diff --git a/libc/test/src/signal/sigprocmask_test.cpp b/libc/test/src/signal/sigprocmask_test.cpp
@@ -0,0 +1,64 @@
+//===--------------------- Unittests for sigprocmask ----------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "include/errno.h"
+#include "include/signal.h"
+#include "src/errno/llvmlibc_errno.h"
+#include "src/signal/raise.h"
+#include "src/signal/sigaddset.h"
+#include "src/signal/sigemptyset.h"
+#include "src/signal/sigprocmask.h"
+
+#include "utils/UnitTest/Test.h"
+
+class SignalTest : public __llvm_libc::testing::Test {
+  sigset_t oldSet;
+
+public:
+  void SetUp() override { __llvm_libc::sigprocmask(0, nullptr, &oldSet); }
+
+  void TearDown() override {
+    __llvm_libc::sigprocmask(SIG_SETMASK, &oldSet, nullptr);
+  }
+};
+
+// This tests for invalid input.
+TEST_F(SignalTest, SigprocmaskInvalid) {
+  sigset_t valid;
+  // 17 and -4 are out of the range for sigprocmask's how paramater.
+  llvmlibc_errno = 0;
+  EXPECT_EQ(__llvm_libc::sigprocmask(17, &valid, nullptr), -1);
+  EXPECT_EQ(llvmlibc_errno, EINVAL);
+
+  llvmlibc_errno = 0;
+  EXPECT_EQ(__llvm_libc::sigprocmask(-4, &valid, nullptr), -1);
+  EXPECT_EQ(llvmlibc_errno, EINVAL);
+
+  // This pointer is out of this processes address range.
+  sigset_t *invalid = reinterpret_cast<sigset_t *>(-1);
+
+  llvmlibc_errno = 0;
+  EXPECT_EQ(__llvm_libc::sigprocmask(SIG_SETMASK, invalid, nullptr), -1);
+  EXPECT_EQ(llvmlibc_errno, EFAULT);
+
+  llvmlibc_errno = 0;
+  EXPECT_EQ(__llvm_libc::sigprocmask(-4, nullptr, invalid), -1);
+  EXPECT_EQ(llvmlibc_errno, EFAULT);
+}
+
+// This tests that when nothing is blocked, a process gets killed and alse tests
+// that when signals are blocked they are not delivered to the process.
+TEST_F(SignalTest, BlockUnblock) {
+  sigset_t sigset;
+  EXPECT_EQ(__llvm_libc::sigemptyset(&sigset), 0);
+  EXPECT_EQ(__llvm_libc::sigprocmask(SIG_SETMASK, &sigset, nullptr), 0);
+  EXPECT_DEATH([] { __llvm_libc::raise(SIGUSR1); }, SIGUSR1);
+  EXPECT_EQ(__llvm_libc::sigaddset(&sigset, SIGUSR1), 0);
+  EXPECT_EQ(__llvm_libc::sigprocmask(SIG_SETMASK, &sigset, nullptr), 0);
+  EXPECT_EXITS([] { __llvm_libc::raise(SIGUSR1); }, 0);
+}