In C programs, dynamic management of memory blocks is normally done with the functions malloc, realloc, and free. Guile has additional functions for dynamic memory allocation that are integrated into the garbage collector and the error reporting system.
Memory blocks that are associated with Scheme objects (for example a
foreign object) should be allocated with scm_gc_malloc
or
scm_gc_malloc_pointerless
. These two functions will either
return a valid pointer or signal an error. Memory blocks allocated this
way may be released explicitly; however, this is not strictly needed,
and we recommend not calling scm_gc_free
. All memory
allocated with scm_gc_malloc
or scm_gc_malloc_pointerless
is automatically reclaimed when the garbage collector no longer sees any
live reference to it18.
When garbage collection occurs, Guile will visit the words in memory
allocated with scm_gc_malloc
, looking for live pointers. This
means that if scm_gc_malloc
-allocated memory contains a pointer
to some other part of the memory, the garbage collector notices it and
prevents it from being reclaimed19. Conversely, memory allocated with
scm_gc_malloc_pointerless
is assumed to be “pointer-less” and
is not scanned for pointers.
For memory that is not associated with a Scheme object, you can use
scm_malloc
instead of malloc
. Like
scm_gc_malloc
, it will either return a valid pointer or signal
an error. However, it will not assume that the new memory block can
be freed by a garbage collection. The memory must be explicitly freed
with free
.
There is also scm_gc_realloc
and scm_realloc
, to be used
in place of realloc
when appropriate, and scm_gc_calloc
and scm_calloc
, to be used in place of calloc
when
appropriate.
The function scm_dynwind_free
can be useful when memory should be
freed with libc’s free
when leaving a dynwind context,
See Dynamic Wind.
void *
scm_malloc (size_t size)
¶void *
scm_calloc (size_t size)
¶Allocate size bytes of memory and return a pointer to it. When
size is 0, return NULL
. When not enough memory is
available, signal an error. This function runs the GC to free up some
memory when it deems it appropriate.
The memory is allocated by the libc malloc
function and can be
freed with free
. There is no scm_free
function to go
with scm_malloc
to make it easier to pass memory back and forth
between different modules.
The function scm_calloc
is similar to scm_malloc
, but
initializes the block of memory to zero as well.
These functions will (indirectly) call
scm_gc_register_allocation
.
void *
scm_realloc (void *mem, size_t new_size)
¶Change the size of the memory block at mem to new_size and
return its new location. When new_size is 0, this is the same
as calling free
on mem and NULL
is returned. When
mem is NULL
, this function behaves like scm_malloc
and allocates a new block of size new_size.
When not enough memory is available, signal an error. This function runs the GC to free up some memory when it deems it appropriate.
This function will call scm_gc_register_allocation
.
void *
scm_gc_malloc (size_t size, const char *what)
¶void *
scm_gc_malloc_pointerless (size_t size, const char *what)
¶void *
scm_gc_realloc (void *mem, size_t old_size, size_t new_size, const char *what);
¶void *
scm_gc_calloc (size_t size, const char *what)
¶Allocate size bytes of automatically-managed memory. The memory is automatically freed when no longer referenced from any live memory block.
When garbage collection occurs, Guile will visit the words in memory
allocated with scm_gc_malloc
or scm_gc_calloc
, looking for
pointers to other memory allocations that are managed by the GC. In
contrast, memory allocated by scm_gc_malloc_pointerless
is not
scanned for pointers.
The scm_gc_realloc
call preserves the “pointerlessness” of the
memory area pointed to by mem. Note that you need to pass the old
size of a reallocated memory block as well. See below for a motivation.
void
scm_gc_free (void *mem, size_t size, const char *what)
¶Explicitly free the memory block pointed to by mem, which was
previously allocated by one of the above scm_gc
functions. This
function is almost always unnecessary, except for codebases that still
need to compile on Guile 1.8.
Note that you need to explicitly pass the size parameter. This is done since it should normally be easy to provide this parameter (for memory that is associated with GC controlled objects) and help keep the memory management overhead very low. However, in Guile 2.x, size is always ignored.
void
scm_gc_register_allocation (size_t size)
¶Informs the garbage collector that size bytes have been allocated, which the collector would otherwise not have known about.
In general, Scheme will decide to collect garbage only after some amount of memory has been allocated. Calling this function will make the Scheme garbage collector know about more allocation, and thus run more often (as appropriate).
It is especially important to call this function when large unmanaged allocations, like images, may be freed by small Scheme allocations, like foreign objects.
void
scm_dynwind_free (void *mem)
¶Equivalent to scm_dynwind_unwind_handler (free, mem,
SCM_F_WIND_EXPLICITLY)
. That is, the memory block at mem will be
freed (using free
from the C library) when the current dynwind is
left.
Return an alist ((what . n) ...) describing number
of malloced objects.
what is the second argument to scm_gc_malloc
,
n is the number of objects of that type currently
allocated.
This function is only available if the GUILE_DEBUG_MALLOC
preprocessor macro was defined when Guile was compiled.
In Guile up to version 1.8, memory
allocated with scm_gc_malloc
had to be freed with
scm_gc_free
.
In Guile up to 1.8, memory
allocated with scm_gc_malloc
was not visited by the
collector in the mark phase. Consequently, the GC had to be told
explicitly about pointers to live objects contained in the memory block,
e.g., via SMOB mark functions (see scm_set_smob_mark
)