Why doesn't it directly invoke malloc(3) to allocate a big block of memory other than calling sbrk() or map() under different conditions?

Why doesn't it directly invoke malloc(3) to allocate a big block of memory other than calling sbrk() or map() under different conditions?

[孙世龙 sunshilong]

Hi, list

As the subject, here is the related code snippet(which could be seen
on https://github.com/ros2/tlsf/blob/master/tlsf/src/tlsf.c):
#if USE_SBRK || USE_MMAP
static __inline__ void *get_new_area(size_t * size)
{
    void *area;

#if USE_SBRK
    area = (void *)sbrk(0);
    if (((void *)sbrk(*size)) != ((void *) -1))
return area;
#endif

#ifndef MAP_ANONYMOUS
/* https://dev.openwrt.org/ticket/322 */
# define MAP_ANONYMOUS MAP_ANON
#endif


#if USE_MMAP
    *size = ROUNDUP(*size, getpagesize());
    if ((area = __STD(mmap(0, *size, PROT_READ | PROT_WRITE,
MAP_PRIVATE | MAP_ANONYMOUS, -1, 0))) != MAP_FAILED)
return area;
#endif
    return ((void *) ~0);
}
#endif

I would be grateful to have some help with this question.
Best Regards.
Sunshilong

Re: Why doesn't it directly invoke malloc(3) to allocate a big block of memory other than calling sbrk() or map() under different conditions?

[Siddhesh Poyarekar]

On Thu, 13 Aug 2020 at 14:06, 孙世龙 sunshilong via Libc-help
<libc-help@sourceware.org> wrote:
> I would be grateful to have some help with this question.

It is a question for the tlsf project, not for glibc.  Projects make
such decisions for a variety of reasons and the most common one is
that system malloc does not perform the way they would like it.

Siddhesh
-- 
http://siddhesh.in

Re: Why doesn't it directly invoke malloc(3) to allocate a big block of memory other than calling sbrk() or map() under different conditions?

[Florian Weimer]

* 孙世龙 sunshilong via Libc-help:

> As the subject, here is the related code snippet(which could be seen
> on https://github.com/ros2/tlsf/blob/master/tlsf/src/tlsf.c):
> #if USE_SBRK || USE_MMAP
> static __inline__ void *get_new_area(size_t * size)
> {
>     void *area;
>
> #if USE_SBRK
>     area = (void *)sbrk(0);
>     if (((void *)sbrk(*size)) != ((void *) -1))
> return area;
> #endif
>
> #ifndef MAP_ANONYMOUS
> /* https://dev.openwrt.org/ticket/322 */
> # define MAP_ANONYMOUS MAP_ANON
> #endif
>
>
> #if USE_MMAP
>     *size = ROUNDUP(*size, getpagesize());
>     if ((area = __STD(mmap(0, *size, PROT_READ | PROT_WRITE,
> MAP_PRIVATE | MAP_ANONYMOUS, -1, 0))) != MAP_FAILED)
> return area;
> #endif
>     return ((void *) ~0);
> }
> #endif

Calling malloc here would lead to infinite recursion if TLSF is used
as a malloc replacement:

  <https://www.gnu.org/software/libc/manual/html_node/Replacing-malloc.html>

It would call into TLSF again, never actually allocating any memory.