Re: SegFault with Bison Grammar

[Kyle Brandt]

Hi Phil,
Thank you for the response and time, regarding what you said:

**GDB**
I have tried GDB, my gdb skills are weak trying to learn how to use it
better, but I am still not sure what is going on.  I will include the gdb
output at the end of the email which include the verbose Bison output.

**String Dumps with Lex:**
I think that is what I am doing:

[a-zA-Z/0-9\-_\.]+ {
    yylval.string=strdup(yytext); return(WORD);
}

**Using Union**
I am using Union and types, here they are:
%union {
    char * string;
    struct node * args;
    struct ext_cmd * cmd_w_args;
}

%token <string> CD WORD PWD EXIT EQUALS
%token ERROR HELP NEWLINE SEMI PVAR GT
%type <args> arg_list
%type <string> variable_assignment redirect
%type <cmd_w_args> external_cmd

**Malloc Tip**
Seems smart, going to implemement when I get a chance.

 **Memory Problems with Yacc/Lex**
"""Your not going to be able to fault memory corruption or problems on
bison or lex"""
My fault for not being clear, I am not trying to blame Bison at all.  I
figure it is myfault, I just meant I wasn't quite sure how things like $$
work, and if I am abusing something by do something like &($1)

***"Not going to debug this for you", and a little more targeted
information***
Well, I can always hope ;-)  I am just a little stuck, becuase I don't get
why my execute_command function, providing my external_cmd (which is typed
cmd_w_args) as an argument, would behave different depending on what rule it
is called from ( Works when I call it from 'command', but not from
'redirect').   They both seem to get reduced the same up the same rule:
line 109 (arg_list: WORD) to line 85( external_cmd: WORD arg_list).

**GDB Output with Bison debug**
Reading a token:
kbsh:/home/kbrandt/projects/svn/practicalc/simpleShell/preConcepts/chdir_and_printdir_lex_and_yacc%
ls /home
Next token is token WORD ()
Reducing stack by rule 4 (line 49):
-> $$ = nterm line ()
Stack now 0
Entering state 2
Next token is token WORD ()
Shifting token WORD ()
Entering state 6
Reading a token: Next token is token WORD ()
Shifting token WORD ()
Entering state 24
Reading a token: Next token is token NEWLINE ()
Reducing stack by rule 21 (line 109):
   $1 = token WORD ()
-> $$ = nterm arg_list ()
Stack now 0 2 6
Entering state 26
Reducing stack by rule 18 (line 85):
   $1 = token WORD ()
   $2 = nterm arg_list ()
-> $$ = nterm external_cmd ()
Stack now 0 2
Entering state 16
Next token is token NEWLINE ()
Reducing stack by rule 11 (line 70):
   $1 = nterm external_cmd ()

Breakpoint 1, execute_command (cmd=0x941a088, redirection=0) at command.c:18
18          struct node ** node_pp = cmd->args_pp;
(gdb) print *(cmd->args_pp)
$6 = (struct node *) 0x941a078
(gdb) print *(cmd->args_pp)->val
$7 = 47 '/'


Reading a token:
kbsh:/home/kbrandt/projects/svn/practicalc/simpleShell/preConcepts/chdir_and_printdir_lex_and_yacc%
ls /home > foo
Next token is token WORD ()
Reducing stack by rule 4 (line 49):
-> $$ = nterm line ()
Stack now 0
Entering state 2
Next token is token WORD ()
Shifting token WORD ()
Entering state 6
Reading a token: Next token is token WORD ()
Shifting token WORD ()
Entering state 24
Reading a token: Next token is token GT ()
Reducing stack by rule 21 (line 109):
   $1 = token WORD ()
-> $$ = nterm arg_list ()
Stack now 0 2 6
Entering state 26
Reducing stack by rule 18 (line 85):
   $1 = token WORD ()
   $2 = nterm arg_list ()
-> $$ = nterm external_cmd ()
Stack now 0 2
Entering state 16
Next token is token GT ()
Shifting token GT ()
Entering state 29
Reading a token: Next token is token WORD ()
Shifting token WORD ()
Entering state 33
Reducing stack by rule 8 (line 60):
   $1 = nterm external_cmd ()
   $2 = token GT ()
   $3 = token WORD ()
Redirecting stdout of ls to foo
DEBUG: GT

Breakpoint 1, execute_command (cmd=0x9b80088, redirection=0) at command.c:18
18          struct node ** node_pp = cmd->args_pp;
(gdb) print cmd->args_pp
$4 = (struct node **) 0xbfd4cc40
(gdb) print *(cmd->args_pp)
$5 = (struct node *) 0x9b80068
(gdb) print *(cmd->args_pp)->val
Cannot access memory at address 0x6d6f682f

Thanks again,
Kyle