malloc(3x)
Name
malloc, free, realloc, calloc, mallopt, mallinfo − fast main memory allocator
Syntax
#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 (max)
int max;
Description
The malloc and free subroutines provide a simple general-purpose memory allocation package, which runs considerably faster than the malloc() package. It is found in the library malloc, and is loaded if the option −lmalloc is used with cc() or ld(.).
The malloc subroutine 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.
Undefined results will occur if the space assigned by malloc is overrun or if some random number is handed to free.
The realloc subroutine 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.
The calloc subroutine allocates space for an array of nelem elements of size elsize. The space is initialized to zeros.
The mallopt subroutine 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 0.
M_NLBLKS
Set numlblks to value . The above mentioned large groups each contain numlblks blocks. The 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 . The 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.
The mallopt subroutine may be called repeatedly, but may not be called after the first small block is allocated.
The mallinfo subroutine provides information describing space usage. It returns the following 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.
Restrictions
This package usually uses more data space than malloc(.).
The code size is also bigger than malloc(.).
Note that unlike malloc(,), 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() have not been duplicated.
Return Value
The malloc, realloc, and calloc subroutines 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, nonzero is returned. Otherwise, it returns zero.