3.2.6.2 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.