Advantages of Alloca (The GNU C Library)

#### 3.2.7.2 Advantages of `alloca` ¶

Here are the reasons why `alloca` may be preferable to `malloc`:

- Using `alloca` wastes very little space and is very fast. (It is open-coded by the GNU C compiler.)
- Since `alloca` does not have separate pools for different sizes of blocks, space used for any size block can be reused for any other size. `alloca` does not cause memory fragmentation.
- Nonlocal exits done with `longjmp` (see Non-Local Exits) automatically free the space allocated with `alloca` when they exit through the function that called `alloca`. This is the most important reason to use `alloca`.

  To illustrate this, suppose you have a function `open_or_report_error` which returns a descriptor, like `open`, if it succeeds, but does not return to its caller if it fails. If the file cannot be opened, it prints an error message and jumps out to the command level of your program using `longjmp`. Let’s change `open2` (see `alloca` Example) to use this subroutine:

      int
      open2 (char *str1, char *str2, int flags, int mode)
      {
        char *name = (char *) alloca (strlen (str1) + strlen (str2) + 1);
        stpcpy (stpcpy (name, str1), str2);
        return open_or_report_error (name, flags, mode);
      }

  Because of the way `alloca` works, the memory it allocates is freed even when an error occurs, with no special effort required.

  By contrast, the previous definition of `open2` (which uses `malloc` and `free`) would develop a memory leak if it were changed in this way. Even if you are willing to make more changes to fix it, there is no easy way to do so.