how to find size of memory allocated to a pointer?

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valloc

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

NAME

       posix_memalign, memalign, valloc - Allocate aligned memory

SYNOPSIS

       #include <stdlib.h>

       int posix_memalign(void **memptr, size_t alignment, size_t size);
       void *memalign(size_t boundary, size_t size);
       void *valloc(size_t size);

DESCRIPTION

       The function posix_memalign() allocates size bytes and places the address of the allocated memory in *memptr.  The address of the allocated
       memory will be a multiple of alignment, which must be a power of two and a multiple of sizeof(void *).

       The obsolete function memalign() allocates size bytes and returns a pointer to the allocated memory.  The memory address will be a multiple
       of boundary, which must be a power of two.

       The  obsolete  function valloc() allocates size bytes and returns a pointer to the allocated memory.  The memory address will be a multiple
       of the page size.  It is equivalent to memalign(sysconf(_SC_PAGESIZE),size).

       For all three routines, the memory is not zeroed.

RETURN VALUE

       memalign() and valloc() return the pointer to the allocated memory, or NULL if the request fails.

       posix_memalign() returns zero on success, or one of the error values listed in the next section on failure. Note that errno is not set.

ERRORS

       EINVAL The alignment parameter was not a power of two, or was not a multiple of sizeof(void *).

       ENOMEM There was insufficient memory to fulfill the allocation request.

NOTES

       posix_memalign() verifies that alignment matches the requirements detailed above.  memalign() may not check that the boundary parameter	is
       correct.

       POSIX  requires	that memory obtained from posix_memalign() can be freed using free().  Some systems provide no way to reclaim memory allo-
       cated with memalign() or valloc() (because one can only pass to free() a pointer gotten from malloc(), while e.g.   memalign()  would  call
       malloc() and then align the obtained value).  GNU libc allows memory obtained from any of these three routines to be reclaimed with free().

       GNU libc malloc() always returns 8-byte aligned memory addresses, so these routines are only needed if you require larger alignment values.

AVAILABILITY

       The  functions  memalign()  and valloc() have been available in all Linux libc libraries.  The function posix_memalign() is available since
       glibc 2.1.91.

CONFORMING TO

       The function valloc() appeared in 3.0 BSD. It is documented as being obsolete in BSD 4.3, and as legacy in SUSv2. It no	longer	occurs	in
       SUSv3.  The function memalign() appears in SunOS 4.1.3 but not in BSD 4.4.  The function posix_memalign() comes from POSIX 1003.1d.

SEE ALSO

       malloc(3), free(3), getpagesize(2), brk(2)

GNU
								    2001-10-11							 POSIX_MEMALIGN(3)

HP-UX