Tribblix: manual page: malloc.3c

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

NAME

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

SYNOPSIS

#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);

DESCRIPTION

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.

RETURN VALUES

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.

ERRORS

The malloc(), calloc(), realloc(), reallocf(), and reallocarray()
functions will fail if:

ENOMEM The physical limits of the system are exceeded by
size bytes of memory which cannot be allocated, or
there's integer overflow in reallocarray().

EAGAIN 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:

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

USAGE

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.

INTERFACE STABILITY

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.

MT-LEVEL
Safe.

WARNINGS

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.

illumos September 12, 2019 illumos