realloc and free causes "double free or corruption"

Question

Bear with me. I have not coded in c in 8 years and am totally baffled why my string manipulation is not working. I am writing a program that loops forever. In the loop I initialize two char pointers each is passed to a function that add text to the char pointer (array). When the functions are done I print the char pointer and free the two char pointers. However the program dies after 7 iterations with the following error message

* glibc detected * ./test: double free or corruption (fasttop): 0x0804a168 ***

#include sys/types.h
#include sys/stat.h
#include fcntl.h
#include string.h
#include stdio.h
#include stdlib.h
#include errno.h
#include time.h

char *SEPERATOR = "|";

void getEvent (char* results);
void getTimeStamp(char* timeStamp, int timeStampSize);
void stringAppend(char* str1, char* str2);

int main (int argc, char *argv[])
{
  int i = 0; 
  while(1)
  { 
    i++;
    printf("%i", i);    

    char* events= realloc(NULL, 1); 
    events[0] = '\0';
    getEvent(events);

    char* timestamp= realloc(NULL, 20);
    timestamp[0] = '\0';
    getTimeStamp(timestamp, 20);

    printf("%s", events);
    printf("timestamp: %s\n", timestamp);

    free(events);
    free(timestamp);
  } 
}

void getEvent (char* results)
{
  stringAppend(results, "a111111111111");
  stringAppend(results, "b2222222222222");
}

void getTimeStamp(char* timeStamp, int timeStampSize)
{
  struct tm *ptr;
  time_t lt;
  lt = time(NULL);
  ptr = localtime(&lt);
  int r = strftime(timeStamp, timeStampSize, "%Y-%m-%d %H:%M:%S", ptr);
}

void stringAppend(char* str1, char* str2)
{   
  int arrayLength = strlen(str1) + strlen(str2) + strlen(SEPERATOR) + 1;
  printf("--%i--",arrayLength);

  str1 = realloc(str1, arrayLength);
  if (str1 != NULL)
  {
    strcat(str1, SEPERATOR);
    strcat(str1, str2);
  }
  else
  {
    printf("UNABLE TO ALLOCATE MEMORY\n");
  }
}

Answer 1

The problem is that while stringAppend reallocates the pointers, only stringAppend is aware of this fact. You need to modify stringAppend to take pointer-to-pointers (char **) so that the original pointers are updated.