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(libc.info)Advantages of Alloca

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Advantages of `alloca'
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   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
     block, space used for any size block can be reused for any other
     size.  `alloca' does not cause memory fragmentation.

   * Nonlocal exits done with `longjmp' (Note: 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' (Note: 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.