diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index 38d740e440..93a2cebe1c 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,3 +1,11 @@
+2019-01-28  John Baldwin  <jhb@FreeBSD.org>
+
+	* Makefile.in (SUBDIR_UNITTESTS_SRCS): Add
+	unittests/child-path-selftests.c.
+	* common/pathstuff.c (child_path): New function.
+	* common/pathstuff.h (child_path): New prototype.
+	* unittests/child-path-selftests.c: New file.
+
 2019-01-28  John Baldwin  <jhb@FreeBSD.org>
 
 	* symfile.c (find_separate_debug_file): Look for separate debug
diff --git a/gdb/Makefile.in b/gdb/Makefile.in
index 72ca855eb0..cec7ad32a4 100644
--- a/gdb/Makefile.in
+++ b/gdb/Makefile.in
@@ -411,6 +411,7 @@ SUBDIR_PYTHON_CFLAGS =
 
 SUBDIR_UNITTESTS_SRCS = \
 	unittests/array-view-selftests.c \
+	unittests/child-path-selftests.c \
 	unittests/cli-utils-selftests.c \
 	unittests/common-utils-selftests.c \
 	unittests/copy_bitwise-selftests.c \
diff --git a/gdb/common/pathstuff.c b/gdb/common/pathstuff.c
index 11675303b3..d6beb8faf1 100644
--- a/gdb/common/pathstuff.c
+++ b/gdb/common/pathstuff.c
@@ -147,6 +147,58 @@ gdb_abspath (const char *path)
 
 /* See common/pathstuff.h.  */
 
+const char *
+child_path (const char *parent, const char *child)
+{
+  /* The child path must start with the parent path.  */
+  size_t parent_len = strlen (parent);
+  if (filename_ncmp (parent, child, parent_len) != 0)
+    return NULL;
+
+  /* The parent path must be a directory and the child must contain at
+     least one component underneath the parent.  */
+  const char *child_component;
+  if (IS_DIR_SEPARATOR (parent[parent_len - 1]))
+    {
+      /* The parent path ends in a directory separator, so it is a
+	 directory.  The first child component starts after the common
+	 prefix.  */
+      child_component = child + parent_len;
+    }
+  else
+    {
+      /* The parent path does not end in a directory separator.  The
+	 first character in the child after the common prefix must be
+	 a directory separator.
+
+	 Note that CHILD must hold at least parent_len characters for
+	 filename_ncmp to return zero.  If the character at parent_len
+	 is nul due to CHILD containing the same path as PARENT, the
+	 IS_DIR_SEPARATOR check will fail here.  */
+      if (!IS_DIR_SEPARATOR (child[parent_len]))
+	return NULL;
+
+      /* The first child component starts after the separator after the
+	 common prefix.  */
+      child_component = child + parent_len + 1;
+    }
+
+  /* The child must contain at least one non-separator character after
+     the parent.  */
+  while (*child_component != '\0')
+    {
+      if (IS_DIR_SEPARATOR (*child_component))
+	{
+	  child_component++;
+	  continue;
+	}
+      return child_component;
+    }
+  return NULL;
+}
+
+/* See common/pathstuff.h.  */
+
 bool
 contains_dir_separator (const char *path)
 {
diff --git a/gdb/common/pathstuff.h b/gdb/common/pathstuff.h
index d43f337550..20a7bdda26 100644
--- a/gdb/common/pathstuff.h
+++ b/gdb/common/pathstuff.h
@@ -48,6 +48,12 @@ extern gdb::unique_xmalloc_ptr<char>
 
 extern gdb::unique_xmalloc_ptr<char> gdb_abspath (const char *path);
 
+/* If the path in CHILD is a child of the path in PARENT, return a
+   pointer to the first component in the CHILD's pathname below the
+   PARENT.  Otherwise, return NULL.  */
+
+extern const char *child_path (const char *parent, const char *child);
+
 /* Return whether PATH contains a directory separator character.  */
 
 extern bool contains_dir_separator (const char *path);
diff --git a/gdb/unittests/child-path-selftests.c b/gdb/unittests/child-path-selftests.c
new file mode 100644
index 0000000000..2621eecef6
--- /dev/null
+++ b/gdb/unittests/child-path-selftests.c
@@ -0,0 +1,64 @@
+/* Self tests for child_path for GDB, the GNU debugger.
+
+   Copyright (C) 2019 Free Software Foundation, Inc.
+
+   This file is part of GDB.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+#include "defs.h"
+#include "common/pathstuff.h"
+#include "common/selftest.h"
+
+namespace selftests {
+namespace child_path {
+
+/* Verify the result of a single child_path test.  */
+
+static bool
+child_path_check (const char *parent, const char *child, const char *expected)
+{
+  const char *result = ::child_path (parent, child);
+  if (result == NULL || expected == NULL)
+    return result == expected;
+  return strcmp (result, expected) == 0;
+}
+
+/* Test child_path.  */
+
+static void
+test ()
+{
+  SELF_CHECK (child_path_check ("/one", "/two", NULL));
+  SELF_CHECK (child_path_check ("/one", "/one", NULL));
+  SELF_CHECK (child_path_check ("/one", "/one/", NULL));
+  SELF_CHECK (child_path_check ("/one", "/one//", NULL));
+  SELF_CHECK (child_path_check ("/one", "/one/two", "two"));
+  SELF_CHECK (child_path_check ("/one/", "/two", NULL));
+  SELF_CHECK (child_path_check ("/one/", "/one", NULL));
+  SELF_CHECK (child_path_check ("/one/", "/one/", NULL));
+  SELF_CHECK (child_path_check ("/one/", "/one//", NULL));
+  SELF_CHECK (child_path_check ("/one/", "/one/two", "two"));
+}
+
+}
+}
+
+void
+_initialize_child_path_selftests ()
+{
+  selftests::register_test ("child_path",
+			    selftests::child_path::test);
+}
+
