c - segmentation fault (core dumped) Error while calling the mknode function -

define yystype struct node1 *

%token int float char double void

start: declaration function declaration1 {$$ = mknode($1, $2, $3,null,0);}

|declaration function             {$$=mknode($1,$2,null,null,0);}     | declaration                 {$$ =mknode($1,null,null,null,0); }  | function                {$$ =mknode($1,null,null,null,0); } ;  declaration1 :function           {$$ =mknode($1,null,null,null,null); } ;   function:   type id '(' arglistopt ')' compoundstmt         {$$ = mknode($1,$2,$4,$6,null); } ; 

type: int {$$ = mknode(null,null,null,null,"int"); }

| float {$$ = mknode(null,null,null,null,"float"); }

| char {$$ = mknode(null,null,null,null,"char"); }

| double {$$ = mknode(null,null,null,null,"double"); }

| void {$$ = mknode(null,null,null,null,"void"); }

;

compoundstmt: '{' stmtlist '}'      {$$ =$2; } ;  stmtlist: stmtlist stmt            {$$ = mknode($1,$2,null,null,null); }  |                   {$$=mknode(null,null,null,null,null);} ;  when running on input   int mian() {}   giving segementation fault @  char* newnode =(char*)malloc(strlen(token));    node1 *mknode(node1 *left1, node1 *left2, node1* left3,node1* left4,char *token) {    /* malloc node */    node1 *newnode = (node1 *)malloc(sizeof(node1));      char *newstr = (char *)malloc(strlen(token));    strcpy(newstr, token);      newnode->left1 = left1;    newnode->left2 = left2;  newnode->left3 = left3;    newnode->left4 = left4;    newnode->token = newstr;    return(newnode); } 

what error here? plz help

while should run program in debugger find out, these 2 lines culprit:

char *newstr = (char *)malloc(strlen(token));  strcpy(newstr, token); 

if token null both strlen , strcpy crash.

dereferencing null pointer undefined behavior , in situations cause crash. before dereferencing pointer, or calling function does, need check if pointer null or not.