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A simple, yet effective way to avoid double-free and access-freed-memory vulnerabilities is to set pointers to NULL after they have been freed. Calling free() on a NULL pointer incurs results in no action being taken by free(). As a result, freed pointers can be safely set to NULL to help eliminate memory related vulnerabilities.

...

if (!strcmp(message,value_1)) {
  process_message(message);
  free(message);    
}
if (!strcmp(message,value_2)) {
  process_message(message);
  free(message);
}

...

if (!strcmp(message,value_1)) {
  process_message(message);
  free(message);
  message = NULL;
}
if (!strcmp(message,value_2)) {
  process_message(message);
  free(message);
  message = NULL;
}

References

• ISO/IEC 9899-1999 Section 7.20.3.2 The free function
• Seacord 05 Chapter 4 Dynamic Memory Management