Learning c getting position pointer out of strtok as int - c

Question

Beginner here,
somewhat confused about an exercise:
Tutorial Last one on the page (it is german). I should read HTML-Lines and print attributes and their values. The declaration of the function which should be used is given.
Two things irritate me:
1. The Line is stored in a const char string, but i would like the User to type in his desired HTML-line. It seems not to be possible to change a const variable at runtime. How can it be achieved without changing the given declaration?
2. The Tutorial wants me to give back the position of strtok-search as an integer, but I read online that this value is stored within strtok, is there a way to cast that, or get it somehow?
To solve the exercise I wrote this code, but the program crashes at runtime with "Segmentation fault (core dumped)"-Message and I don't know why, could someone please explain that to me? (I probably need malloc, but for which variable?)
//cHowTo Uebung Teil 2 Nr. 4
//HTMLine.c
#include<stdio.h>
#include<stdlib.h>
#include<string.h>
//char getHTMLline ();
int getHtmlAttributes(const char *string, int start, char *attrNamem,char *attrValue); //given by Tutorial
int main(int argc, char *argv) //because i want user-input later on, if possible
{
const char strg[]= {"<img src=\"kurt.jpg\" width=\"250\" alt=\"Kurt Kanns\" />"}; //given example line by tutorial
char attriN[255]={0}, attriV[255]={0};
int pos=99;
//printf("Please type the tag for analysis.\n");
//fgets(strg, 255, stdin);
printf("attribute\tvalue\n\n");
do
{
pos = getHtmlAttributes(strg, pos, attriN, attriV); //pos should be strtok-search-position
printf("%s\t\t%s\n", attriN, attriV);
}
while(pos!=1);
return EXIT_SUCCESS;
}
int getHtmlAttributes(const char *string, int start, char *attrNamem, char *attrValue)
{
int i, len;
char *ptr;
len = strlen(string);
char stringT[len]; //variable used to be split by strtok
for(i=0; i<len; i++)
stringT[i]=string[i];//copy string to stringT
if(start==99)
ptr = strtok(stringT, "<="); //get first attribute as whole
else
ptr = strtok(NULL, "= "); // get following attributes
for(i=0; i<len; i++)
attrNamem[i] = ptr[i];
ptr = strtok(NULL, "\""); //get values
for(i=0; i<len; i++)
attrValue[i] = ptr[i];
if(ptr == NULL) //if search complete
{
return 1;
}
else // if search continues
{
return 0;
}
}
//char getHTMLline ()
//{
// char user_input;
// scanf("%s", &user_input);
// return user_input;
//}

Answer 1

What strtok() does is that if you call it with a string different to NULL it stores a pointer to that string internally and returns the first token. A subsequent call with NULL then uses that internally stored pointer to determine the next token.
Now what happens in your code is:
When you call getHtmlAttributes() for the first time you create a copy of the given string in stringT and pass that copy to strtok(). The next time you call strtok( NULL, ... ). And there we have two bugs:
your loop to copy string() to stringT() is not correct. You don't copy
the teminating '\0'. Just use strcpy() in such cases
the important one: When you call getHtmlAttributes() for the second time, you call strtok( NULL, ... ) but the lifetime of the stringT that it has been called with originally has ended with the first call of getHtmlAttributes() returning, because stringT is a local variable that is created anew on the stack every time the function is called. You could solve that problem by either
declaring static char stringT[N] where N must be a constant (like 255), you can't use len (what should have been len+1 anyway) in that case
creating a dynamically allocated copy of string by char *stringT = strdup( string );. Please do that only if you call strtok( stringT, ... ) aferwards and be aware that without additional code you have a memory leak because you are not able to free that memory again.
what I would pefer: use string directly instead of stringT. In that case you should not declare string as const and create a copy of strg in main() that you pass to the function
Edit
I think the request "give back the position of strtok-search as an integer" means, you should return the offset of the found token in the complete string. That's quite easy to achieve, if you use the suggested solution static char stringT[N] from above:
As strtok() works on the string passed by the first call, after receiving a ptr different from NULL, you can calculate and return int offset = ptr - stringT;
Edit 2
And now something completely different
I've just read the Tutorial you have linked to (greetings from Hessen :-) ) and I think the idea is to use a new strtok() loop every time the function is called. That could like this:
int getHtmlAttributes(const char *string, int start, char *attrName, char *attrValue)
{
char *buf;
char *attrptr;
char *ptr;
// copy the substring starting at start to buf
// you should add error checking (buf may become NULL)
buf = strdup( string+start );
// first step: tokenize on whitespace; we stop at the first token that contains a "="
for( attrptr = strtok( buf, " \t" ); attrptr && (ptr = strchr( attrptr, '=' )) != NULL; attrptr = strtok( NULL, " \t" ) ) ;
if( attrptr ) {
// copy everything before "=" to attrName
sprintf( attrName, "%.*s", ptr-attrptr, attrptr );
// copy everything after "=" to attrValue
strcpy( attrValue, ptr+1 );
// the next start is the current start + the offset of the attr found
// + the length of the complete attr
start += (attrptr - buf) + strlen( attrptr );
free( buf );
return start;
} else {
// no more attribute
free( buf );
return -1;
}
}