4

我正在为 STM32F7 编写嵌入式软件，我的 libc 是 newlib-2.4.0.20160527。

我已经实现_sbrk()如下：

extern intptr_t g_bss_end; /* value after the last byte in .bss */
extern intptr_t g_msp_lim; /* stack buffer starts at this address */

intptr_t _sbrk(ptrdiff_t heap_incr)
{
    static intptr_t heap_end = 0;

    intptr_t prev_heap_end;
    intptr_t new_heap_end;

    if(heap_end == 0) {
        heap_end = (intptr_t)&g_bss_end;
    }

    prev_heap_end = heap_end;
    new_heap_end = prev_heap_end + heap_incr;

    if(new_heap_end >= g_msp_lim) {
        errno = ENOMEM;

        return -1;
    }

    heap_end = new_heap_end;

    return prev_heap_end;
}

然后，当我执行以下操作时：

/* total capacity of my heap is 0x40000 */
void * mem = malloc(0x40000);
free(mem); mem = 0;
mem = malloc(0x40000);

一切正常（即 malloc 两次返回非零值）。

但是当我执行以下操作时（出于测试目的）：

for(int32_t sz = 0x50000; sz >= 0; sz--) {
    void * mem = malloc(sz);

    if(mem != 0) {
        __BKPT();
        free(mem);

        break;
    }
}

每个malloc()失败，甚至malloc(0)（即，__BKPT()从未达到）。所以，实际上堆上没有分配的内存（我没有得到任何内存，mem != 0所以我什free()至什么都做不到），也没有可用的内存。

我希望malloc()每一次都失败sz > 0x40000并为每一次成功sz <= 0x40000（假设在每一次free()之后都可以正常工作malloc()）。

我错过了什么，或者这是 newlib 中的错误或预期行为？

4

2 回答 2

6

由于. malloc()_ malloc_extend_top()_ newlib/libc/stdlib/mallocr.c:2137调用成功后_sbrk()

  brk = (char*)(MORECORE (sbrk_size)); /* MORECORE = _sbrk */

  /* Fail if sbrk failed or if a foreign sbrk call killed our space */
  if (brk == (char*)(MORECORE_FAILURE) || 
      (brk < old_end && old_top != initial_top))
    return;

它尝试计算校正以适应页面对齐：

/* Guarantee alignment of first new chunk made from this space */
front_misalign = (POINTER_UINT)chunk2mem(brk) & MALLOC_ALIGN_MASK;
if (front_misalign > 0) 
{
  correction = (MALLOC_ALIGNMENT) - front_misalign;
  brk += correction;
}
else
  correction = 0;

/* Guarantee the next brk will be at a page boundary */
correction += pagesz - ((POINTER_UINT)(brk + sbrk_size) & (pagesz - 1));

修正总是积极的，因为即使分配完全适合，它也会尝试分配下一页。例如，如果页面大小为4096并且brk + sbrk_size = 4096*n, 表达式4096 - ((brk + sbrk_size) & 4095)将给出4096，因此需要下一个空白页面，但没有空间。

该例程没有正确处理这种情况，只留下分配的数据（brk 值），导致永久“不可释放”的整个堆分配。太浪费了 ：-）

于 2016-08-22T23:13:45.123 回答
2

经测试，工作正常。

From 1473e08d2a16ad448afedb7036a476231a785643 Mon Sep 17 00:00:00 2001
From: Jeff Johnston <jjohnstn@redhat.com>
Date: Thu, 24 May 2018 23:53:15 -0400
Subject: [PATCH] Fix issue with malloc_extend_top

- when calculating a correction to align next brk to page boundary,
  ensure that the correction is less than a page size
- if allocating the correction fails, ensure that the top size is
  set to brk + sbrk_size (minus any front alignment made)

Signed-off-by: Jeff Johnston <jjohnstn@redhat.com>
---
 newlib/libc/stdlib/mallocr.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/newlib/libc/stdlib/mallocr.c b/newlib/libc/stdlib/mallocr.c
index ecc445f..26d1c89 100644
--- a/newlib/libc/stdlib/mallocr.c
+++ b/newlib/libc/stdlib/mallocr.c
@@ -2198,13 +2198,18 @@ static void malloc_extend_top(RARG nb) RDECL INTERNAL_SIZE_T nb;
     /* Guarantee the next brk will be at a page boundary */
     correction += pagesz - ((POINTER_UINT)(brk + sbrk_size) & (pagesz - 1));

+    /* To guarantee page boundary, correction should be less than pagesz */
+    correction &= (pagesz - 1);
+
     /* Allocate correction */
     new_brk = (char*)(MORECORE (correction));
     if (new_brk == (char*)(MORECORE_FAILURE))
       {
    correction = 0;
    correction_failed = 1;
-   new_brk = brk;
+   new_brk = brk + sbrk_size;
+   if (front_misalign > 0)
+     new_brk -= (MALLOC_ALIGNMENT) - front_misalign;
       }

     sbrked_mem += correction;
-- 
1.8.3.1
于 2018-05-25T04:34:29.227 回答