bison - YACC - strlen of $1 is 0 although string is there -


i having strange error in program

the structure of yystype is

%union {         char *text;         node *n; } %token <text> number

and grammar rule is

p:         number         {                 cout<<"$1 : "<<$1<<endl;                 int = 0;                 while($1[i])                 {                         cout<<"char : "<<$1[i++]<<endl;                 }                 $<n>$->left = $<n>$->right = null;                 char *test1 = new char[strlen($1)];                 strcpy(test1, $1);                 cout<<"len : "<<strlen($1)<<"test1 : "<<test1<<endl;                 char *lolz = strdup($1);                 cout<<"dup : "<<((uint64_t)lolz)<<' '<<((int)lolz[1])<<" : dup"<<endl;                 $<n>$->data = string($1);                 cout<<"nd : "<<$<n>$->data<<endl;                 print_tree($<n>$);         }         ;

i can print contents of $1, when strlen($1), returns 0 length causing strdup , string initialisation fail.

output:

$1 : 65301 char : 6 char : 5 char : 3 char : 0 char : 1 len : 0test1 : dup : 26935504 0 : dup segmentation fault (core dumped)

am missing obvious here?

when execute:

$<n>$->left = $<n>$->right = null;

what suppose value of $<n>$ is? have assigned address of node object?

to save time: haven't assigned think of uninitialised pointer; dereferencing uninitialised pointer undefined behaviour , corresponds see.

but analysis not quite accurate.

the bison-generated parser initialises $$ $1 prior executing action. in case, $1 union text member has been assigned to, using n member (a different) ub. result same in common compilers more predictable: suppose left element of node @ offset 0, assignment above overwrites first 16 bytes of character string zeros (8 if have 32-bit architecture). that's buffer overrun, if doesn't segfault, end result first byte of $1 0, hence return value of strlen. (when try use data element, segfault, apparently, presumably because not initialised std::string. using zero-length c-string not problem either strdup or std::string constructor.)

moral: never assign through pointer if don't know points to.

by way, strcpy test1 buffer overrun of 1 byte. seem have gotten away time it's bad habit.


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