[Top][All Lists]

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: Using alloca

From: Akim Demaille
Subject: Re: Using alloca
Date: 14 Dec 2000 14:03:24 +0100
User-agent: Gnus/5.0807 (Gnus v5.8.7) XEmacs/21.1 (Channel Islands)

>>>>> "Hans" == Hans Aberg <address@hidden> writes:

Hans> What is the advantage of using alloca over malloc?


Automatic Storage with Variable Size

   The function `alloca' supports a kind of half-dynamic allocation in
which blocks are allocated dynamically but freed automatically.

   Allocating a block with `alloca' is an explicit action; you can
allocate as many blocks as you wish, and compute the size at run time.
But all the blocks are freed when you exit the function that `alloca'
was called from, just as if they were automatic variables declared in
that function.  There is no way to free the space explicitly.

   The prototype for `alloca' is in `stdlib.h'.  This function is a BSD

 - Function: void * alloca (size_t SIZE);
     The return value of `alloca' is the address of a block of SIZE
     bytes of memory, allocated in the stack frame of the calling

   Do not use `alloca' inside the arguments of a function call--you
will get unpredictable results, because the stack space for the
`alloca' would appear on the stack in the middle of the space for the
function arguments.  An example of what to avoid is `foo (x, alloca
(4), y)'.

`alloca' Example

   As an example of the use of `alloca', here is a function that opens
a file name made from concatenating two argument strings, and returns a
file descriptor or minus one signifying failure:

     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 (name, flags, mode);

Here is how you would get the same results with `malloc' and `free':

     open2 (char *str1, char *str2, int flags, int mode)
       char *name = (char *) malloc (strlen (str1) + strlen (str2) + 1);
       int desc;
       if (name == 0)
         fatal ("virtual memory exceeded");
       stpcpy (stpcpy (name, str1), str2);
       desc = open (name, flags, mode);
       free (name);
       return desc;

   As you can see, it is simpler with `alloca'.  But `alloca' has
other, more important advantages, and some disadvantages.

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
     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:

          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.

Disadvantages of `alloca'

   These are the disadvantages of `alloca' in comparison with `malloc':

   * If you try to allocate more memory than the machine can provide,
     you don't get a clean error message.  Instead you get a fatal
     signal like the one you would get from an infinite recursion;
     probably a segmentation violation (*note Program Error Signals::).

   * Some non-GNU systems fail to support `alloca', so it is less
     portable.  However, a slower emulation of `alloca' written in C is
     available for use on systems with this deficiency.

reply via email to

[Prev in Thread] Current Thread [Next in Thread]