因为项目需要，最近重新拾起来STM32，正好趁着这个机会好好的梳理下遇到的知识细节。
STM32CubeMx默认移植和链接newlib，因此理论上我们可以方便的使用printf，malloc，free等功能，如下图：
这里我们只关注内存这块，移植newlib时需要_sbrk函数，这里STM32CubeMX在生成工程的时候帮忙生成了代码，如下：
//sysmem.c
/**
* Pointer to the current high watermark of the heap usage
*/
static uint8_t *__sbrk_heap_end = NULL;
/**
* @brief _sbrk() allocates memory to the newlib heap and is used by malloc
* and others from the C library
*
* @verbatim
* ############################################################################
* # .data # .bss # newlib heap # MSP stack #
* # # # # Reserved by _Min_Stack_Size #
* ############################################################################
* ^-- RAM start ^-- _end _estack, RAM end --^
* @endverbatim
*
* This implementation starts allocating at the '_end' linker symbol
* The '_Min_Stack_Size' linker symbol reserves a memory for the MSP stack
* The implementation considers '_estack' linker symbol to be RAM end
* NOTE: If the MSP stack, at any point during execution, grows larger than the
* reserved size, please increase the '_Min_Stack_Size'.
*
* @param incr Memory size
* @return Pointer to allocated memory
*/
void *_sbrk(ptrdiff_t incr)
{
extern uint8_t _end; /* Symbol defined in the linker script */
extern uint8_t _estack; /* Symbol defined in the linker script */
extern uint32_t _Min_Stack_Size; /* Symbol defined in the linker script */
const uint32_t stack_limit = (uint32_t)&_estack - (uint32_t)&_Min_Stack_Size;
const uint8_t *max_heap = (uint8_t *)stack_limit;
uint8_t *prev_heap_end;
/* Initalize heap end at first call */
if (NULL == __sbrk_heap_end)
{
__sbrk_heap_end = &_end;
}
/* Protect heap from growing into the reserved MSP stack */
if (__sbrk_heap_end + incr > max_heap)
{
errno = ENOMEM;
return (void *)-1;
}
prev_heap_end = __sbrk_heap_end;
__sbrk_heap_end += incr;
return (void *)prev_heap_end;
}
这里 __sbrk_heap_end 指向的是当前堆的指针位置，max_heap指向的是堆默认的最大值。这里各个指针偏移如下图：
地址偏移是如何计算的呢，这里可以看下最终的链接脚本：
/* XXXX_FLASH.ld */
/* Highest address of the user mode stack */
_estack = ORIGIN(RAM) + LENGTH(RAM); /* end of "RAM" Ram type memory */
_Min_Heap_Size = 0x200 ; /* required amount of heap */
_Min_Stack_Size = 0x400 ; /* required amount of stack */
。。。。
。。。。
/* Initialized data sections into "RAM" Ram type memory */
.data :
{
. = ALIGN(4);
_sdata = .; /* create a global symbol at data start */
*(.data) /* .data sections */
*(.data*) /* .data* sections */
. = ALIGN(4);
_edata = .; /* define a global symbol at data end */
} >RAM AT> FLASH /* data第一个链接进RAM */
/* Uninitialized data section into "RAM" Ram type memory */
. = ALIGN(4);
.bss :
{
/* This is used by the startup in order to initialize the .bss section */
_sbss = .; /* define a global symbol at bss start */
__bss_start__ = _sbss;
*(.bss)
*(.bss*)
*(COMMON)
. = ALIGN(4);
_ebss = .; /* define a global symbol at bss end */
__bss_end__ = _ebss;
} >RAM /* bss第二个链接进RAM */
/* User_heap_stack section, used to check that there is enough "RAM" Ram type memory left */
._user_heap_stack :
{
. = ALIGN(8);
PROVIDE ( end = . );
PROVIDE ( _end = . );
. = . + _Min_Heap_Size;
. = . + _Min_Stack_Size;
. = ALIGN(8);
} >RAM /* _user_heap_stack第三个链接进RAM */
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