Why unknown variable "mystring"?

Question

I'm trying to debug why my variable mystring is not known when I think it should be according to an earlier question

Is the bug in the grammar or in the code?

(gdb) run
The program being debugged has been started already.
Start it from the beginning? (y or n) y
Starting program: /home/dac/ClionProjects/openshell/openshell 
'PATH' is set to /home/dac/proj/google-cloud-sdk/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games.
> echo 'a b'
lexcode 3 Text echo mystring (null)
Becho
testlexcode 4 Text ' mystring (null)
lexcode 1 Text 
 mystring (null)
argument ::= ARGUMENT .
argumentList ::= argument .
command ::= FILENAME argumentList .
commandList ::= command .
 {(null)} {echo} {(null)}

Program received signal SIGSEGV, Segmentation fault.
0x0000000000402308 in main ()
(gdb) 

My grammar is

%{
    #include "shellparser.h"
    #include <string.h>
    char *mystring;
%}

%option reentrant
%option noyywrap

%x SINGLE_QUOTED
%x DOUBLE_QUOTED

%%

"|"                     { return PIPE; }

[ \t\r]                 { }
[\n]                    { return EOL; }

[a-zA-Z0-9_\.\-]+       { return FILENAME; }

[']                     { BEGIN(SINGLE_QUOTED); }
<SINGLE_QUOTED>[^']+    { printf("test");mystring = strdup(yytext); }

<SINGLE_QUOTED>[']      { BEGIN(INITIAL);
      /*  mystring contains the whole string now,
           yytext contains only "'" */
                          return ARGUMENT; }
<SINGLE_QUOTED><<EOF>>  { return -1; }

["]                     { BEGIN(DOUBLE_QUOTED); }
<DOUBLE_QUOTED>[^"]+    { }
<DOUBLE_QUOTED>["]      { BEGIN(INITIAL); return ARGUMENT; }
<DOUBLE_QUOTED><<EOF>>  { return -1; }

[^ \t\r\n|'"]+          { return ARGUMENT; }

%%

Then my main loop is

yylex_init(&scanner);
yyset_in(stdin, scanner);

shellParser = ParseAlloc(malloc);

params[0] = NULL;
printf("> ");
i=1;
do {
    lexCode = yylex(scanner);
    text = strdup(yyget_text(scanner));
    printf("lexcode %i Text %s mystring %s\n", lexCode, text, mystring);
    if (lexCode == 4) {
        params[i++] = mystring;
        if (strcmp(text, "\'\0")) {
            params[i++] = mystring;
        }
    } else
    if (lexCode != EOL) {
        params[i++] = text;
        printf("B%s\n", text);
    }
    Parse(shellParser, lexCode, text);
    if (lexCode == EOL) {
        dump_argv("Before exec_arguments", i, params);
        exec_arguments(i, params);
        corpse_collector();
        Parse(shellParser, 0, NULL);
        i=1;
    }
} while (lexCode > 0);
if (-1 == lexCode) {
    fprintf(stderr, "The scanner encountered an error.\n");
}
yylex_destroy(scanner);
ParseFree(shellParser, free);

Why is mystring null when I expect it to be something? I get a segmentation fault:

$ ./openshell 
'PATH' is set to /home/dac/proj/google-cloud-sdk/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games.
> echo 'a b'
lexcode 3 Text echo mystring (null)
Becho
testlexcode 4 Text ' mystring (null)
lexcode 1 Text 
 mystring (null)
argument ::= ARGUMENT .
argumentList ::= argument .
command ::= FILENAME argumentList .
commandList ::= command .
 {(null)} {echo} {(null)}
Segmentation fault (core dumped)

The whole project is on my github.

Answer 1

Because in

    lexCode = yylex(scanner);
    text = strdup(yyget_text(scanner));
    printf("lexcode %i Text %s mystring %s\n", lexCode, text, mystring);

mystring has not necessarilly been set by yylex. There is only one rule that sets it, so generally it will (still) be NULL, causing the segfault.

Answer 2

@PaulOgilvie explained pretty clearly why mystring might still be NULL after yylex() returns. As an otherwise-uninitialized global, mystring's initial value is NULL. After scanning the text "echo" from your example command, yylex() returns, not having set mystring, so at that point it is still NULL.

Do note, however, that that does not appear to be the proximal cause of your segfault. Your output shows the program proceeding past that point and in fact executing the printf() call immediately preceding the assignment to mystring. That strdup() with which you compute a value for mystring is a problem, however, because although yytext is a pointer to the start of the text of the token, it is not a pointer to a C string containing the token. Rather, it is a pointer to the location of the text in flex's buffer, and that text is not, generally, terminated at the end of the token.

flex provides global variable yyleng to tell you how long the text is, and you can use that to make a copy. For example, you could do this:

mystring = strndup(yytext, yyleng);

With that said, your output seems to show the scanning proceeding to completion (receiving token EOL, with value 1), in which case the crash is probably occurring in dump_argv() or even after. From the output, I'm guessing that you have a wild pointer or perhaps a pointer to an unterminated string somewhere in there. It's hard to tell, because you do not present the code for those functions.

Update: You do, still, seem to have mystring in the main loop not seeing the assignment performed by your scanner. The only plausible explanation for this is that they are not the same mystring. Perhaps you are declaring a static or local mystring in the scope of the main loop you presented. Note, too, that using flex's %reentrant option is intended to produce a scanner that avoids communicating via global variables, but you defeat that by introducing your own (mystring).