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man pages section 3: Basic Library Functions

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Updated: Wednesday, July 27, 2022



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


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

void *malloc(size_t size);
void *calloc(size_t nelem, size_t elsize);
void *memalign(size_t alignment, size_t size);
void *realloc(void *ptr, size_t size);
void *valloc(size_t size);
size_t malloc_usable_size(void *ptr);
void free(void *ptr);
#include <malloc.h>

int mallopt(int cmd, int value);
struct mallinfo mallinfo(void);


These functions provide a simple general-purpose memory allocation package. These routines are space-efficient but have lower performance than other malloc implementations. Their usage can result in serious performance degradation.

They operate as described on the malloc(3C) manual page, except for the following differences:

  • The memory allocation algorithm may be tuned by using the mallopt() function described below, and observed by using the mallinfo() function described below.

  • The mallopt() can control whether the contents of the block at ptr have been destroyed after free() is performed, as described below.

  • Support for using adi(7) is not available.

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


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.


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.


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.


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.


Unlike some historical implementations of 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(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.

See malloc(3C) for an overview and comparison of all the allocation libraries provided by Oracle Solaris.


See attributes(7) for descriptions of the following attributes:


See Also

brk(2), malloc(3C), libmalloc(3LIB), attributes(7)