Advanced Memory Handling

The allocation scheme used in libmpdec allows for mixing statically and dynamically allocated decimals. If a decimal with a static coefficient needs to grow beyond the fixed array size, the coefficient automatically switches to dynamic memory. This is very fast in practice and eliminates the need for alloca.

It is not necessary to read this section if only dynamically allocated decimals are used.

Static and Dynamic Decimals

Static and dynamic decimals may be mixed freely. In particular, they may be used in the position of the result operand or passed as an argument to mpd_del.

The coefficient of a static decimal must have the size MPD_MINALLOC_MAX (see below).

MPD_STATIC if set, the mpd_t struct is static
MPD_STATIC_DATA if set, the coefficient is static

Shared and Constant Decimals

The coefficient of a decimal can be marked shared or constant. Decimals with such coefficients must not be result operands or passed to mpd_del. Since these flags are intended for internal use, the memory reallocation functions only check for their presence using assert.

MPD_SHARED_DATA if set, the coefficient is shared
MPD_CONST_DATA if set, the coefficient is constant


In order to avoid frequent resizing operations, the global variable MPD_MINALLOC guarantees a minimum amount of allocated words for the coefficient of each mpd_t. The variable can be set once at program start and all memory functions respect it. The minimum value for MPD_MINALLOC is MPD_MINALLOC_MIN=2, the maximum value is MPD_MINALLOC_MAX=64.

void mpd_setminalloc(mpd_ssize_t n);

At program start, set MPD_MINALLOC to n. If n is outside the limits, the function prints an error and exits. If an attempt is made to set MPD_MINALLOC a second time, the function does nothing but print a warning.

Resizing Decimals

When writing functions that operate directly on the data of a decimal, it is necessary to resize the coefficient. All the above functions may be called on static decimals, since they are smart enough to allocate dynamic storage if the static array is too small. The functions must not be called on constant or shared decimals.

int mpd_qresize(mpd_t *result, mpd_ssize_t size, uint32_t *status);
int mpd_resize(mpd_t *result, mpd_ssize_t size, mpd_context_t *ctx);

int mpd_qresize_zero(mpd_t *result, mpd_ssize_t size, uint32_t *status);
int mpd_resize_zero(mpd_t *result, mpd_ssize_t size, mpd_context_t *ctx);

mpd_qresize tries to resize result to size words. If the size is increased, the existing coefficient data is left untouched. On success, 1 is returned. On failure, 0 is returned and MPD_Malloc_error is added to the status parameter.

mpd_qresize_zero is the same, but initializes the complete coefficient - including the old memory area - to zero.

void mpd_minalloc(mpd_t *result);

Reduce the size of the coefficient to MPD_MINALLOC words. This function cannot fail.

Custom Allocation Functions

extern void *(* mpd_mallocfunc)(size_t size);
extern void *(* mpd_callocfunc)(size_t nmemb, size_t size);
extern void *(* mpd_reallocfunc)(void *ptr, size_t size);
extern void (* mpd_free)(void *ptr);

At program start, these variables can be set to custom memory allocation functions. By default, they are set to the standard C functions.

void *mpd_alloc(mpd_size_t nmemb, mpd_size_t size);
void *mpd_calloc(mpd_size_t nmemb, mpd_size_t size);

Memory allocation with overflow checking, using the custom allocation functions. If the allocation fails, NULL is returned.

In the overflow case, the functions print an error and exit. Note that overflow can only happen due to a serious programming error, like using a context with invalid values.

void *mpd_realloc(void *ptr, mpd_size_t nmemb, mpd_size_t size,
                  uint8_t *err);

If successful, return the pointer to the new memory area. Otherwise, return ptr and set err to 1.

Overflow handling as above.

/* struct hack alloc */
void *mpd_sh_alloc(mpd_size_t struct_size, mpd_size_t nmemb,
                   mpd_size_t size);

Like mpd_alloc, with an additional parameter for struct hack allocations.