Since include_file was just added, why not go one extra step and
allow things like (adapted from etc/kamailio.cfg):

#!define PRESENCE

#!ifdef PRESENCE
  loadmodule "presence.so"
  loadmodule "presence_xml.so"
#!else
  #!ifndef KAMAILIO
    ...
  #!endif
#!endif

M4 is already used and a better alternative IMO, but cfg is clearly not a
typical language, so I imagine someone may find this useful. Patch against
commit 7362f824.

For a minimal test input, this can be used:
=====
listen=::1

#!define xyz

#!ifdef xyz
  listen=::2
  #!ifndef abc
    listen=::3
  #!else
    listen=::4
  #!endif
  listen=::5
#!endif
=====

While wr're at it, the ID token seemed wrong -- it misses
_ids_that_start_with_underscores (?).

Bogdan


diff --git a/cfg.lex b/cfg.lex
index 8490364..df4a2ee 100644
--- a/cfg.lex
+++ b/cfg.lex
@@ -105,6 +105,12 @@
     #define PVAR_P_S                7  /* pvar: $(...)  or $foo(...)*/
     #define PVARID_S                8  /* $foo.bar...*/
     #define STR_BETWEEN_S        9
+    #define LINECOMMENT_S            10
+    #define DEFINE_S                11
+    #define DEFINE_EOL_S            12
+    #define IFDEF_S                    13
+    #define IFDEF_EOL_S                14
+    #define IFDEF_SKIP_S            15
 
     #define STR_BUF_ALLOC_UNIT    128
     struct str_buf{
@@ -152,11 +158,18 @@
         struct sr_yy_fname *next;
     } *sr_yy_fname_list = 0;
 
+    static int  pp_define(int len, const char * text);
+    static int  pp_ifdef_type(int pos);
+    static void pp_ifdef_var(int len, const char * text);
+    static void pp_ifdef();
+    static void pp_else();
+    static void pp_endif();
 %}
 
 /* start conditions */
 %x STRING1 STRING2 STR_BETWEEN COMMENT COMMENT_LN ATTR SELECT AVP_PVAR PVAR_P
 %x PVARID INCLF
+%x LINECOMMENT DEFINE_ID DEFINE_EOL IFDEF_ID IFDEF_EOL IFDEF_SKIP
 
 /* config script types : #!SER  or #!KAMAILIO or #!MAX_COMPAT */
 SER_CFG            SER
@@ -474,8 +487,9 @@ TLSv1            "tlsv1"|"TLSv1"|"TLSV1"
 
 LETTER        [a-zA-Z]
 DIGIT        [0-9]
-ALPHANUM    {LETTER}|{DIGIT}|[_]
-ID            {LETTER}{ALPHANUM}*
+LETTER_        {LETTER}|[_]
+ALPHANUM    {LETTER_}|{DIGIT}
+ID            {LETTER_}{ALPHANUM}*
 NUM_ID        {ALPHANUM}+
 HEX            [0-9a-fA-F]
 HEXNUMBER    0x{HEX}+
@@ -506,6 +520,12 @@ COM_LINE    #
 COM_START    "/\*"
 COM_END        "\*/"
 
+DEFINE        define
+IFDEF        ifdef
+IFNDEF        ifndef
+ENDIF        endif
+/* else is already defined */
+
 EAT_ABLE    [\ \t\b\r]
 
 %%
@@ -1121,7 +1141,36 @@ EAT_ABLE    [\ \t\b\r]
                                             sr_cfg_compat=SR_COMPAT_KAMAILIO;}
 <INITIAL>{COM_LINE}!{MAXCOMPAT_CFG}{CR}    { count();
                                                 sr_cfg_compat=SR_COMPAT_MAX;}
-<INITIAL>{COM_LINE}.*{CR}    { count(); }
+
+<INITIAL>{COM_LINE}!{DEFINE}{EAT_ABLE}+        { count();
+                                                state = DEFINE_S; BEGIN(DEFINE_ID); }
+<DEFINE_ID>{ID}                                { count();
+                                                if (pp_define(yyleng, yytext)) return 1;
+                                                state = DEFINE_EOL_S; BEGIN(DEFINE_EOL); }
+<DEFINE_EOL>{EAT_ABLE}*{CR}                    { count();
+                                                state = INITIAL; BEGIN(INITIAL); }
+
+<INITIAL,IFDEF_SKIP>{COM_LINE}!{IFDEF}{EAT_ABLE}+    { count();
+                                                        if (pp_ifdef_type(1)) return 1;
+                                                        state = IFDEF_S; BEGIN(IFDEF_ID); }
+<INITIAL,IFDEF_SKIP>{COM_LINE}!{IFNDEF}{EAT_ABLE}+    { count();
+                                                        if (pp_ifdef_type(0)) return 1;
+                                                        state = IFDEF_S; BEGIN(IFDEF_ID); }
+<IFDEF_ID>{ID}                { count();
+                                pp_ifdef_var(yyleng, yytext);
+                                state = IFDEF_EOL_S; BEGIN(IFDEF_EOL); }
+<IFDEF_EOL>{EAT_ABLE}*{CR}    { count(); pp_ifdef(); }
+
+<INITIAL,IFDEF_SKIP>{COM_LINE}!{ELSE}{EAT_ABLE}*{CR}    { count(); pp_else(); }
+
+<INITIAL,IFDEF_SKIP>{COM_LINE}!{ENDIF}{EAT_ABLE}*{CR}    { count(); pp_endif(); }
+
+ /* we're in an ifdef that evaluated to false -- throw it away */
+<IFDEF_SKIP>.|{CR}    { count(); }
+
+ /* this is split so the shebangs match more, giving them priority */
+<INITIAL>{COM_LINE}        { count(); state = LINECOMMENT_S; BEGIN(LINECOMMENT); }
+<LINECOMMENT>.*{CR}        { count(); state = INITIAL_S; BEGIN(INITIAL); }
 
 <INITIAL>{ID}            { count(); addstr(&s_buf, yytext, yyleng);
                                     yylval.strval=s_buf.s;
@@ -1472,3 +1521,102 @@ static int sr_pop_yy_state()
     return 0;
 }
 
+/* define/ifdef support */
+
+#define MAX_DEFINES    1024
+static str pp_defines[MAX_DEFINES];
+static int pp_num_defines = 0;
+
+/* pp_ifdef_stack[i] is 1 if the ifdef test at depth i is either
+ * ifdef(defined), ifndef(undefined), or the opposite of these
+ * two, but in an else branch
+ */
+#define MAX_IFDEFS    16
+static int pp_ifdef_stack[MAX_IFDEFS];
+static int pp_sptr = 0; /* stack pointer */
+
+static int pp_lookup(int len, const char * text)
+{
+    str var = {(char *)text, len};
+    int i;
+
+    for (i=0; i<pp_num_defines; i++)
+        if (STR_EQ(pp_defines[i], var))
+            return i;
+
+    return -1;
+}
+
+static int pp_define(int len, const char * text)
+{
+    if (pp_num_defines == MAX_DEFINES) {
+        LOG(L_CRIT, "ERROR: too many defines -- adjust MAX_DEFINES\n");
+        return -1;
+    }
+
+    if (pp_lookup(len, text) >= 0) {
+        LOG(L_CRIT, "ERROR: already defined: %.*s\n", len, text);
+        return -1;
+    }
+
+    pp_defines[pp_num_defines].len = len;
+    pp_defines[pp_num_defines].s = (char*)pkg_malloc(len+1);
+    memcpy(pp_defines[pp_num_defines].s, text, len);
+    pp_num_defines++;
+
+    return 0;
+}
+
+static int pp_ifdef_type(int type)
+{
+    if (pp_sptr == MAX_IFDEFS) {
+        LOG(L_CRIT, "ERROR: too many nested ifdefs -- adjust MAX_IFDEFS\n");
+        return -1;
+    }
+   
+    pp_ifdef_stack[pp_sptr] = type;
+    return 0;
+}
+
+/* this sets the result of the if[n]def expr:
+ * ifdef  defined   -> 1
+ * ifdef  undefined -> 0
+ * ifndef defined   -> 0
+ * ifndef undefined -> 1
+ */
+static void pp_ifdef_var(int len, const char * text)
+{
+    pp_ifdef_stack[pp_sptr] ^= (pp_lookup(len, text) < 0);
+}
+
+static void pp_update_state()
+{
+    int i;
+   
+    for (i=0; i<pp_sptr; i++)
+        if (! pp_ifdef_stack[i]) {
+            state = IFDEF_SKIP_S; BEGIN(IFDEF_SKIP);
+            return;
+        }
+
+    state = INITIAL; BEGIN(INITIAL);
+}
+
+static void pp_ifdef()
+{
+    pp_sptr++;
+    pp_update_state();
+}
+
+static void pp_else()
+{
+    pp_ifdef_stack[pp_sptr-1] ^= 1;
+    pp_update_state();
+}
+
+static void pp_endif()
+{
+    pp_sptr--;
+    pp_update_state();
+}
+