MALLOC(3X) (Specialized Libraries) MALLOC(3X)
NAME
malloc, free, realloc, calloc, mallopt, mallinfo - fast main
memory allocator
SYNOPSIS
#include <malloc.h>
char *malloc (size)
unsigned size;
void free (ptr)
char *ptr;
char *realloc (ptr, size)
char *ptr;
unsigned size;
char *calloc (nelem, elsize)
unsigned nelem, elsize;
int mallopt (cmd, value)
int cmd, value;
struct mallinfo mallinfo()
DESCRIPTION
malloc and free provide a simple general-purpose memory
allocation package, which runs considerably faster than the
malloc(3C) package. It is found in the library ``malloc'',
and is loaded if the option ``-lmalloc'' is used with cc(1)
or ld(1).
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).
Page 1 May 1989
MALLOC(3X) (Specialized Libraries) MALLOC(3X)
Undefined results will 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 will be unchanged up to the lesser of
the new and old sizes.
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 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 which 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
Page 2 May 1989
MALLOC(3X) (Specialized Libraries) MALLOC(3X)
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.
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.
WARNINGS
This package usually uses more data space than malloc(3C).
The code size is also bigger than malloc(3C).
Note that unlike malloc(3C), this package does not preserve
the contents of a block when it is freed, unless the M_KEEP
Page 3 May 1989
MALLOC(3X) (Specialized Libraries) MALLOC(3X)
option of mallopt is used.
Undocumented features of malloc(3C) have not been
duplicated.
Page 4 May 1989