For static-duration and automatic-duration variables, the size of the allocation must be compile-time constant (except for the case of variable-length automatic arrays[5]).
In many situations the programmer requires greater flexibility in managing the lifetime of allocated memory.
[8][9] Some platforms provide library or intrinsic function calls which allow run-time dynamic allocation from the C stack rather than the heap (e.g. alloca()[10]).
Because malloc might not be able to service the request, it might return a null pointer and it is good programming practice to check for this: When the program no longer needs the dynamic array, it must eventually call free to return the memory it occupies to the free store: The memory set aside by malloc is not initialized and may contain cruft: the remnants of previously used and discarded data.
The command calloc will return an allocation that has already been cleared: With realloc we can resize the amount of memory a pointer points to.
Most common errors are as follows:[15] In addition, as an interface that precedes ANSI C standardization, malloc and its associated functions have behaviors that were intentionally left to the implementation to define for themselves.
[18] A way to wrap these functions to make them safer is by simply checking for 0-size allocations and turning them into those of size 1.
The GNU C library (glibc) is derived from Wolfram Gloger's ptmalloc ("pthreads malloc"), a fork of dlmalloc with threading-related improvements.
Allocated memory contains an 8- or 16-byte overhead for the size of the chunk and usage flags (similar to a dope vector).
Bins are sorted by size into three classes:[22][24]: Overlaid data structures Game developer Adrian Stone argues that dlmalloc, as a boundary-tag allocator, is unfriendly for console systems that have virtual memory but do not have demand paging.
Since FreeBSD 7.0 and NetBSD 5.0, the old malloc implementation (phkmalloc by Poul-Henning Kamp) was replaced by jemalloc, written by Jason Evans.
[29][better source needed] On a call to free, memory is released and unmapped from the process address space using munmap.
This system is designed to improve security by taking advantage of the address space layout randomization and gap page features implemented as part of OpenBSD's mmap system call, and to detect use-after-free bugs—as a large memory allocation is completely unmapped after it is freed, further use causes a segmentation fault and termination of the program.
The GrapheneOS project initially started out by porting OpenBSD's memory allocator to Android's Bionic C Library.
Like OpenBSD's allocator, Hoard uses mmap exclusively, but manages memory in chunks of 64 kilobytes called superblocks.
[31] An open-source compact general-purpose memory allocator from Microsoft Research with focus on performance.
[36] This necessitates a malloc implementation tightly integrated with the virtual memory subsystem of the operating system kernel.
[37] The most common form on POSIX-like systems is to set the environment variable LD_PRELOAD with the path of the allocator, so that the dynamic linker uses that version of malloc/calloc/free instead of the libc implementation.
Theoretically, the largest number should be the maximum value that can be held in a size_t type, which is an implementation-dependent unsigned integer representing the size of an area of memory.
On glibc systems, the largest possible memory block malloc can allocate is only half this size, namely 2^(CHAR_BIT * sizeof(ptrdiff_t) - 1) - 1.
[38] The C library implementations shipping with various operating systems and compilers may come with alternatives and extensions to the standard malloc interface.