Add -e flag for checked #exec

doc/gpp.pp

@@ -179,7 +179,7 @@
 $define{SYNTAX}{
 $pre$
 gpp [$dp$$bra$o$pipe$O$ket$ $I{outfile}$] [$dp$I$I{/include/path}$ ...]
-    [$dp$D$I{name=val}$ ...] [$dp$z$pipe$+z] [$dp$x] [$dp$m]
+    [$dp$D$I{name=val}$ ...] [$dp$z$pipe$+z] [$dp$e] [$dp$x] [$dp$m]
     [$dp$C$pipe$$dp$T$pipe$$dp$H$pipe$$dp$X$pipe$$dp$P$pipe$$dp$U ... [$dp$M ...]]
     [$dp$n$pipe$+n] [+c$I{$l$n$g$}$ $I{str1}$ $I{str2}$] [+s$I{$l$n$g$}$ $I{str1}$ $I{str2}$ $I{c}$]
     [$dp$c $I{str1}$] [$dp$$dp$nostdinc] [$dp$$dp$nocurinc]
@@ -316,10 +316,15 @@
 converted to CR$ndash$LF. This is the default if GPP is compiled with the
 WIN$und$NT option.
 $li$
+$BI{$d$e}$
+Enable the use of the $I{$dz$exec}$ meta-macro. Since $I{$dz$exec}$ includes
+the output of an arbitrary shell command line, it may cause a potential
+security threat, and is thus disabled unless this or the $I{$d$x}$ option is specified. If a shell command fails, GPP emits an error message and exits.
+$li$
 $BI{$d$x}$
 Enable the use of the $I{$dz$exec}$ meta-macro. Since $I{$dz$exec}$ includes
 the output of an arbitrary shell command line, it may cause a potential
-security threat, and is thus disabled unless this option is specified.
+security threat, and is thus disabled unless this or the $I{$d$e}$ option is specified. If a shell command fails, GPP ignores it. 
 $li$
 $BI{$d$m}$
 Enable automatic mode switching to the cpp compatibility mode if the name
@@ -871,12 +876,9 @@
 $BI{$dz$exec }{command}$
 This causes GPP to execute the specified command line and include its standard
 output in the current output. Note that, for security reasons, this meta-macro
-is disabled unless the $I{$d$x}$ command line flag was specified.
+is disabled unless the $I{$d$e}$ or $I{$d$x}$ command line flag was specified.
 If use of $I{$dz$exec}$ is not allowed, a warning message is printed
-and the output is left blank. Note that the specified command line is
-evaluated before being executed, thus allowing the use of macros in the
-command-line. However, the output of the command is included verbatim and
-not evaluated. If you need the output to be evaluated, you must use
+and the output is left blank. If $I{$dz$exec}$ fails and the $I{$d$e}$ command line flag was specified, GPP emits an error message and exits. If the $I{$d$x}$ command line flag was specified, GPP ignores the failure.  Note that the specified command line is evaluated before being executed, thus allowing the use of macros in the command-line. However, the output of the command is included verbatim and not evaluated. If you need the output to be evaluated, you must use
 $I{$dz$defeval}$ (see above) to cause a double evaluation.
 $li$
 $BI{$dz$eval }{expr}$

src/gpp.c

@@ -168,6 +168,7 @@ int nmacros, nalloced;
 char *includedir[MAXINCL];
 int nincludedirs;
 int execallowed;
+int popenchecked;
 int dosmode;
 int autoswitch;
 /* must be a format-like string that has % % % in it.
@@ -345,7 +346,7 @@ void display_version(void) {
 }
 
 void usage(void) {
-    printf("Usage : gpp [-{o|O} outfile] [-I/include/path] [-Dname=val ...] [-z] [-x] [-m]\n");
+    printf("Usage : gpp [-{o|O} outfile] [-I/include/path] [-Dname=val ...] [-z] [-e] [-x] [-m]\n");
     printf("            [-n] [-C | -T | -H | -X | -P | -U ... [-M ...]] [+c<n> str1 str2]\n");
     printf("            [+s<n> str1 str2 c] [long options] [infile]\n\n");
     printf("      default:    #define x y           macro(arg,...)\n");
@@ -359,7 +360,8 @@ void usage(void) {
     printf(" -o : output to outfile\n");
     printf(" -O : output to outfile and stdout\n");
     printf(" -z : line terminator is CR-LF (MS-DOS style)\n");
-    printf(" -x : enable #exec built-in macro\n");
+    printf(" -e : enable #exec built-in macro; exit if #exec fails.\n");
+    printf(" -x : enable #exec built-in macro; warn if #exec fails.\n");
     printf(" -m : enable automatic mode switching upon including .h/.c files\n");
     printf(" -n : send LF characters serving as macro terminators to output\n");
     printf(" +c : use next 2 args as comment start and comment end sequences\n");
@@ -1244,6 +1246,7 @@ void initthings(int argc, char **argv) {
     commented[0] = 0;
     iflevel = 0;
     execallowed = 0;
+    popenchecked = 0;
     autoswitch = 0;
     dosmode = DEFAULT_CRLF;
 
@@ -1475,6 +1478,9 @@ void initthings(int argc, char **argv) {
                 parseCmdlineDefine(s);
                 free(s);
                 break;
+            case 'e':
+                popenchecked = 1;
+                /* fallthrough */
             case 'x':
                 execallowed = 1;
                 break;
@@ -2687,12 +2693,17 @@ int ParsePossibleMeta(void) {
                 }
                 f = popen(s, "r");
                 free(s);
-                if (f == NULL )
+                if (f == NULL ) {
+                  if (popenchecked)
+                    bug("Cannot #exec. Command not found(?)");
+                  else
                     warning("Cannot #exec. Command not found(?)");
-                else {
+                } else {
                     while ((c = fgetc(f)) != EOF)
                         outchar((char) c);
-                    pclose(f);
+
+                    if (pclose(f) != 0 && popenchecked)
+                          bug("Failed #exec process");
                 }
             }
         }