Tribblix: manual page: malloc.3malloc

MALLOC(3MALLOC) Memory Allocation Library Functions MALLOC(3MALLOC)

NAME

malloc, free, memalign, realloc, valloc, calloc, mallopt, mallinfo -
memory allocator

SYNOPSIS

cc [ flag ... ] file ... -lmalloc [ library ... ]
#include <stdlib.h>

void *malloc(size_t size);

void free(void *ptr);

void *memalign(size_t alignment, size_t size);

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

void *valloc(size_t size);

void *calloc(size_t nelem, size_t elsize);

#include <malloc.h>

int mallopt(int cmd, int value);

struct mallinfo mallinfo(void);

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.

The argument to free() is a pointer to a block previously allocated
by malloc(). After free() is performed, this space is made available
for further allocation, and its contents have been destroyed. See
mallopt() below for a way to change this behavior. If ptr is a null
pointer, no action occurs.

Undefined results occur if the space assigned by malloc() is overrun
or if some random number is handed to free().

The free() function does not set errno.

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

The mallopt() function provides for control over the allocation
algorithm. The available values for cmd are:

M_MXFAST
Set maxfast to value. The algorithm allocates all blocks
below the size of maxfast in large groups and then doles
them out very quickly. The default value for maxfast is
24.

M_NLBLKS
Set numlblks to value. The above mentioned ``large
groups'' each contain numlblks blocks. numlblks must be
greater than 0. The default value for numlblks is 100.

M_GRAIN
Set grain to value. The sizes of all blocks smaller than
maxfast are considered to be rounded up to the nearest
multiple of grain. grain must be greater than 0. The
default value of grain is the smallest number of bytes
that will allow alignment of any data type. Value will be
rounded up to a multiple of the default when grain is
set.

M_KEEP
Preserve data in a freed block until the next malloc(),
realloc(), or calloc(). This option is provided only for
compatibility with the old version of malloc(), and it is
not recommended.

These values are defined in the <malloc.h> header.

The mallopt() function can be called repeatedly, but cannot be called
after the first small block is allocated.

The mallinfo() function provides instrumentation describing space
usage. It returns the mallinfo structure with the following members:

unsigned long arena; /* total space in arena */
unsigned long ordblks; /* number of ordinary blocks */
unsigned long smblks; /* number of small blocks */
unsigned long hblkhd; /* space in holding block headers */
unsigned long hblks; /* number of holding blocks */
unsigned long usmblks; /* space in small blocks in use */
unsigned long fsmblks; /* space in free small blocks */
unsigned long uordblks; /* space in ordinary blocks in use */
unsigned long fordblks; /* space in free ordinary blocks */
unsigned long keepcost; /* space penalty if keep option */
/* is used */

The mallinfo structure is defined in the <malloc.h> header.

Each of the allocation routines returns a pointer to space suitably
aligned (after possible pointer coercion) for storage of any type of
object.

RETURN VALUES

The malloc(), memalign(), realloc(), valloc(), and calloc() functions
return a null pointer if there is not enough available memory. When
realloc() returns NULL, the block pointed to by ptr is left intact.
If size, nelem, or elsize is 0, either a null pointer or a unique
pointer that can be passed to free() is returned. If mallopt() is
called after any allocation or if cmd or value are invalid, a non-
zero value is returned. Otherwise, it returns 0.

ERRORS

If malloc(), calloc(), or realloc() returns unsuccessfully, errno is
set to indicate the error:

ENOMEM
size bytes of memory exceeds the physical limits of your
system, and cannot be allocated.

EAGAIN
There is not enough memory available at this point in time
to allocate size bytes of memory; but the application could
try again later.

USAGE

Unlike malloc(3C), this package does not preserve the contents of a
block when it is freed, unless the M_KEEP option of mallopt() is
used.

Undocumented features of malloc(3C) have not been duplicated.

Function prototypes for malloc(), realloc(), calloc(), and free() are
also defined in the <malloc.h> header for compatibility with old
applications. New applications should include <stdlib.h> to access
the prototypes for these functions.

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

ATTRIBUTES