On 06/05/2021 19.41, Simon Glass wrote:
Hi Pratyush,
On Thu, 6 May 2021 at 10:07, Pratyush Yadav p.yadav@ti.com wrote:
On 06/05/21 08:23AM, Simon Glass wrote:
Add a function to duplicate a memory region, a little like strdup().
Signed-off-by: Simon Glass sjg@chromium.org
Changes in v2:
- Add a patch to introduce a memdup() function
include/linux/string.h | 13 +++++++++++++ lib/string.c | 13 +++++++++++++ test/lib/string.c | 32 ++++++++++++++++++++++++++++++++ 3 files changed, 58 insertions(+)
diff --git a/include/linux/string.h b/include/linux/string.h index dd255f21633..3169c93796e 100644 --- a/include/linux/string.h +++ b/include/linux/string.h @@ -129,6 +129,19 @@ extern void * memchr(const void *,int,__kernel_size_t); void *memchr_inv(const void *, int, size_t); #endif
+/**
- memdup() - allocate a buffer and copy in the contents
- Note that this returns a valid pointer even if @len is 0
I'm uneducated about U-Boot's memory allocator. But I wonder how it returns a valid pointer even on 0 length allocations. What location does it point to? What are users expected to do with that pointer? They obviously can't read/write to it since it is supposed to be a 0 byte long allocation. If another positive length allocation happens before the said pointer is freed, will it point to the same memory location? If not, isn't the 0-length pointer actually at least a 1-length pointer?
I think it is just a 0-length pointer and that the only thing you can do with it is call free().
I am certainly no expert on this sort of thing though. It seems that some implementations return NULL for a zero size, some return a valid pointer which can be passed to free().
It's implementation-defined, which means that one cannot know for certain that a given malloc implementation won't return NULL for a request of 0 bytes. The linux kernel solved that problem by introducing ZERO_SIZE_PTR which is basically just (void*)16L or something like that - that way callers don't have to write their "did the allocation succeed" test in the ugly
if (!p && size != 0) error_out;
way. Of course kfree() must then accept that in addition to NULL, but it's not really more expensive to have that early nop check be
if ((unsigned long)ptr <= 16) return;
instead of
if (!ptr) return;
"man malloc" says
RETURN VALUE The malloc() and calloc() functions return a pointer to the allocated memory, which is suitably aligned for any built-in type. On error, these functions return NULL. NULL may also be returned by a successful call to malloc() with a size of zero, or by a successful call to calloc() with nmemb or size equal to zero.
Anyway, I don't think this helper should be put in string.c - it needs to be in some C file that's easily compiled for both board, sandbox and host tools (for the latter probably via the "tiny one-line wrapper that just includes the whole real C file"). I see there's linux_string.c already.
Rasmus