argp: Distinguish NULL and '\0' from 0

[Bruno Haible]

The argp code still uses 0 to denote a null pointer, like in the 1980ies.
It makes the code harder to understand than necessary.


2023-09-04  Bruno Haible  <br...@clisp.org>

        argp: Distinguish NULL and '\0' from 0.
        * lib/argp-pin.c: Include <stddef.h>.
        (program_invocation_short_name, program_invocation_name): Use NULL
        instead of 0.
        * lib/argp-parse.c (convert_options, __argp_parse, __argp_input):
        Likewise.
        * lib/argp-fmtstream.c (__argp_make_fmtstream): Likewise.
        * lib/argp-help.c (make_hol, hol_entry_first_long, hol_find_entry,
        hol_append, hol_entry_help, argp_doc, _help): Likewise.
        (hol_usage): Use '\0' instead of 0.

diff --git a/lib/argp-fmtstream.c b/lib/argp-fmtstream.c
index 7febb76c50..0f6798137b 100644
--- a/lib/argp-fmtstream.c
+++ b/lib/argp-fmtstream.c
@@ -74,7 +74,7 @@ __argp_make_fmtstream (FILE *stream,
       if (! fs->buf)
         {
           free (fs);
-          fs = 0;
+          fs = NULL;
         }
       else
         {
diff --git a/lib/argp-help.c b/lib/argp-help.c
index 7a277ebf13..15d94993c1 100644
--- a/lib/argp-help.c
+++ b/lib/argp-help.c
@@ -449,7 +449,7 @@ make_hol (const struct argp *argp, struct hol_cluster 
*cluster)
   assert (hol);
 
   hol->num_entries = 0;
-  hol->clusters = 0;
+  hol->clusters = NULL;
 
   if (opts)
     {
@@ -634,7 +634,7 @@ hol_entry_first_long (const struct hol_entry *entry)
   for (opt = entry->opt, num = entry->num; num > 0; opt++, num--)
     if (opt->name && ovisible (opt))
       return opt->name;
-  return 0;
+  return NULL;
 }
 
 /* Returns the entry in HOL with the long option name NAME, or NULL if there is
@@ -664,7 +664,7 @@ hol_find_entry (struct hol *hol, const char *name)
       while (--num_entries > 0);
     }
 
-  return 0;
+  return NULL;
 }
 
 /* If an entry with the long option NAME occurs in HOL, set its special
@@ -945,7 +945,7 @@ hol_append (struct hol *hol, struct hol *more)
   while (*cl_end)
     cl_end = &(*cl_end)->next;
   *cl_end = more->clusters;
-  more->clusters = 0;
+  more->clusters = NULL;
 
   /* Merge entries.  */
   if (more->num_entries > 0)
@@ -1317,7 +1317,7 @@ hol_entry_help (struct hol_entry *entry, const struct 
argp_state *state,
     {
       const char *tstr = real->doc ? dgettext (state == NULL ? NULL
                                                : state->root_argp->argp_domain,
-                                               real->doc) : 0;
+                                               real->doc) : NULL;
       const char *fstr = filter_doc (tstr, real->key, entry->argp, state);
       if (fstr && *fstr)
         {
@@ -1479,7 +1479,7 @@ hol_usage (struct hol *hol, argp_fmtstream_t stream)
                                  entry->argp->argp_domain, &snao_end);
       if (snao_end > short_no_arg_opts)
         {
-          *snao_end++ = 0;
+          *snao_end++ = '\0';
           __argp_fmtstream_printf (stream, " [-%s]", short_no_arg_opts);
         }
 
@@ -1593,7 +1593,7 @@ argp_doc (const struct argp *argp, const struct 
argp_state *state,
 {
   const char *text;
   const char *inp_text;
-  void *input = 0;
+  void *input = NULL;
   int anything = 0;
   size_t inp_text_limit = 0;
   const char *doc = argp->doc ? dgettext (argp->argp_domain, argp->doc) : NULL;
@@ -1681,7 +1681,7 @@ _help (const struct argp *argp, const struct argp_state 
*state, FILE *stream,
        unsigned flags, char *name)
 {
   int anything = 0;             /* Whether we've output anything.  */
-  struct hol *hol = 0;
+  struct hol *hol = NULL;
   argp_fmtstream_t fs;
 
   if (! stream)
diff --git a/lib/argp-parse.c b/lib/argp-parse.c
index dfc4351f9b..a76295feb3 100644
--- a/lib/argp-parse.c
+++ b/lib/argp-parse.c
@@ -355,9 +355,9 @@ convert_options (const struct argp *argp,
       group->args_processed = 0;
       group->parent = parent;
       group->parent_index = parent_index;
-      group->input = 0;
-      group->hook = 0;
-      group->child_inputs = 0;
+      group->input = NULL;
+      group->hook = NULL;
+      group->child_inputs = NULL;
 
       if (children)
         /* Assign GROUP's CHILD_INPUTS field some space from
@@ -373,7 +373,7 @@ convert_options (const struct argp *argp,
       parent = group++;
     }
   else
-    parent = 0;
+    parent = NULL;
 
   if (children)
     {
@@ -912,7 +912,7 @@ __argp_parse (const struct argp *argp, int argc, char 
**argv, unsigned flags,
       (child++)->argp = &argp_default_argp;
       if (argp_program_version || argp_program_version_hook)
         (child++)->argp = &argp_version_argp;
-      child->argp = 0;
+      child->argp = NULL;
 
       argp = top_argp;
     }
@@ -949,7 +949,7 @@ __argp_input (const struct argp *argp, const struct 
argp_state *state)
           return group->input;
     }
 
-  return 0;
+  return NULL;
 }
 #ifdef weak_alias
 weak_alias (__argp_input, _argp_input)
diff --git a/lib/argp-pin.c b/lib/argp-pin.c
index 76006cb7a1..056e360186 100644
--- a/lib/argp-pin.c
+++ b/lib/argp-pin.c
@@ -18,11 +18,13 @@
 # include <config.h>
 #endif
 
+#include <stddef.h>
+
 #ifndef HAVE_PROGRAM_INVOCATION_SHORT_NAME
-char *program_invocation_short_name = 0;
+char *program_invocation_short_name = NULL;
 #endif
 #ifndef HAVE_PROGRAM_INVOCATION_NAME
-char *program_invocation_name = 0;
+char *program_invocation_name = NULL;
 #endif
 
 #if (defined HAVE_PROGRAM_INVOCATION_SHORT_NAME \