mulle-allocator

🔄 Flexible C memory allocation scheme

... provides a way to pass around the memory scheme du jour

... has identical API to malloc, realloc, free

... frees your code from having to check for errors when allocating memory

... contains a safer and portable alloca

mulle-allocator has a companion project: mulle-testallocator. mulle-testallocator provides the error and leak detection, that was formerly a part of mulle-allocator..

Release Version	Release Notes
	RELEASENOTES

API

• Allocator

Use mulle_malloc instead of malloc to reduce code size

Instead of:

   if( ! malloc( 1848))
   {
      perror( "malloc:");
      exit( 1);
   }
   if( ! calloc( 18, 48))
   {
      perror( "calloc:");
      exit( 1);
   }
   s = strdup( "VfL Bochum 1848");
   if( ! s)
   {
      perror( "strdup:");
      exit( 1);
   }
   if( ! realloc( s, 18);
   {
      perror( "realloc:");
      exit( 1);
   }
   free( s);

write

   mulle_malloc( 1848);
   mulle_calloc( 18, 48);
   s = mulle_strdup( "VfL Bochum 1848");
   mulle_realloc( s, 18);
   mulle_free( s);

You don't have to check for out of memory error conditions anymore. Otherwise your code will run the same and a possible performance degradation because of the indirection will be hardly measurable.

How mulle-allocator deals with memory shortage

By the C standard malloc returns NULL and sets errno to ENOMEM, if it can't satisfy the memory request. Here are the two most likely scenarios why this happens:

The caller specified a huge amount of memory, that the OS can't give you. Typically (size_t) -1 can never work. This is considered to be a bug on the callers part.

Or the program has exhausted all memory space available to the process. Here is what happens on various OS:

The gist is, that in portable programs it doesn't really make sense to rely on malloc returning NULL and doing something clever based on it.

If we define the mulle-allocator malloc to always return a valid memory block - discounting erroneous parameters as programmers error, to be caught during development - then memory calling code simplifies from:

   p = malloc( size);
   if( ! p)
      return( -1);
   memcpy( p, q, size);

to

   p = mulle_malloc( size);
   memcpy( p, q, size);

Use mulle_allocator to make your code more flexible

You can make your code, and especially your data structures, more flexible by using a mulle_allocator. This decouples your data structure from stdlib. It enables your data structure to reside in shared or wired memory with no additional code. Your API consumers just have to pass their own allocators.

Also it can be helpful to isolate your data structure memory allocation during tests. This way, other, possibly benign, code leaks, do not obscure the test.

What is an allocator ?

The mulle_allocator struct is a collection of function pointers, with one added pointer for aba and looks like this:

struct mulle_allocator
{
   void   *(*calloc)( size_t n, size_t size, struct mulle_allocator *p);
   void   *(*realloc)( void *block, size_t size, struct mulle_allocator *p);
   void   (*free)( void *block, struct mulle_allocator *p);
   void   (*fail)( struct mulle_allocator *p, void *block, size_t size) MULLE_C_NO_RETURN;
   int    (*abafree)( void *aba, void (*free)( void *), void *block);
   void   *aba;
};

By default .aba and .abafree are not available. If you need ABA safe freeing, it is recommended to use mulle-aba.

You should not jump through the vectors directly, but use supplied inline functions like mulle_allocator_malloc, as they perform the necessary return value checks (see below: Dealing with memory shortage).

The shortcut functions like mulle_malloc use the mulle_default_allocator by default and save you some type-work. You can also use NULL as the allocator for mulle_allocator_malloc with the same effect as choosing the mulle_default_allocator.

Embedding the allocator in your data structure

A pointer to the allocator could be kept in your data structure. This simplifies your API, as the allocator is only needed during creation. Here is an example how to use the allocator in this fashion:

struct my_string
{
   struct mulle_allocator   *allocator;
   char                     s[ 1];
};


struct my_string   *my_string_alloc( char *s, struct mulle_allocator *allocator)
{
   size_t              len;
   struct my_string    *p;

   len = s ? strlen( s) : 0;
   p   = mulle_allocator_malloc( allocator, sizeof( struct my_string) + len);
   dst->allocator = allocator;
   memcpy( p->s, s, len);
   p->s[ len] = 0;
   return( p);
}


static inline void   my_string_free( struct my_string *p)
{
   mulle_allocator_free( p->allocator, p);
}

Not embedding the allocator in your data structure

But if you don't want to store the allocator inside the data structure, you can pass it in again:

struct my_other_string
{
   char   s[ 1];
};


struct my_other_string   *my_other_string_alloc( char *s, struct mulle_allocator *allocator)
{
   size_t                    len;
   struct my_other_string    *p;

   len = s ? strlen( s) : 0;
   p   = mulle_allocator_malloc( allocator, sizeof( struct my_other_string) + len);
   memcpy( p->s, s, len);
   p->s[ len] = 0;
   return( p);
}


static inline void   my_other_string_free( struct my_other_string *p,
                                           struct mulle_allocator *allocator)
{
   mulle_allocator_free( allocator, p);
}

The disadvantage is, that this opens the door for bugs, as you may be using different allocators accidentally.

Use mulle_alloca_do like a better alloca

mulle-allocator provides the mulle_alloca_do macro for stack based storage, that gets swapped to heap based when the size exceeds a certain compile time size.

What is alloca ? Excerpts from man alloca

The alloca() function allocates size bytes of space in the stack frame of the caller. This temporary space is automatically freed the function that called alloca() returns to its caller... There is no error indication, if the stack frame cannot be extended... For certain applications, its use can improve efficiency compared to the use of malloc plus free... Otherwise, its use is discouraged...

Here is an example function, where the use of alloca avoids a malloc and free call. Getting stack space is super cheap and the automatic reclamation makes the code easier to write:

void  print_uppercase( char *s)
{
   char     *copy;
   size_t   i;
   size_t   len;

   len  = strlen( s) + 1;
   copy = alloca( len);

   for( i = 0; i < len; i++)
     copy[ i] = toupper( s[ i]);

   printf( "%s\n", copy);
}

The problems are two-fold: alloca may not be available (or is "hidden" in a non standard header, which decreases portability). The length of s is unknown and the available stack size is also unknown. So this code is likely to crash for large strings. How large ? Hard to say...

mulle_alloca_do uses a certain safe amount (the default is 128 bytes) of stack space for a temporary allocation.

Note

What is a safe amount ? The author thinks double[8] should be fine. If your system is very tiny or very large, you can set MULLE_ALLOCA_STACKSIZE to a size of your liking. Remember though, that if you call other functions that also use mulle_alloca_do the reasonable stack size is halved (and so on). Use mulle_alloca_do_flexible if you want to set the stack size per macro invocation.

But if the requests exceeds this amount mulle_alloca_do will fallback to malloc/free. See above code transformed to mulle_alloca_do:

void  print_uppercase( char *s)
{
   // char     *copy;  // no longer needed, will be declared in mulle_alloca_do
   size_t   i;
   size_t   len;

   len  = strlen( s) + 1;
   mulle_alloca_do( copy, char, len) // need type to alloca for alignment
   {
      for( i = 0; i < len; i++)
        copy[ i] = toupper( s[ i]);

      printf( "%s\n", copy);
   }
}

The scope of copy is now tied to mulle_alloca_do and not the function!

/* copy not yet in scope */

mulle_alloca_do( copy, char, size)
{
   /* code block, where copy is valid */
}

/* copy no longer in scope (and freed) */

There is quite a lot more to read about mulle_alloca_do, so check out the header mulle-alloca.h for more details, including break protection.

Caveats

The mulle_default_allocator and mulle_stdlib_allocator never return, if the allocation went bad. If a mulle-allocator function detects, that an allocation could not be satisfied, it jumps through its fail vector. This will print an error message and exit the program.

You can not pass a zero size to either mulle_realloc or mulle_malloc without getting a failure. If you want to free memory with realloc - by passing a zero block size - you need to use mulle_realloc_strict. If you pass a zero block size and a zero block to mulle_realloc_strict, it will return NULL.

You are here

Add

This project is a component of the mulle-core library. As such you usually will not add or install it individually, unless you specifically do not want to link against mulle-core.

Add as an individual component

Use mulle-sde to add mulle-allocator to your project:

mulle-sde add github:mulle-c/mulle-allocator

To only add the sources of mulle-allocator with dependency sources use clib:

clib install --out src/mulle-c mulle-c/mulle-allocator

Add -isystem src/mulle-c to your CFLAGS and compile all the sources that were downloaded with your project.

Install

Install with mulle-sde

Use mulle-sde to build and install mulle-allocator and all dependencies:

mulle-sde install --prefix /usr/local \
   https://github.com/mulle-c/mulle-allocator/archive/latest.tar.gz

Manual Installation

Install the requirements:

Download the latest tar or zip archive and unpack it.

Install mulle-allocator into /usr/local with cmake:

cmake -B build \
      -DCMAKE_INSTALL_PREFIX=/usr/local \
      -DCMAKE_PREFIX_PATH=/usr/local \
      -DCMAKE_BUILD_TYPE=Release &&
cmake --build build --config Release &&
cmake --install build --config Release

Author

Nat! for Mulle kybernetiK