Talk:C dynamic memory allocation

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 * Sunmist3 (talk) 22:00, 6 July 2016 (UTC)

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"heap", "free store", etc.
Neither C90 nor C99 nor C11 nor C18 appear to indicate that there is a given region containing all memory allocated by malloc, calloc, realloc, or aligned_alloc. For example, an implementation could, conceivably, allocate objects larger than the page size used by the OS from one region of the address space, by rounding up the size to a multiple of a page size and allocating n/page_size pages aligned on a page boundary, and could allocate smaller objects from a separate pool of memory. (If I remember correctly, the Darwin memory allocator works that way.)

So allocating from a "heap" is best thought of as meaning "these allocations come from one or more places, and all of those places, considered together, constitute a 'heap'".

Note also that none of those standards use the term "heap".

(The term "heap" predates both C and Unix; it dates back at least as far as Algol 68, the Report for which says, in section 5.2.2 "Generators", that

"The use of a local-generator implies (with most implementations) the reservation of storage on a run-time stack, whereas heap-generators imply the reservation of storage in another region, termed the "heap", in which garbage-collection techniques may be used for storage retrieval. Since this is less efficient, local-generators are preferable; this is why only LOC may be omitted from sample-generators of variable-declarations."

In an answer to a StackOverflow question "What is the origin of the term 'heap' for the free store?", somebody notes that even K&R, Second Edition, didn't use the term "heap":

"N.B. I don't think it's right to blame C, as neither K&R (2nd Ed) nor the C standard say "heap" anywhere. The C++ standard only uses heap to mean the data structure, and refers to the source of dynamic memory as "the freestore"."

So perhaps neither "free store" nor "freestore" nor "heap" should be used here, as this is about C.

As for C++, C++2017 uses the phrase "free store" in three places:


 * on page 1, it says "In addition to the facilities provided by C, C++ provides additional data types, classes, templates, exceptions, namespaces, operator overloading, function name overloading, references, free store management operators, and additional library facilities.", where I assume "free store management operators" means new and delete;
 * section 15.5 "Free store", on pages 289 to 291, uses it in the section title but doesn't seem to say what "free store" is;
 * the index has, on page 1518, an entry "free store, see also new, delete, 289".

So I'm not even sure what should be said about C++.

(Darwin also offers a set of "zone" routines that take a malloc_zone_t * as an argument, where a "zone" is allocated with malloc_create_zone and destroyed, along with everything allocated in it, with malloc_destroy_zone. malloc_default_zone returns the zone from which malloc etc. allocate memory, so, at least on Darwin, I guess you could say that stuff is allocated from a "zone".

However, none of that is a reason to use "zone" here, unless it's commonly used in platforms that aren't Darwin-based.) Guy Harris (talk) 06:29, 27 July 2022 (UTC)

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The above message was substituted from by PrimeBOT (talk) on 19:56, 1 February 2023 (UTC)

It is the compiler not glibc
The article says that glibc limits allocation up to 2^(CHAR_BIT * sizeof(ptrdiff_t) - 1) - 1. This is a compiler limitation, GCC generates non-sense (by design cannot deal with objects bigger than ptrdiff_t) for any allocation over that value, clang DCEs the code because "it can never  succeed".. 181.160.10.238 (talk) 22:23, 4 October 2023 (UTC)