linux man page for malloc_usable_size

MALLOC_USABLE_SIZE(3)					     Linux Programmer's Manual					     MALLOC_USABLE_SIZE(3)

NAME
       malloc_usable_size - obtain size of block of memory allocated from heap

SYNOPSIS
       #include <malloc.h>

       size_t malloc_usable_size (void *ptr);

DESCRIPTION
       The  malloc_usable_size()  function returns the number of usable bytes in the block pointed to by ptr, a pointer to a block of memory allo-
       cated by malloc(3) or a related function.

RETURN VALUE
       malloc_usable_size() returns the number of usable bytes in the block of allocated memory pointed to by ptr.  If ptr is NULL, 0 is returned.

ATTRIBUTES
       For an explanation of the terms used in this section, see attributes(7).

       +---------------------+---------------+---------+
       |Interface	     | Attribute     | Value   |
       +---------------------+---------------+---------+
       |malloc_usable_size() | Thread safety | MT-Safe |
       +---------------------+---------------+---------+
CONFORMING TO
       This function is a GNU extension.

NOTES
       The value returned by malloc_usable_size() may be greater than the requested size of the allocation because of alignment and  minimum  size
       constraints.   Although	the excess bytes can be overwritten by the application without ill effects, this is not good programming practice:
       the number of excess bytes in an allocation depends on the underlying implementation.

       The main use of this function is for debugging and introspection.

SEE ALSO
       malloc(3)

GNU
								    2015-03-02						     MALLOC_USABLE_SIZE(3)