Freeing after Malloc - The GNU C Library

Next: Changing Block Size, Previous: Malloc Examples, Up: Unconstrained Allocation

##### 3.2.2.3 Freeing Memory Allocated with `malloc`

When you no longer need a block that you got with `malloc`, use the function `free` to make the block available to be allocated again. The prototype for this function is in `stdlib.h`.

— Function: void free (void *ptr)

> The `free` function deallocates the block of memory pointed at by ptr.

— Function: void cfree (void *ptr)

> This function does the same thing as `free`. It's provided for backward compatibility with SunOS; you should use `free` instead.

Freeing a block alters the contents of the block. Do not expect to find any data (such as a pointer to the next block in a chain of blocks) in the block after freeing it. Copy whatever you need out of the block before freeing it! Here is an example of the proper way to free all the blocks in a chain, and the strings that they point to:

         struct chain
           {
             struct chain *next;
             char *name;
           }

         void
         free_chain (struct chain *chain)
         {
           while (chain != 0)
             {
               struct chain *next = chain->next;
               free (chain->name);
               free (chain);
               chain = next;
             }
         }

Occasionally, `free` can actually return memory to the operating system and make the process smaller. Usually, all it can do is allow a later call to `malloc` to reuse the space. In the meantime, the space remains in your program as part of a free-list used internally by `malloc`.

There is no point in freeing blocks at the end of a program, because all of the program's space is given back to the system when the process terminates.