free(3) - OpenBSD manual pages (original) (raw)
MALLOC(3) OpenBSD Programmer's Manual MALLOC(3)
NAME malloc, calloc, realloc, free, cfree - memory allocation and deallocation
SYNOPSIS #include <stdlib.h>
_void_ _*_
**malloc**(_sizet_ _size_);
_void_ _*_
**calloc**(_sizet_ _nmemb_, _sizet_ _size_);
_void_ _*_
**realloc**(_void_ _*ptr_, _sizet_ _size_);
_void_
**free**(_void_ _*ptr_);
_void_
**cfree**(_void_ _*ptr_);
_char_ _*_ _mallocoptions_DESCRIPTION The malloc() function allocates uninitialized space for an object whose size is specified by size. The malloc() function maintains multiple lists of free blocks according to size, allocating space from the appropriate list.
The allocated space is suitably aligned (after possible pointer coercion)
for storage of any type of object. If the space is of _pagesize_ or larg-
er, the memory returned will be page-aligned.
Allocation of a zero size object returns a pointer to a zero size object.
The **calloc**() function allocates space for an array of _nmemb_ objects, each
of whose size is _size_. The space is initialized to all bits zero.
The **free**() function causes the space pointed to by _ptr_ to be deallocated,
that is, at least made available for further allocation, but if possible,
it will passed back to the kernel with sbrk(2). If _ptr_ is a null pointer,
no action occurs.
A **cfree**() function is also provided for compatibility with old systems
and other **malloc** libraries; it is simply an alias for **free**().
The **realloc**() function changes the size of the object pointed to by _ptr_
to _size_ bytes and returns a pointer to the (possibly moved) object. The
contents of the object are unchanged up to the lesser of the new and old
sizes. If the new size is larger, the value of the newly allocated por-
tion of the object is indeterminate and uninitialized. If _ptr_ is a null
pointer, the **realloc**() function behaves like the **malloc**() function for
the specified size. If the space cannot be allocated, the object pointed
to by _ptr_ is unchanged. If _size_ is zero and _ptr_ is not a null pointer,
the object it points to is freed and a new zero size object is returned.
When using **realloc**() one must be careful to avoid the following idiom:
if ((p = realloc(p, nsize)) == NULL)
return NULL;
In most cases, this will result in a leak of memory. As stated earlier,
a return value of NULL indicates that the old object still remains allo-
cated. Better code looks like this:
if ((p2 = realloc(p, nsize)) == NULL) {
if (p)
free(p);
p = NULL;
return NULL;
}
p = p2;
Malloc will first look for a symbolic link called _/etc/malloc.conf_ and
next check the environment for a variable called MALLOC_OPTIONS and fi-
nally for the global variable _mallocoptions_ and scan them for flags in
that order. Flags are single letters, uppercase means on, lowercase
means off.
**A** ``Abort''. **malloc**() will coredump the process, rather than toler-
ate failure. This is a very handy debugging aid, since the core
file will represent the time of failure, rather than when the
null pointer was accessed.
**D** ``Dump''. **malloc**() will dump statistics in a file called
_malloc.out_ at exit. This option requires the library to have
been compiled with -DMALLOC_STATS in order to have any effect.
**J** ``Junk''. Fill some junk into the area allocated. Currently junk
is bytes of 0xd0; this is pronounced ``Duh''. :-)
**H** ``Hint''. Pass a hint to the kernel about pages we don't use. If
the machine is paging a lot this may help a bit.
**N** Do not output warning messages when encountering possible corrup-
tion or bad pointers.
**R** ``realloc''. Always reallocate when **realloc**() is called, even if
the initial allocation was big enough. This can substantially
aid in compacting memory.
**U** ``utrace''. Generate entries for ktrace(1) for all operations.
Consult the source for this one.
**X** ``xmalloc''. rather than return failure, abort(3) the program
with a diagnostic message on stderr. It is the intention that
this option be set at compile time by including in the source:
extern char *malloc_options;
malloc_options = "X";
**Z** ``Zero''. Fill some junk into the area allocated (see **J**), except
for the exact length the user asked for, which is zeroed.
**<** ``Half the cache size''. Reduce the size of the cache by a factor
of two.
**>** ``Double the cache size''. Double the size of the cache by a fac-
tor of two.
So to set a systemwide reduction of cache size and coredumps on problems
one would: ln -s 'A<' /etc/malloc.conf
The **J** and **Z** is mostly for testing and debugging. If a program changes
behavior if either of these options are used, it is buggy.
The default cache size is 16 pages.ENVIRONMENT See above.
RETURN VALUES The malloc() and calloc() functions return a pointer to the allocated space if successful; otherwise, a null pointer is returned and errno is set to ENOMEM.
The **free**() and **cfree**() functions return no value.
The **realloc**() function returns a pointer to the (possibly moved) allocat-
ed space if successful; otherwise, a null pointer is returned and _errno_
is set to ENOMEM.DIAGNOSTICS If malloc(), calloc(), realloc(), or free() detect an error or warning condition, a message will be printed to file descriptor 2 (not using stdio). Errors will always result in the process being abort(3)'ed. If the A option has been specified, warnings will also abort(3) the process.
Here is a brief description of the error messages and what they mean:
``(ES): mumble mumble mumble''
**malloc**() has been compiled with -DEXTRA_SANITY and something
looks fishy in there. Consult sources and/or wizards.
``allocation failed''
If the **A** option is specified it is an error for **malloc**(),
**calloc**(), or **realloc**() to return NULL.
``mmap(2) failed, check limits.''
This is a rather weird condition that is most likely to indi-
cate a seriously overloaded system or a ulimit(1) restric-
tion.
``freelist is destroyed.''
**malloc**()'s internal freelist has been stomped on.
Here is a brief description of the warning messages and what they mean:
``chunk/page is already free.''
A pointer to a free chunk is attempted freed again.
``junk pointer, too high to make sense.''
The pointer doesn't make sense. It's above the area of memo-
ry that **malloc**() knows something about. This could be a
pointer from some mmap(2)'ed memory.
``junk pointer, too low to make sense.''
The pointer doesn't make sense. It's below the area of memo-
ry that **malloc**() knows something about. This pointer proba-
bly came from your data or bss segments.
``malloc() has never been called.''
Nothing has ever been allocated, yet something is being freed
or realloc'ed.
``modified (chunk-/page-) pointer.''
The pointer passed to free or realloc has been modified.
``pointer to wrong page.''
The pointer that **malloc**() is trying to free is not pointing
to a sensible page.
``recursive call.''
An attempt was made to call recursively into these functions,
i.e., from a signal handler. This behavior is not supported.
In particular, signal handlers should _not_ use any of the
**malloc**() functions nor utilize any other functions which may
call **malloc**() (e.g., stdio(3) routines).
``unknown char in MALLOC_OPTIONS''
We found something we didn't understand.FILES /etc/malloc.conf symbolic link to filename containing option flags
SEE ALSO brk(2), alloca(3), getpagesize(3), memory(3) /usr/share/doc/papers/malloc.ascii.gz
STANDARDS The malloc() function conforms to ANSI X3.159-1989 (``ANSI C'').
HISTORY The present implementation of malloc() started out as a filesystem on a drum attached to a 20-bit binary challenged computer built with discrete germanium transistors, and it has since graduated to handle primary stor- age rather than secondary.
The main difference from other **malloc**() implementations are believed to
be that the free pages are not accessed until allocated. Most **malloc**()
implementations will store a data structure containing a, possibly dou-
ble-, linked list in the free chunks of memory, used to tie all the free
memory together. That is a quite suboptimal thing to do. Every time the
free-list is traversed, all the otherwise unused, and very likely paged
out, pages get faulted into primary memory, just to see what lies after
them in the list.
On systems which are paging, this can make a factor five in difference on
the page-faults of a process.OpenBSD 2.9 August 27, 1996 4