Use of freed memory

If your program tries to read from or write to memory that was previously freed, the program will generate a memory error. For example, if it calls the free function for a particular block and then continues to use that block, the program will create a memory reuse problem when a malloc call is next made.


Using freed memory generates the following runtime errors:
  • memory corruption (may cause an unpredictable failure in the future)
  • random data read (when a heap block is re-used, other data can be in that location)

Detecting the error

Depending on which error checks are enabled under Memory Errors in the configuration, the Memory Analysis tool can detect when free memory is read from or written to in the following conditions:
  • Verify parameters in string and memory functions—when library functions read memory referred to by a pointer that's known to be free
  • Enabled bounds checking (where possible)—when a newly allocated block contains altered data, meaning it was modified after it was previously deallocated

For the list of functions that trap these error conditions, see the Trap Function field, below.

Information returned by the Memory Problems view

The notification for this type of memory error includes the following details:
  • Severity: ERROR
  • Description: data in freed memory block has been modified
  • Pointer: address of the freed memory block
  • Trap Function: one of: malloc calloc realloc free strcat strdup strncat strcmp strncmp strcpy strncpy strlen strchr strrchr index rindex strpbrk strspn strcspn strstr strtok memccpy memchr memmove memcpy memcmp memset bcopy bzero bcmp
  • Alloc Kind: how memory was allocated for this block (malloc, calloc, or realloc)
  • Location: source file and line of code where the error occurred (e.g., main.c:59)
  • Count: number of blocks involved

How to address freed memory usage

Set the pointer of the freed memory to NULL immediately after the call to free, unless it's a local variable that goes out of scope in the next line of the program.


The following code shows an example of using freed memory:
int main(int argc, char ** argv) {
  char * ptr = NULL;
  ptr = malloc(13);
  strcpy(ptr,"Hello World!");
  return 0;