Improve documentation of set-default-file-modes

* src/fileio.c (Fset_default_file_modes): Doc fix.  (Bug#28875)

* doc/lispref/files.texi (Changing Files): Clarify how the bits in
the argument of set-default-file-modes are used for setting file
permissions.

1 files changed, 9 insertions, 1 deletions

diff --git a/src/fileio.c b/src/fileio.c
index b7df38c8570..92cc39496c4 100644
--- a/src/fileio.c
+++ b/src/fileio.c
@@ -3125,7 +3125,15 @@ symbolic notation, like the `chmod' command from GNU Coreutils.  */)
 DEFUN ("set-default-file-modes", Fset_default_file_modes, Sset_default_file_modes, 1, 1, 0,
        doc: /* Set the file permission bits for newly created files.
 The argument MODE should be an integer; only the low 9 bits are used.
-This setting is inherited by subprocesses.  */)
+On Posix hosts, this setting is inherited by subprocesses.
+
+This function works by setting the Emacs's file mode creation mask.
+Each bit that is set in the mask means that the corresponding bit
+in the the permissions of newly created files will be disabled.
+
+Note that when `write-region' creates a file, it resets the
+execute bit, even if the mask set by this function allows that bit
+by having the corresponding bit in the mask reset.  */)
   (Lisp_Object mode)
 {
   mode_t oldrealmask, oldumask, newumask;