Assigning Array Elements By Reference

**bhenderson** · 08-22-2009

Hi All,
Can you spot an error in my code? It shows no warnings but causes a seg fault. I've isolated it down to the wrd[] lines in red. Rest of the code available if needed.

Thanks in advance.

Code:

char * fetchword( FILE* ifp, char* *wrd, int* wrd_count ) {
  unsigned int c ;      /*character to be added to end of current string*/
  int wrd_len = 0 ;     /*number of characters to be in new string*/

  /*other stuff done here*/

  *wrd = realloc( *wrd, wrd_len * sizeof(char) ) ;    /*expand current array by one char*/
      if( *wrd == NULL ) {
        perror("realloc returned NULL. Unable to allocate memory.") ;
        exit (-1) ;
      }
     *wrd[wrd_len-1] = c ;            /*replace existing '\0' with new char*/
     *wrd[wrd_len] = '\0' ;             /*add the string terminator after new char*/

  return *wrd ;

**tabstop** · 08-22-2009

If you alloc wrd_len number of elements, *wrd[wrd_len] doesn't exist.

**bhenderson** · 08-22-2009

if *wrd[] doesn't exist, what can I use?

Thanks

**tabstop** · 08-22-2009

I never said *wrd[] didn't exist. I said *wrd[wrd_len] didn't exist. If you ask for six things, and then try to use the seventh, what do you expect?

**bhenderson** · 08-22-2009

Oh right, yes, that would be obvious then. Sorry for being dumb!

**bhenderson** · 08-23-2009

I'm pulling my hair out with this one now. I thought I had it sorted last night, but seemingly not! Any clues guys? They'd be really appreciated.

Code:

char * fetchword( FILE* ifp, char* *wrd, int* wrd_count ) {

  /*other stuff here*/

  *wrd = realloc( *wrd, (wrd_len+1) * sizeof(char) ) ;   
      *wrd[(wrd_len-1)] = (char)c ; /*seg fault*/
      *wrd[(wrd_len)] = '\0' ;             /*seg fault*/
      /* *wrd[0] = (char)c; */              /*works fine*/

return *wrd ;

**itCbitC** · 08-23-2009

An array and index expression is the same as a pointer and an offset so don't throw 'em into the same mix

Code:

*wrd[(wrd_len-1)] = (char)c;

Edit: Whoops! didn't realize that wrd is a double-indirect pointer. Have you allocated storage for wrd before moving onto *wrd?

**tabstop** · 08-23-2009

Originally Posted by bhenderson

I'm pulling my hair out with this one now. I thought I had it sorted last night, but seemingly not! Any clues guys? They'd be really appreciated.

Code:

char * fetchword( FILE* ifp, char* *wrd, int* wrd_count ) {

  /*other stuff here*/

  *wrd = realloc( *wrd, (wrd_len+1) * sizeof(char) ) ;   
      *wrd[(wrd_len-1)] = (char)c ; /*seg fault*/
      /* (*wrd)[wrd_len-1] = (char)c; */
      *wrd[(wrd_len)] = '\0' ;             /*seg fault*/
      /* (*wrd)[wrd_len] = '\0'; */
      /* *wrd[0] = (char)c; */              /*works fine*/

return *wrd ;

Check the blue bits -- I bet we need to specify order of operations, otherwise the indexing will happen first and then the * will happen at the end.

**bhenderson** · 08-23-2009

Thanks tapstop, that makes sense to me and fixes the issue. Hurray!

-The array and index are mixed because wrd is a char**.

Working Code:

Code:

(*wrd)[wrd_len-1] = (char)c ;
(*wrd)[wrd_len] = '\0' ;

Thanks to everyone else for their effort as well!

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