是否可以在不搜索malloc语句的情况下找到分配给指针的内存
假设我已将内存分配给函数 foo 中的某个指针：
void foo()
{
// ...
int *ptr = malloc(20*sizeof(int));
bar (ptr);
}
从 foo() 中，我将此指针传递给 bar() ，比方说从 bar() 到另一个函数。
现在，在某个时间点，我想检查： 为指针分配了多少内存。
有没有可能的方法，无需搜索语句：
int *ptr = malloc(20*sizeof(int));
使用 GDB 找出分配给指针的内存量？
谢谢。
Suppose I have allocated memory to some pointer in a function foo:
void foo()
{
// ...
int *ptr = malloc(20*sizeof(int));
bar (ptr);
}
From foo(), I pass this pointer to bar() and let's say from bar() to another function.
Now, at some point of time, I want to check: How much memory was allocated to the pointer.
Is there any possible way, without searching for the statement:
int *ptr = malloc(20*sizeof(int));
to figure out how much memory is allocated to the pointer, using GDB?
Thanks.
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答案是：这取决于。
许多系统提供
msize()[1]、malloc_usable_size()[2] 或类似函数。如果您使用的是这样的系统，您只需要(gdb) print malloc_usable_size(ptr)即可。2023 更新：
msize()文档已失效，但 _msize() 仍然可用。[1] http://msdn.microsoft。 com/en-us/library/z2s077bc(v=vs.80).aspx
[2] http ://www.slac.stanford.edu/comp/unix/package/rtems/doc/html/libc/libc.info.malloc.html
The answer is: it depends.
Many systems provide
msize()[1],malloc_usable_size()[2], or similar function. If you are on such a system,(gdb) print malloc_usable_size(ptr)is all you need.2023 Update:
msize()documentation is dead, but _msize() is still available.[1] http://msdn.microsoft.com/en-us/library/z2s077bc(v=vs.80).aspx
[2] http://www.slac.stanford.edu/comp/unix/package/rtems/doc/html/libc/libc.info.malloc.html
一般来说，没有。 C 没有提供获取已分配内存块大小的方法。您需要跟踪您自己分配了多少内存。
但是，在某些 C 库上，有一个函数可以获取内存块的可用大小 - malloc_usable_size （在 Linux 系统上位于
为了完整起见，我最初的答案深入探讨了低级堆元数据，在 @Employed Russian 指出
malloc_usable_size之前：但是，您可以手动提取它。但请注意，这一切可能会因操作系统、CPU 架构和 C 库的不同而有所不同。我假设您使用的是eglibc 2.12.1；在其他地方，您的结果可能会有所不同。
警告：说真的，除了在 gdb 中调试之外，不要使用它。真的。我是认真的。
glibc 内存分配器像这样存储内存块（来自 malloc/malloc.c)：
您的数据位于此处的“mem”，块的大小包括标头。 P 标志指示前一个块数据是否有效，M 指示这是一个 mmap 映射（对于大型 malloc）。所有这些都不太重要；重要的是该大小在内存之前存在一个指针大小的增量；您只需屏蔽这些标志并减去标头大小：
警告：实际分配的大小可能比您请求的大。不要试图变得聪明并使用多余的东西。一开始就询问您需要多少。
注意事项 2：这只适用于 glibc。它仅适用于某些版本的 glibc。因此可能会在没有任何警告的情况下随时破裂。我怎么强调都不为过； 不要在您的实际代码中使用它；仅用于当您用尽所有其他选项时进行调试。您的代码需要自行跟踪其缓冲区大小。
In general, no. C doesn't provide a way to get the size of an allocated block of memory. You need to keep track of how much memory you allocated yourself.
BUT, on some C libraries, there is a function to get the usable size of a block of memory - malloc_usable_size (found in
<malloc.h>on Linux systems, with no manpage). Note that this does not work on all libcs, and may report a value larger than you requested. Please, use it only for debugging.For completeness, my original answer, which dives into the low-level heap metadata, prior to @Employed Russian pointing out
malloc_usable_size:BUT, you may be able to extract this manually. Note, however, that this all can vary depending on your OS, CPU architecture, and C library. I will assume you're using eglibc 2.12.1; your results may vary anywhere else.
WARNING: Seriously, DO NOT use this except for debugging in gdb. Really. I mean it.
The glibc memory allocator stores chunks of memory like this (out of a doc comment in malloc/malloc.c):
Your data is at 'mem' here, and the size of chunk includes the header. The P flag indicates whether the previous-chunk data is valid, and M indicates this is a mmap mapping (for large mallocs). All of that isn't too important; what's important is that the size lives one pointer-sized increment before your memory; you just have to mask out those flags and subtract the header size:
Caveat: The actually allocated size may be larger than you requested. Don't try to get smart and use the excess. Ask for how much you need at the start.
Caveat 2: This only works with glibc. And it only works with certain versions of glibc. and thus may break at any moment without any warning. I can't stress this enough; DO NOT use this in your actual code; only for debugging when you have exhausted every other option. Your code needs to keep track of its buffer sizes on its own.
不。当您使用
malloc()时，您必须自己存储该信息。No. You have to store that information yourself when you
malloc().