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In C, the library function malloc is used to allocate a block of memory on the heap. The program accesses this block of memory via a pointer that malloc returns. When the memory is no longer needed, the pointer is passed to free which deallocates the memory so that it can be used for other purposes.
Many programming languages use manual techniques to determine when to allocate a new object from the free store. C uses the malloc function; C++ and Java use the new operator; and many other languages (such as Python) allocate all objects from the free store.
At any given time, some parts of the heap are in use, while some are "free" (unused) and thus available for future allocations. In the C language, the function which allocates memory from the heap is called malloc and the function which takes previously allocated memory and marks it as "free" (to be used by future allocations) is called free ...
See C programming language for more discussion. The void pointer, or void*, is supported in ANSI C and C++ as a generic pointer type. A pointer to void can store the address of any object (not function), [a] and, in C, is implicitly converted to any other object pointer type on assignment, but it must be explicitly cast if dereferenced.
One thread could temporarily remove the hook while another thread could malloc memory at the sametime leading to missed allocations in a multithreaded application! The function mtrace installs handlers for malloc, realloc and free; the function muntrace disables these handlers.
These functions are typically called from a higher-level memory management library function such as malloc. In the original Unix system, brk and sbrk were the only ways in which applications could acquire additional heap space; later versions allowed this to also be done using the mmap call.
sizeof cannot be used in C preprocessor expressions, such as #if, because it is an element of the programming language, not of the preprocessor syntax, which has no data types. The following example in C++ uses the operator sizeof with variadic templates.
Many Unix-like systems as well as Microsoft Windows implement a function called alloca for dynamically allocating stack memory in a way similar to the heap-based malloc.A compiler typically translates it to inlined instructions manipulating the stack pointer, similar to how variable-length arrays are handled. [4]