diff: prefer nullptr to NULL

* bootstrap.conf (gnulib_modules): Add c-nullptr.
* src/cmp.c (main):
* src/context.c (pr_context_hunk, pr_unidiff_hunk)
(mark_ignorable, find_function):
* src/diff.c (longopts, main, specify_value, specify_style)
(specify_colors_style, compare_files):
* src/diff3.c (main, process_diff):
* src/dir.c (find_dir_file_pathname):
* src/ifdef.c (scan_char_literal):
* src/sdiff.c (main):
* src/util.c (process_signals, install_signal_handlers)
(color_ext_list, color_indicator, indicator_name)
(parse_diff_color):
Prefer C23-style nullptr to NULL, as nullptr is a bit better.

9 files changed, 52 insertions, 50 deletions

diff --git a/bootstrap.conf b/bootstrap.conf
index 7670cae..4ddd0cc 100644
--- a/bootstrap.conf
+++ b/bootstrap.conf
@@ -27,6 +27,7 @@ announce-gen
 attribute
 argmatch
 binary-io
+c-nullptr
 c-stack
 config-h
 diffseq
diff --git a/src/cmp.c b/src/cmp.c
index d3e1aec..979408b 100644
--- a/src/cmp.c
+++ b/src/cmp.c
@@ -251,7 +251,7 @@ main (int argc, char **argv)
 
       case 'v':
         version_etc (stdout, PROGRAM_NAME, PACKAGE_NAME, Version,
-                     AUTHORS, (char *) NULL);
+                     AUTHORS, nullptr);
         check_stdout ();
         return EXIT_SUCCESS;
 
diff --git a/src/context.c b/src/context.c
index 2ed7478..a4c76ca 100644
--- a/src/context.c
+++ b/src/context.c
@@ -196,7 +196,7 @@ pr_context_hunk (struct change *hunk)
     last1 = files[1].valid_lines - 1;
 
   /* If desired, find the preceding function definition line in file 0.  */
-  function = NULL;
+  function = nullptr;
   if (function_regexp.fastmap)
     function = find_function (files[0].linbuf, first0);
 
@@ -345,7 +345,7 @@ pr_unidiff_hunk (struct change *hunk)
     last1 = files[1].valid_lines - 1;
 
   /* If desired, find the preceding function definition line in file 0.  */
-  function = NULL;
+  function = nullptr;
   if (function_regexp.fastmap)
     function = find_function (files[0].linbuf, first0);
 
@@ -379,7 +379,7 @@ pr_unidiff_hunk (struct change *hunk)
           char const *const *line = &files[0].linbuf[i++];
           if (! (suppress_blank_empty && **line == '\n'))
             putc (initial_tab ? '\t' : ' ', out);
-          print_1_line (NULL, line);
+          print_1_line (nullptr, line);
           j++;
         }
       else
@@ -395,7 +395,7 @@ pr_unidiff_hunk (struct change *hunk)
               putc ('-', out);
               if (initial_tab && ! (suppress_blank_empty && **line == '\n'))
                 putc ('\t', out);
-              print_1_line_nl (NULL, line, true);
+              print_1_line_nl (nullptr, line, true);
 
               set_color_context (RESET_CONTEXT);
 
@@ -414,7 +414,7 @@ pr_unidiff_hunk (struct change *hunk)
               putc ('+', out);
               if (initial_tab && ! (suppress_blank_empty && **line == '\n'))
                 putc ('\t', out);
-              print_1_line_nl (NULL, line, true);
+              print_1_line_nl (nullptr, line, true);
 
               set_color_context (RESET_CONTEXT);
 
@@ -481,7 +481,7 @@ mark_ignorable (struct change *script)
       lin first0, last0, first1, last1;
 
       /* Turn this change into a hunk: detach it from the others.  */
-      script->link = NULL;
+      script->link = nullptr;
 
       /* Determine whether this change is ignorable.  */
       script->ignore = ! analyze_hunk (script,
@@ -497,7 +497,7 @@ mark_ignorable (struct change *script)
 
 /* Find the last function-header line in LINBUF prior to line number LINENUM.
    This is a line containing a match for the regexp in 'function_regexp'.
-   Return the address of the text, or NULL if no function-header is found.  */
+   Return the address of the text, or null if no function-header is found.  */
 
 static char const *
 find_function (char const * const *linbuf, lin linenum)
@@ -516,7 +516,7 @@ find_function (char const * const *linbuf, lin linenum)
 	 to LEN = LINELEN and no machine code is generated.  */
       regoff_t len = MIN (linelen, TYPE_MAXIMUM (regoff_t));
 
-      if (0 <= re_search (&function_regexp, line, len, 0, len, NULL))
+      if (0 <= re_search (&function_regexp, line, len, 0, len, nullptr))
         {
           find_function_last_match = i;
           return line;
@@ -527,5 +527,5 @@ find_function (char const * const *linbuf, lin linenum)
   if (find_function_last_match != LIN_MAX)
     return linbuf[find_function_last_match];
 
-  return NULL;
+  return nullptr;
 }
diff --git a/src/diff.c b/src/diff.c
index 375adad..dd6f63b 100644
--- a/src/diff.c
+++ b/src/diff.c
@@ -224,7 +224,7 @@ static struct option const longopts[] =
   {"width", 1, 0, 'W'},
 
   /* This is solely for testing.  Do not document.  */
-  {"-presume-output-tty", no_argument, NULL, PRESUME_OUTPUT_TTY_OPTION},
+  {"-presume-output-tty", no_argument, nullptr, PRESUME_OUTPUT_TTY_OPTION},
   {0, 0, 0, 0}
 };
 
@@ -283,8 +283,8 @@ main (int argc, char **argv)
   bool explicit_context = false;
   size_t width = 0;
   bool show_c_function = false;
-  char const *from_file = NULL;
-  char const *to_file = NULL;
+  char const *from_file = nullptr;
+  char const *to_file = nullptr;
   intmax_t numval;
   char *numend;
 
@@ -305,7 +305,7 @@ main (int argc, char **argv)
 
   /* Decode the options.  */
 
-  while ((c = getopt_long (argc, argv, shortopts, longopts, NULL)) != -1)
+  while ((c = getopt_long (argc, argv, shortopts, longopts, nullptr)) != -1)
     {
       switch (c)
         {
@@ -477,7 +477,7 @@ main (int argc, char **argv)
 
         case 'l':
           if (!pr_program[0])
-            try_help ("pagination not supported on this host", NULL);
+            try_help ("pagination not supported on this host", nullptr);
           paginate = true;
 #ifdef SIGCHLD
           /* Pagination requires forking and waiting, and
@@ -544,7 +544,7 @@ main (int argc, char **argv)
 
         case 'v':
           version_etc (stdout, PROGRAM_NAME, PACKAGE_NAME, Version,
-                       AUTHORS, (char *) NULL);
+                       AUTHORS, nullptr);
           check_stdout ();
           return EXIT_SUCCESS;
 
@@ -697,7 +697,7 @@ main (int argc, char **argv)
           break;
 
         default:
-          try_help (NULL, NULL);
+          try_help (nullptr, nullptr);
         }
       prev = c;
     }
@@ -824,7 +824,7 @@ main (int argc, char **argv)
       else
         for (; optind < argc; optind++)
           {
-            int status = compare_files (NULL, from_file, argv[optind]);
+            int status = compare_files (nullptr, from_file, argv[optind]);
             if (exit_status < status)
               exit_status = status;
           }
@@ -834,7 +834,7 @@ main (int argc, char **argv)
       if (to_file)
         for (; optind < argc; optind++)
           {
-            int status = compare_files (NULL, argv[optind], to_file);
+            int status = compare_files (nullptr, argv[optind], to_file);
             if (exit_status < status)
               exit_status = status;
           }
@@ -848,7 +848,7 @@ main (int argc, char **argv)
                 try_help ("extra operand '%s'", argv[optind + 2]);
             }
 
-          exit_status = compare_files (NULL, argv[optind], argv[optind + 1]);
+          exit_status = compare_files (nullptr, argv[optind], argv[optind + 1]);
         }
     }
 
@@ -1081,7 +1081,7 @@ specify_value (char const **var, char const *value, char const *option)
   if (*var && ! STREQ (*var, value))
     {
       error (0, 0, _("conflicting %s option value '%s'"), option, value);
-      try_help (NULL, NULL);
+      try_help (nullptr, nullptr);
     }
   *var = value;
 }
@@ -1093,7 +1093,7 @@ specify_style (enum output_style style)
   if (output_style != style)
     {
       if (output_style != OUTPUT_UNSPECIFIED)
-        try_help ("conflicting output style options", NULL);
+        try_help ("conflicting output style options", nullptr);
       output_style = style;
     }
 }
@@ -1102,7 +1102,7 @@ specify_style (enum output_style style)
 static void
 specify_colors_style (char const *value)
 {
-  if (value == NULL || STREQ (value, "auto"))
+  if (value == nullptr || STREQ (value, "auto"))
     colors_style = AUTO;
   else if (STREQ (value, "always"))
     colors_style = ALWAYS;
@@ -1194,17 +1194,17 @@ compare_files (struct comparison const *parent,
 
   if (!parent)
     {
-      free0 = NULL;
-      free1 = NULL;
+      free0 = nullptr;
+      free1 = nullptr;
       cmp.file[0].name = name0;
       cmp.file[1].name = name1;
     }
   else
     {
       cmp.file[0].name = free0
-        = file_name_concat (parent->file[0].name, name0, NULL);
+        = file_name_concat (parent->file[0].name, name0, nullptr);
       cmp.file[1].name = free1
-        = file_name_concat (parent->file[1].name, name1, NULL);
+        = file_name_concat (parent->file[1].name, name1, nullptr);
     }
 
   /* Stat the files.  */
@@ -1370,7 +1370,7 @@ compare_files (struct comparison const *parent,
             {
               char const *dir;
 
-              /* PARENT must be non-NULL here.  */
+              /* PARENT must be non-null here.  */
               assert (parent);
               dir = parent->file[cmp.file[0].desc == NONEXISTENT].name;
 
@@ -1405,12 +1405,12 @@ compare_files (struct comparison const *parent,
           && S_ISLNK (cmp.file[1].stat.st_mode))
         {
           /* Compare the values of the symbolic links.  */
-          char *link_value[2] = { NULL, NULL };
+          char *link_value[2] = { nullptr, nullptr };
 
           for (f = 0; f < 2; f++)
             {
               link_value[f] = xreadlink (cmp.file[f].name);
-              if (link_value[f] == NULL)
+              if (link_value[f] == nullptr)
                 {
                   perror_with_name (cmp.file[f].name);
                   status = EXIT_TROUBLE;
diff --git a/src/diff3.c b/src/diff3.c
index c4bd7b8..aef0044 100644
--- a/src/diff3.c
+++ b/src/diff3.c
@@ -329,7 +329,7 @@ main (int argc, char **argv)
           break;
         case 'v':
           version_etc (stdout, PROGRAM_NAME, PACKAGE_NAME, Version,
-                       AUTHORS, (char *) NULL);
+                       AUTHORS, nullptr);
           check_stdout ();
           return EXIT_SUCCESS;
         case DIFF_PROGRAM_OPTION:
@@ -1091,7 +1091,7 @@ process_diff (char const *filea,
       block_list_end = &bptr->next;
     }
 
-  *block_list_end = NULL;
+  *block_list_end = nullptr;
   return block_list;
 }
 
diff --git a/src/dir.c b/src/dir.c
index 7721228..802060a 100644
--- a/src/dir.c
+++ b/src/dir.c
@@ -342,8 +342,8 @@ find_dir_file_pathname (char const *dir, char const *file)
 
   char *val;
   struct dirdata dirdata;
-  dirdata.names = NULL;
-  dirdata.data = NULL;
+  dirdata.names = nullptr;
+  dirdata.data = nullptr;
 
   if (ignore_file_name_case)
     {
@@ -373,7 +373,7 @@ find_dir_file_pathname (char const *dir, char const *file)
         }
     }
 
-  val = file_name_concat (dir, match, NULL);
+  val = file_name_concat (dir, match, nullptr);
   free (dirdata.names);
   free (dirdata.data);
   return val;
diff --git a/src/ifdef.c b/src/ifdef.c
index 0939c1a..d83a387 100644
--- a/src/ifdef.c
+++ b/src/ifdef.c
@@ -395,7 +395,7 @@ scan_char_literal (char const *lit, char *valptr)
     {
       case 0:
       case '\'':
-        return NULL;
+        return nullptr;
 
       case '\\':
         value = 0;
@@ -403,18 +403,18 @@ scan_char_literal (char const *lit, char *valptr)
           {
             unsigned int digit = c - '0';
             if (8 <= digit)
-              return NULL;
+              return nullptr;
             value = 8 * value + digit;
           }
         digits = p - lit - 2;
         if (! (1 <= digits && digits <= 3))
-          return NULL;
+          return nullptr;
         break;
 
       default:
         value = c;
         if (*p++ != '\'')
-          return NULL;
+          return nullptr;
         break;
     }
 
diff --git a/src/sdiff.c b/src/sdiff.c
index ded1837..1633565 100644
--- a/src/sdiff.c
+++ b/src/sdiff.c
@@ -518,7 +518,7 @@ main (int argc, char *argv[])
 
         case 'v':
           version_etc (stdout, PROGRAM_NAME, PACKAGE_NAME, Version,
-                       AUTHORS, (char *) NULL);
+                       AUTHORS, nullptr);
           check_stdout ();
           return EXIT_SUCCESS;
 
diff --git a/src/util.c b/src/util.c
index b25dbe1..8ea528e 100644
--- a/src/util.c
+++ b/src/util.c
@@ -287,7 +287,7 @@ process_signals (void)
       /* Exit or suspend the program.  */
       if (raise (sig) != 0)
 	pfatal_with_name ("raise");
-      xsigprocmask (SIG_SETMASK, &oldset, NULL);
+      xsigprocmask (SIG_SETMASK, &oldset, nullptr);
 
       /* If execution reaches here, then the program has been
          continued (after being suspended).  */
@@ -351,7 +351,7 @@ install_signal_handlers (void)
   for (int j = 0; j < nsigs; j++)
     {
       struct sigaction actj;
-      if (sigaction (sig[j], NULL, &actj) == 0 && actj.sa_handler != SIG_IGN)
+      if (sigaction (sig[j], nullptr, &actj) == 0 && actj.sa_handler != SIG_IGN)
 	xsigaddset (&caught_signals, sig[j]);
     }
 
@@ -363,7 +363,7 @@ install_signal_handlers (void)
     if (xsigismember (&caught_signals, sig[j]))
       {
 	act.sa_handler = is_tstp_index (j) ? stophandler : sighandler;
-	if (sigaction (sig[j], &act, NULL) != 0)
+	if (sigaction (sig[j], &act, nullptr) != 0)
 	  pfatal_with_name ("sigaction");
 	some_signals_caught = true;
       }
@@ -403,7 +403,7 @@ static char const *current_name1;
 static bool currently_recursive;
 static bool colors_enabled;
 
-static struct color_ext_type *color_ext_list = NULL;
+static struct color_ext_type *color_ext_list = nullptr;
 
 struct bin_str
   {
@@ -642,7 +642,7 @@ static struct bin_str color_indicator[] =
   {
     { LEN_STR_PAIR ("\033[") },		/* lc: Left of color sequence */
     { LEN_STR_PAIR ("m") },		/* rc: Right of color sequence */
-    { 0, NULL },			/* ec: End color (replaces lc+rs+rc) */
+    { 0, nullptr },			/* ec: End color (replaces lc+rs+rc) */
     { LEN_STR_PAIR ("0") },		/* rs: Reset to ordinary colors */
     { LEN_STR_PAIR ("1") },		/* hd: Header */
     { LEN_STR_PAIR ("32") },		/* ad: Add line */
@@ -652,7 +652,7 @@ static struct bin_str color_indicator[] =
 
 static const char *const indicator_name[] =
   {
-    "lc", "rc", "ec", "rs", "hd", "ad", "de", "ln", NULL
+    "lc", "rc", "ec", "rs", "hd", "ad", "de", "ln", nullptr
   };
 ARGMATCH_VERIFY (indicator_name, color_indicator);
 
@@ -674,10 +674,11 @@ parse_diff_color (void)
   char label[] = "??";		/* Indicator label */
   struct color_ext_type *ext;	/* Extension we are working on */
 
-  if ((p = color_palette) == NULL || *p == '\0')
+  p = color_palette;
+  if (p == nullptr || *p == '\0')
     return;
 
-  ext = NULL;
+  ext = nullptr;
 
   /* This is an overly conservative estimate, but any possible
      --palette string will *not* generate a color_buf longer than
@@ -739,7 +740,7 @@ parse_diff_color (void)
           state = PS_FAIL;	/* Assume failure...  */
           if (*(p++) == '=')/* It *should* be...  */
             {
-              for (ind_no = 0; indicator_name[ind_no] != NULL; ++ind_no)
+              for (ind_no = 0; indicator_name[ind_no] != nullptr; ++ind_no)
                 {
                   if (STREQ (label, indicator_name[ind_no]))
                     {
@@ -783,7 +784,7 @@ parse_diff_color (void)
       error (0, 0,
              _("unparsable value for --palette"));
       free (color_buf);
-      for (e = color_ext_list; e != NULL; /* empty */)
+      for (e = color_ext_list; e != nullptr; /* empty */)
         {
           e2 = e;
           e = e->next;