From mboxrd@z Thu Jan 1 00:00:00 1970 From: Sean Anderson Date: Thu, 6 May 2021 13:57:31 -0400 Subject: [PATCH v2 01/50] lib: Add memdup() In-Reply-To: References: <20210506142438.1310977-1-sjg@chromium.org> <20210506082420.v2.1.I1d417387eb1e7273b536017f4a8920fc4e2369a9@changeid> <20210506170739.4tz7yumpfp2kfli4@ti.com> Message-ID: List-Id: MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: u-boot@lists.denx.de On 5/6/21 1:41 PM, Simon Glass wrote: > Hi Pratyush, > > On Thu, 6 May 2021 at 10:07, Pratyush Yadav 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 >>> --- >>> >>> 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(). Of course, U-Boot lets you pass > NULL to free() anyway. dlmalloc has a minimum allocation size of 2*sizeof(void *) (e.g. MINSIZE - 2*SIZE_SZ). If you request less than this, you will get an allocation of this size. This is the same as other requests, which are rounded up the the nearest multiple of MALLOC_ALIGNMENT. Of course, malloc_simple will actually give you a zero-byte allocation, so don't rely on being able to store anything there ;) --Sean