MALLOC(3C) Standard C Library Functions MALLOC(3C)

malloc, calloc, free, freezero, memalign, realloc, reallocf, reallocarray, recallocarray, valloc, alloca —
memory allocator

#include <stdlib.h>

void *
malloc(size_t size);

void *
calloc(size_t nelem, size_t elsize);

void
free(void *ptr);

void
freezero(void *ptr, size_t size);

void *
memalign(size_t alignment, size_t size);

void *
realloc(void *ptr, size_t size);

void *
reallocf(void *ptr, size_t size);

void *
reallocarray(void *ptr, size_t nelem, size_t elsize);

void *
recallocarray(void *ptr, size_t oldnelem, size_t newnelem, size_t elsize);

void *
valloc(size_t size);

#include <alloca.h>

void *
alloca(size_t size);

The malloc() and free() functions provide a simple, general-purpose memory allocation package. The malloc() function returns a pointer to a block of at least size bytes suitably aligned for any use. If the space assigned by malloc() is overrun, the results are undefined.

The argument to free() is a pointer to a block previously allocated by malloc(), calloc(), realloc(), reallocf(), reallocarray(), or recallocarray(). After free() is executed, this space is made available for further allocation by the application, though not returned to the system. Memory is returned to the system only upon termination of the application. If ptr is a null pointer, no action occurs. If a random number is passed to free(), the results are undefined.

The freezero() function is similar to the free() function except it ensures memory is explicitly discarded. If ptr is NULL, no action occurs. If ptr is not NULL, the size argument must be equal or smaller than the size of the earlier allocation that returned ptr. freezero() guarantees the memory range starting at ptr with length size is discarded while deallocating the whole object originally allocated.

The calloc() function allocates space for an array of nelem elements of size elsize. The space is initialized to zeros.

The memalign() function allocates size bytes on a specified alignment boundary and returns a pointer to the allocated block. The value of the returned address is guaranteed to be an even multiple of alignment. The value of alignment must be a power of two and must be greater than or equal to the size of a word.

The realloc() function changes the size of the block pointed to by ptr to size bytes and returns a pointer to the (possibly moved) block. The contents will be unchanged up to the lesser of the new and old sizes. If the new size of the block requires movement of the block, the space for the previous instantiation of the block is freed. If the new size is larger, the contents of the newly allocated portion of the block are unspecified. If ptr is NULL, realloc() behaves like malloc() for the specified size. If size is 0 and ptr is not a null pointer, the space pointed to is freed.

The reallocf() function behaves in the same way as realloc() except that the passed pointer is freed automatically on failure.

The reallocarray() function is similar to realloc(), but operates on nelem elements of size elsize and checks for overflow in nelem*elsize calculation.

The recallocarray() function is similar to reallocarray() except it ensures newly allocated memory is cleared similar to calloc(). If ptr is NULL, oldnelem is ignored and the call is equivalent to calloc(). If ptr is not NULL, oldnelem must be a value such that oldnelem*elsize is the size of the earlier allocation that returned ptr, otherwise the behaviour is undefined.

The valloc() function has the same effect as malloc(), except that the allocated memory will be aligned to a multiple of the value returned by sysconf(_SC_PAGESIZE).

The alloca() function allocates size bytes of space in the stack frame of the caller, and returns a pointer to the allocated block. This temporary space is automatically freed when the caller returns. If the allocated block is beyond the current stack limit, the resulting behavior is undefined.

Upon successful completion, each of the allocation functions returns a pointer to space suitably aligned (after possible pointer coercion) for storage of any type of object.

If there is no available memory, malloc(), calloc(), realloc(), reallocf(), reallocarray(), recallocarray(), memalign(), and valloc() return a null pointer.

When realloc() is called with size > 0 and returns NULL, the block pointed to by ptr is left intact. By contrast, when reallocf() is called with size > 0 and returns NULL, the block pointed to by ptr will have been freed.

If size, nelem, or elsize is 0, either a null pointer or a unique pointer that can be passed to free() is returned.

If malloc(), calloc(), realloc(), reallocf(), reallocarray(), or recallocarray() returns unsuccessfully, errno will be set to indicate the error. The free() and freezero() functions do not set errno.

The malloc(), calloc(), realloc(), reallocf(), and reallocarray() functions will fail if:
The physical limits of the system are exceeded by size bytes of memory which cannot be allocated, or there's integer overflow in reallocarray().
There is not enough memory available to allocate size bytes of memory; but the application could try again later.

The recallocarray() function will fail if:

ptr is not NULL and multiplying oldnelem and elsize results in integer overflow.

Portable applications should avoid using valloc() but should instead use malloc() or mmap(2). On systems with a large page size, the number of successful valloc() operations might be 0.

These default memory allocation routines are safe for use in multithreaded applications but are not scalable. Concurrent accesses by multiple threads are single-threaded through the use of a single lock. Multithreaded applications that make heavy use of dynamic memory allocation should be linked with allocation libraries designed for concurrent access, such as libumem(3LIB) or libmtmalloc(3LIB). Applications that want to avoid using heap allocations (with brk(2)) can do so by using either libumem(3LIB) or libmapmalloc(3LIB). The allocation libraries libmalloc(3LIB) and libbsdmalloc(3LIB) are available for special needs.

Comparative features of the various allocation libraries can be found in the umem_alloc(3MALLOC) manual page.

The malloc(), calloc(), free(), realloc(), valloc() functions are Standard.

The freezero(), reallocf(), reallocarray(), and recallocarray() functions are Committed.

The memalign() and alloca() functions are Stable.

Safe.

brk(2), getrlimit(2), libbsdmalloc(3LIB), libmalloc(3LIB), libmapmalloc(3LIB), libmtmalloc(3LIB), libumem(3LIB), umem_alloc(3MALLOC), watchmalloc(3MALLOC), attributes(7)

Undefined results will occur if the size requested for a block of memory exceeds the maximum size of a process's heap, which can be obtained with getrlimit(2).

The alloca() function is machine-, compiler-, and most of all, system-dependent. Its use is strongly discouraged.

September 12, 2019 OmniOS