C dynamic memory allocation

C dynamic memory allocation refers to performing manual memory management for dynamic memory allocation in the C programming language via a group of functions in the C standard library, namely malloc, realloc, calloc, aligned_alloc and free.

The C++ programming language includes these functions; however, the operators new and delete provide similar functionality and are recommended by that language's authors. Still, there are several situations in which using new/delete is not applicable, such as garbage collection code or performance-sensitive code, and a combination of malloc and placement new may be required instead of the higher-level new operator.

Many different implementations of the actual memory allocation mechanism, used by malloc, are available. Their performance varies in both execution time and required memory.

The C programming language manages memory statically, automatically, or dynamically. Static-duration variables are allocated in main memory, usually along with the executable code of the program, and persist for the lifetime of the program; automatic-duration variables are allocated on the stack and come and go as functions are called and return. 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). If the required size is not known until run-time (for example, if data of arbitrary size is being read from the user or from a disk file), then using fixed-size data objects is inadequate.

The lifetime of allocated memory can also cause concern. Neither static- nor automatic-duration memory is adequate for all situations. Automatic-allocated data cannot persist across multiple function calls, while static data persists for the life of the program whether it is needed or not. In many situations the programmer requires greater flexibility in managing the lifetime of allocated memory.

These limitations are avoided by using dynamic memory allocation, in which memory is more explicitly (but more flexibly) managed, typically by allocating it from the free store (informally called the "heap"),^{[citation needed]} an area of memory structured for this purpose. 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.

The original description of C indicated that calloc and cfree were in the standard library, but not malloc. Code for a simple model implementation of a storage manager for Unix was given with alloc and free as the user interface functions, and using the sbrk system call to request memory from the operating system. The 6th Edition Unix documentation gives alloc and free as the low-level memory allocation functions. The malloc and free routines in their modern form are completely described in the 7th Edition Unix manual.

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()). This memory is automatically freed when the calling function ends.