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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  allocated
       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)
COLOPHON
       This page is part of release 4.15 of the Linux  man-pages  project.   A
       description  of  the project, information about reporting bugs, and the
       latest    version    of    this    page,    can     be     found     at
       https://www.kernel.org/doc/man-pages/.
GNU                               2017-09-15             MALLOC_USABLE_SIZE(3)