Memory Allocation

Memory Allocation — general memory-handling

Synopsis


#include <glib.h>


#define             g_new                               (struct_type, n_structs)
#define             g_new0                              (struct_type, n_structs)
#define             g_renew                             (struct_type, mem, n_structs)
#define             g_try_new                           (struct_type, n_structs)
#define             g_try_new0                          (struct_type, n_structs)
#define             g_try_renew                         (struct_type, mem, n_structs)

gpointer            g_malloc                            (gsize n_bytes);
gpointer            g_malloc0                           (gsize n_bytes);
gpointer            g_realloc                           (gpointer mem,
                                                         gsize n_bytes);
gpointer            g_try_malloc                        (gsize n_bytes);
gpointer            g_try_malloc0                       (gsize n_bytes);
gpointer            g_try_realloc                       (gpointer mem,
                                                         gsize n_bytes);

void                g_free                              (gpointer mem);
extern              gboolean g_mem_gc_friendly;

#define             g_alloca                            (size)
#define             g_newa                              (struct_type, n_structs)

#define             g_memmove                           (dest,src,len)
gpointer            g_memdup                            (gconstpointer mem,
                                                         guint byte_size);

                    GMemVTable;
void                g_mem_set_vtable                    (GMemVTable *vtable);
gboolean            g_mem_is_system_malloc              (void);

extern              GMemVTable	*glib_mem_profiler_table;
void                g_mem_profile                       (void);

Description

These functions provide support for allocating and freeing memory.

Note

If any call to allocate memory fails, the application is terminated. This also means that there is no need to check if the call succeeded.

Note

It's important to match g_malloc() with g_free(), plain malloc() with free(), and (if you're using C++) new with delete and new[] with delete[]. Otherwise bad things can happen, since these allocators may use different memory pools (and new/delete call constructors and destructors). See also g_mem_set_vtable().

Details

g_new()

#define             g_new(struct_type, n_structs)

Allocates n_structs elements of type struct_type. The returned pointer is cast to a pointer to the given type. If n_structs is 0 it returns NULL.

Since the returned pointer is already casted to the right type, it is normally unnecessary to cast it explicitly, and doing so might hide memory allocation errors.

struct_type :

 the type of the elements to allocate

n_structs :

 the number of elements to allocate

Returns :

 a pointer to the allocated memory, cast to a pointer to struct_type

g_new0()

#define             g_new0(struct_type, n_structs)

Allocates n_structs elements of type struct_type, initialized to 0's. The returned pointer is cast to a pointer to the given type. If n_structs is 0 it returns NULL.

Since the returned pointer is already casted to the right type, it is normally unnecessary to cast it explicitly, and doing so might hide memory allocation errors.

struct_type :

 the type of the elements to allocate.

n_structs :

 the number of elements to allocate.

Returns :

 a pointer to the allocated memory, cast to a pointer to struct_type.

g_renew()

#define             g_renew(struct_type, mem, n_structs)

Reallocates the memory pointed to by mem, so that it now has space for n_structs elements of type struct_type. It returns the new address of the memory, which may have been moved.

struct_type :

 the type of the elements to allocate

mem :

 the currently allocated memory

n_structs :

 the number of elements to allocate

Returns :

 a pointer to the new allocated memory, cast to a pointer to struct_type

g_try_new()

#define             g_try_new(struct_type, n_structs)

Attempts to allocate n_structs elements of type struct_type, and returns NULL on failure. Contrast with g_new(), which aborts the program on failure. The returned pointer is cast to a pointer to the given type. If n_structs is 0 it returns NULL.

struct_type :

 the type of the elements to allocate

n_structs :

 the number of elements to allocate

Returns :

 a pointer to the allocated memory, cast to a pointer to struct_type

Since 2.8

g_try_new0()

#define             g_try_new0(struct_type, n_structs)

Attempts to allocate n_structs elements of type struct_type, initialized to 0's, and returns NULL on failure. Contrast with g_new0(), which aborts the program on failure. The returned pointer is cast to a pointer to the given type. The function returns NULL when n_structs is 0.

struct_type :

 the type of the elements to allocate

n_structs :

 the number of elements to allocate

Returns :

 a pointer to the allocated memory, cast to a pointer to struct_type

Since 2.8

g_try_renew()

#define             g_try_renew(struct_type, mem, n_structs)

Attempts to reallocate the memory pointed to by mem, so that it now has space for n_structs elements of type struct_type, and returns NULL on failure. Contrast with g_renew(), which aborts the program on failure. It returns the new address of the memory, which may have been moved.

struct_type :

 the type of the elements to allocate

mem :

 the currently allocated memory

n_structs :

 the number of elements to allocate

Returns :

 a pointer to the new allocated memory, cast to a pointer to struct_type

Since 2.8

g_malloc ()

gpointer            g_malloc                            (gsize n_bytes);

Allocates n_bytes bytes of memory. If n_bytes is 0 it returns NULL.

n_bytes :

 the number of bytes to allocate

Returns :

 a pointer to the allocated memory

g_malloc0 ()

gpointer            g_malloc0                           (gsize n_bytes);

Allocates n_bytes bytes of memory, initialized to 0's. If n_bytes is 0 it returns NULL.

n_bytes :

 the number of bytes to allocate

Returns :

 a pointer to the allocated memory

g_realloc ()

gpointer            g_realloc                           (gpointer mem,
                                                         gsize n_bytes);

Reallocates the memory pointed to by mem, so that it now has space for n_bytes bytes of memory. It returns the new address of the memory, which may have been moved. mem may be NULL, in which case it's considered to have zero-length. n_bytes may be 0, in which case NULL will be returned and mem will be freed unless it is NULL.

mem :

 the memory to reallocate

n_bytes :

 new size of the memory in bytes

Returns :

 the new address of the allocated memory

g_try_malloc ()

gpointer            g_try_malloc                        (gsize n_bytes);

Attempts to allocate n_bytes, and returns NULL on failure. Contrast with g_malloc(), which aborts the program on failure.

n_bytes :

 number of bytes to allocate.

Returns :

 the allocated memory, or NULL.

g_try_malloc0 ()

gpointer            g_try_malloc0                       (gsize n_bytes);

Attempts to allocate n_bytes, initialized to 0's, and returns NULL on failure. Contrast with g_malloc0(), which aborts the program on failure.

n_bytes :

 number of bytes to allocate

Returns :

 the allocated memory, or NULL

Since 2.8

g_try_realloc ()

gpointer            g_try_realloc                       (gpointer mem,
                                                         gsize n_bytes);

Attempts to realloc mem to a new size, n_bytes, and returns NULL on failure. Contrast with g_realloc(), which aborts the program on failure. If mem is NULL, behaves the same as g_try_malloc().

mem :

 previously-allocated memory, or NULL.

n_bytes :

 number of bytes to allocate.

Returns :

 the allocated memory, or NULL.

g_free ()

void                g_free                              (gpointer mem);

Frees the memory pointed to by mem. If mem is NULL it simply returns.

mem :

 the memory to free

g_mem_gc_friendly

extern gboolean g_mem_gc_friendly;

This variable is TRUE if the G_DEBUG environment variable includes the key gc-friendly.

g_alloca()

#define             g_alloca(size)

Allocates size bytes on the stack; these bytes will be freed when the current stack frame is cleaned up. This macro essentially just wraps the alloca() function present on most UNIX variants. Thus it provides the same advantages and pitfalls as alloca():

+ alloca() is very fast, as on most systems it's implemented by just adjusting the stack pointer register.

+ It doesn't cause any memory fragmentation, within its scope, separate alloca() blocks just build up and are released together at function end.

- Allocation sizes have to fit into the current stack frame. For instance in a threaded environment on Linux, the per-thread stack size is limited to 2 Megabytes, so be sparse with alloca() uses.

- Allocation failure due to insufficient stack space is not indicated with a NULL return like e.g. with malloc(). Instead, most systems probably handle it the same way as out of stack space situations from infinite function recursion, i.e. with a segmentation fault.

- Special care has to be taken when mixing alloca() with GNU C variable sized arrays. Stack space allocated with alloca() in the same scope as a variable sized array will be freed together with the variable sized array upon exit of that scope, and not upon exit of the enclosing function scope.

size :

 number of bytes to allocate.

Returns :

 space for size bytes, allocated on the stack

g_newa()

#define             g_newa(struct_type, n_structs)

Wraps g_alloca() in a more typesafe manner.

struct_type :

 Type of memory chunks to be allocated

n_structs :

 Number of chunks to be allocated

Returns :

 Pointer to stack space for n_structs chunks of type struct_type

g_memmove()

#define             g_memmove(dest,src,len)

Copies a block of memory len bytes long, from src to dest. The source and destination areas may overlap.

In order to use this function, you must include string.h yourself, because this macro will typically simply resolve to memmove() and GLib does not include string.h for you.

dest :

 the destination address to copy the bytes to.

src :

 the source address to copy the bytes from.

len :

 the number of bytes to copy.

g_memdup ()

gpointer            g_memdup                            (gconstpointer mem,
                                                         guint byte_size);

Allocates byte_size bytes of memory, and copies byte_size bytes into it from mem. If mem is NULL it returns NULL.

mem :

 the memory to copy.

byte_size :

 the number of bytes to copy.

Returns :

 a pointer to the newly-allocated copy of the memory, or NULL if mem is NULL.

GMemVTable

typedef struct {
  gpointer (*malloc)      (gsize    n_bytes);
  gpointer (*realloc)     (gpointer mem,
			   gsize    n_bytes);
  void     (*free)        (gpointer mem);
  /* optional; set to NULL if not used ! */
  gpointer (*calloc)      (gsize    n_blocks,
			   gsize    n_block_bytes);
  gpointer (*try_malloc)  (gsize    n_bytes);
  gpointer (*try_realloc) (gpointer mem,
			   gsize    n_bytes);
} GMemVTable;

A set of functions used to perform memory allocation. The same GMemVTable must be used for all allocations in the same program; a call to g_mem_set_vtable(), if it exists, should be prior to any use of GLib.

malloc ()

 function to use for allocating memory.

realloc ()

 function to use for reallocating memory.

free ()

 function to use to free memory.

calloc ()

 function to use for allocating zero-filled memory.

try_malloc ()

 function to use for allocating memory without a default error handler.

try_realloc ()

 function to use for reallocating memory without a default error handler.

g_mem_set_vtable ()

void                g_mem_set_vtable                    (GMemVTable *vtable);

Sets the GMemVTable to use for memory allocation. You can use this to provide custom memory allocation routines. This function must be called before using any other GLib functions. The vtable only needs to provide malloc(), realloc(), and free() functions; GLib can provide default implementations of the others. The malloc() and realloc() implementations should return NULL on failure, GLib will handle error-checking for you. vtable is copied, so need not persist after this function has been called.

vtable :

 table of memory allocation routines.

g_mem_is_system_malloc ()

gboolean            g_mem_is_system_malloc              (void);

Checks whether the allocator used by g_malloc() is the system's malloc implementation. If it returns TRUE memory allocated with malloc() can be used interchangeable with memory allocated using g_malloc(). This function is useful for avoiding an extra copy of allocated memory returned by a non-GLib-based API.

A different allocator can be set using g_mem_set_vtable().

Returns :

 if TRUE, malloc() and g_malloc() can be mixed.

glib_mem_profiler_table

extern GMemVTable	*glib_mem_profiler_table;

A GMemVTable containing profiling variants of the memory allocation functions. Use them together with g_mem_profile() in order to get information about the memory allocation pattern of your program.

g_mem_profile ()

void                g_mem_profile                       (void);

Outputs a summary of memory usage.

It outputs the frequency of allocations of different sizes, the total number of bytes which have been allocated, the total number of bytes which have been freed, and the difference between the previous two values, i.e. the number of bytes still in use.

Note that this function will not output anything unless you have previously installed the glib_mem_profiler_table with g_mem_set_vtable().