strcat segmentation fault

hi, i am getting a segmentation fault in the following program.

Code:

---

...
  FILE *fp;
  int helpSize=sizeof(char), i;
  char *helpText;
  char line[255];
  char *filename="HELP";
  GladeXML *xml;
  GtkWidget *mainTextbox;
 
  if ((fp = fopen(filename, "r")) == NULL) {
    fprintf(stdout, "Unable to open file <%s>\n", filename);
    exit(EXIT_FAILURE);
  }
 
  helpText = malloc(1);
  i = 1;
  while(fgets(line, sizeof(line), fp) != NULL) {
    printf("%d)", i++); //segmentation fault after the second append
    helpSize += strlen(line) * sizeof(char);
    realloc(helpText, helpSize);
    printf("%d\n", sizeof(helpText)); //size is always 4, what 4 though  and why?
    strcat(helpText, line);
  }
 
  //here goes the statement to display the help
  //xml = glade_get_widget_tree(GTK_WIDGET(button));
  //mainTextbox = glade_xml_get_widget (xml, "main_textbox");

  //gtk_text_insert(main_textbox, help, -1, -1);
  //gtk_text_insert(GTK_ENTRY(mainTextbox), NULL, NULL, NULL, "HELP!!", -1);
  //gtk_entry_set_text(mainTextbox, "HELP!!");
 
  free(helpText);
  fclose(fp);

---

when i am printf("%s", sixeof(helpText)) after the reallocation of space the answer is always 4. what does that mean? and why i am getting a segmentation fault? any help?

thanks

sorry for that but i dont know howto del this thread. anyway, needed to have a better look to my c reference... traced my error

Code:

---

helpText = realloc(helpText, sizeof(line));

---

>> and why i am getting a segmentation fault

you have to capture the return value of realloc because C passes variables by value:

>> after the reallocation of space the answer is always 4

when sizeof() is applied to a pointer, it returns the size of a pointer (4, in your case) - not the size of the memory it points to.

here is how you might recode all that:

Code:

---

char *helpText = NULL; // very important
 // ...
 int size = helpSize + (strlen(line) * sizeof(char));
 char * newblock = realloc(helpText, helpSize);
 if(newblock != NULL) {
  helpText = newblock;
  helpSize = size;
  } else {
  // handle error
  }

---

also, you might look up the documentation on realloc() to see why it works that way...

thanks sebastiani, your reply has given me even a deeper insight.