Question

我正在为STM32F7编写一个嵌入式软件，我的libc是newlib-2.4.0.20160527。

我已按照以下方式实施_sbrk()：

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（用于测试目的）时：

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

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

        break;
    }
}

每{{}}}失败，即使malloc()（即malloc(0) 从未到达）。所以，事实上堆上没有分配的内存（我没有得到任何__BKPT()因此我甚至不能mem != 0某些东西）并且也没有可用的内存。

我希望每个free() malloc()失败并且sz > 0x40000成功{假设sz <= 0x40000在每个free()之后正常工作。

我是否错过了某些内容，或者这是newlib中的错误或预期行为？

Answer 1

由于malloc()中的malloc_extend_top()例程不正确，分配整个堆内存时，newlib的newlib/libc/stdlib/mallocr.c:2137无法正常工作。成功拨打_sbrk()

后

  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));

校正始终是正的，因为即使分配完全匹配，它也会尝试分配下一整页。例如，如果页面大小为4096而brk + sbrk_size = 4096*n，则表达式4096 - ((brk + sbrk_size) & 4095)将提供4096，因此需要下一个空白页面，但没有空间。< / p>

例程正在处理这种情况并不正确，只留下分配的数据（brk值），导致永久的“不合理”的整个堆分配。这样的浪费： - ）

Answer 2

经过测试，效果很好。

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

newlib中的malloc（）：在一次大的故障分配后会浪费内存吗？

2 个答案: