABI compliant.
+If a signal is delivered while such state is not yet finished, signal
+handlers would misbehave.
+Using standard functions
+.Pq Fn sigprocmask
+to establish critical section might be much slower, because
+.Fn sigprocmask
+is syscall, while
+.Fn pthread_signals_block_np
+consists of a single atomic memory write.
+.Sh RETURN VALUES
+The functions do not return a value.
+.Sh ERRORS
+There are no errors reported by the functions.
+.Sh SEE ALSO
+.Xr sigfastblock 2 ,
+.Xr sigprocmask 2 ,
+.Xr pthread_sigmask 3 ,
+.Xr pthread_np 3