malloc(3X) MISCELLANEOUS LIBRARY FUNCTIONS malloc(3X)
NAME
malloc, free, realloc, calloc, mallopt, mallinfo - memory
allocator
SYNOPSIS
cc [flag ...] file ... -lmalloc [library ...]
#include <stdlib.h>
void *malloc (size_t size)
void free (void *ptr)
void *realloc (void *ptr, 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
malloc and free provide a simple general-purpose memory
allocation package.
malloc 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 (but 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.
realloc changes the size of the block pointed to by ptr to
size bytes and returns a pointer to the (possibly moved)
block. The contents are unchanged up to the lesser of the
new and old sizes. If ptr is a null pointer, realloc
behaves like malloc for the specified size. If size is zero
and ptr is not a null pointer, the object it points to is
freed.
calloc allocates space for an array of nelem elements of
size elsize. The space is initialized to zeros.
mallopt provides for control over the allocation algorithm.
The available values for cmd are:
M_MXFAST
Last change: Specialized Libraries 1
malloc(3X) MISCELLANEOUS LIBRARY FUNCTIONS malloc(3X)
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_GRAINSet 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 is
not recommended.
These values are defined in the malloc.h header file.
mallopt may be called repeatedly, but may not be called
after the first small block is allocated.
mallinfo provides instrumentation describing space usage.
It returns the structure:
struct mallinfo {
int arena; /* total space in arena */
int ordblks; /* number of ordinary blocks */
int smblks; /* number of small blocks */
int hblkhd; /* space in holding block headers */
int hblks; /* number of holding blocks */
int usmblks; /* space in small blocks in use */
int fsmblks; /* space in free small blocks */
int uordblks; /* space in ordinary blocks in use */
int fordblks; /* space in free ordinary blocks */
int keepcost; /* space penalty if keep option */
/* is used */
}
This structure is defined in the malloc.h header file.
Each of the allocation routines returns a pointer to space
suitably aligned (after possible pointer coercion) for
storage of any type of object.
Last change: Specialized Libraries 2
malloc(3X) MISCELLANEOUS LIBRARY FUNCTIONS malloc(3X)
SEE ALSO
brk(2), malloc(3C).
DIAGNOSTICS
malloc, realloc, and calloc 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 mallopt is
called after any allocation or if cmd or value are invalid,
non-zero is returned. Otherwise, it returns zero.
NOTES
Note that 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 file for compatibility with old applica-
tions. New applications should include <stdlib.h> to access
the prototypes for these functions.
Last change: Specialized Libraries 3