Using changeword in M4-1.4.1 can cause Segmentation Fault

[Frank Schwidom]

Dear Developers.

The following code causes an Segfault in M4 if it is compiled with the 
'configure --enable-changeword' switch:

changequote( [, ])dnl
changeword( [[]])dnl
A

The Error resides in the following code in m4-1.4.1/src/input.c

...

set_word_regexp (const char *regexp)
{
 int i;
 char test[2];
 const char *msg;

 if (!strcmp (regexp, DEFAULT_WORD_REGEXP))
   {
     default_word_regexp = TRUE;
     return;
   }

 default_word_regexp = FALSE; ///////////// <= rash status-Change (maybe)

 msg = re_compile_pattern (regexp, strlen (regexp), &word_regexp);

 if (msg != NULL) ////////////////  Regexcompilation failed, and M4 is 
after this function in an bad State
   {
     M4ERROR ((warning_status, 0,
               "Bad regular expression `%s': %s", regexp, msg));
       /////////// fix-suggestion1: here could stay: set_word_regexp( 
old_regex); old_regex must be saved before and it must be clear that 
this will not fail
       /////////// fix-suggestion2: here could stay: 
default_word_regexp = TRUE;
     return; //////////// fix-suggestion3: here could be an 'exit(1)'
   }

 if (word_start == NULL)
   word_start = xmalloc (256);

 word_start[0] = '\0';
 test[1] = '\0';
 for (i = 1; i < 256; i++)
   {
     test[0] = i;
     if (re_search (&word_regexp, test, 1, 0, 0, &regs) >= 0)
       strcat (word_start, test);
   }
}

...

Regards, Frank Schwidom