MirOS Manual: calloc(3), cfree(3), free(3), malloc(3), realloc(3), reallocarray(3), malloc.conf(5)

MALLOC(3)                  BSD Programmer's Manual                   MALLOC(3)


     malloc, calloc, reallocarray, realloc, free, cfree - memory allocation
     and deallocation


     #include <stdlib.h>

     void *
     malloc(size_t size);

     void *
     calloc(size_t nmemb, size_t size);

     void *
     reallocarray(void *ptr, size_t nmemb, size_t size);

     void *
     realloc(void *ptr, size_t size);

     free(void *ptr);

     cfree(void *ptr);

     char *malloc_options;


     Note: if using brk(2) malloc or mmap(2) malloc instead of omalloc, this
     manual page is inaccurate, especially regarding the protection functions.

     The malloc() function allocates uninitialised space for an object of the
     specified size. malloc() maintains multiple lists of free blocks accord-
     ing 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 larger, the memory
     returned will be page-aligned.

     The calloc() function allocates space for an array of nmemb objects, each
     of the specified size. The space is initialised to zero.

     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 uninitialised. If the space can-
     not be allocated, the object pointed to by ptr is unchanged. If ptr is
     NULL, realloc() behaves like malloc() and allocates a new object.

     The reallocarray() function is similar to realloc() except it operates on
     nmemb members of size size and checks for integer overflow in the calcu-
     lation nmemb * size.

     The free() function causes the space pointed to by ptr to be either
     placed on a list of free pages to make it available for future allocation
     or, if required, to be returned to the kernel using munmap(2). If ptr is
     a NULL pointer, no action occurs. If ptr was previously freed by free(),
     realloc(), or reallocarray(), the behavior is undefined and the double
     free is a security concern.

     A cfree() function is also provided for compatibility with old systems
     and other malloc libraries; it is simply an alias for free().


     Upon successful completion, the functions malloc(), calloc(), realloc(),
     and reallocarray() return a pointer to the allocated space; otherwise, a
     NULL pointer is returned and errno is set to ENOMEM.

     If size or nmemb is equal to 0, a unique pointer to an access protected,
     zero sized object is returned. Access via this pointer will generate a
     SIGSEGV exception.

     If multiplying nmemb and size results in integer overflow, calloc() and
     reallocarray() return NULL and set errno to ENOMEM.

     The free() and cfree() functions return no value.


     Consider calloc() or the extension reallocarray() when there is multipli-
     cation in the size argument of malloc() or realloc(). For example, avoid
     this common idiom as it may lead to integer overflow:

           if ((p = malloc(num * size)) == NULL)
                   err(1, "malloc");

     A drop-in replacement is the OpenBSD extension reallocarray():

           if ((p = reallocarray(NULL, num, size)) == NULL)
                   err(1, "reallocarray");

     Alternatively, calloc() may be used at the cost of possible initialisa-
     tion overhead.

     When using realloc(), be careful to avoid the following idiom:

           size += 50;
           if ((p = realloc(p, size)) == NULL)
                   return (NULL);

     Do not adjust the variable describing how much memory has been allocated
     until the allocation has been successful. This can cause aberrant program
     behavior if the incorrect size value is used. In most cases, the above
     sample will also result in a leak of memory. As stated earlier, a return
     value of NULL indicates that the old object still remains allocated.
     Better code looks like this:

           newsize = size + 50;
           if ((newp = realloc(p, newsize)) == NULL) {
                   p = NULL;
                   size = 0;
                   return (NULL);
           p = newp;
           size = newsize;

     As with malloc(), it is important to ensure the new size value will not
     overflow; i.e. avoid allocations like the following:

           if ((newp = realloc(p, num * size)) == NULL) {

     Instead, use reallocarray():

           if ((newp = reallocarray(p, num, size)) == NULL) {

     Calling realloc() with a NULL ptr is equivalent to calling malloc(). In-
     stead of this idiom:

           if (p == NULL)
                   newp = malloc(newsize);
                   newp = realloc(p, newsize);

     Use the following:

           newp = realloc(p, newsize);


     MALLOC_OPTIONS   See below.


     /etc/malloc.conf  symbolic link to filename containing option flags


     If malloc() must be used with multiplication, be sure to test for over-

           size_t num, size;

           /* Check for size_t overflow */
           if (size && num > SIZE_MAX / size)
                   errc(1, EOVERFLOW, "overflow");

           if ((p = malloc(size * num)) == NULL)
                   err(1, "malloc");

     The above test is not sufficient in all cases. For example, multiplying
     signed integers requires a different set of checks:

           int num, size;

           /* Avoid invalid requests */
           if (size < 0 || num < 0)
                   errc(1, EOVERFLOW, "overflow");

           /* Check for signed int overflow */
           if (size && num > INT_MAX / size)
                   errc(1, EOVERFLOW, "overflow");

           if ((p = malloc(size * num)) == NULL)
                   err(1, "malloc");

     Assuming the implementation checks for integer overflow as OpenBSD does,
     it is much easier to use calloc() or reallocarray().

     The above examples could be simplified to:

           if ((p = reallocarray(NULL, num, size)) == NULL)
                   err(1, "reallocarray");

     or, at the cost of possible initialisation:

           if ((p = calloc(num, size)) == NULL)
                   err(1, "calloc");


     If malloc(), calloc(), realloc(), reallocarray(), or free() detect an er-
     ror condition, a message will be printed to file descriptor 2 (not using
     stdio). Errors will result in the process being aborted, unless the a op-
     tion has been specified.

     Here is a brief description of the error messages and what they mean:

     "out of memory"
             If the X option is specified it is an error for malloc(), cal-
             loc(), realloc(), or reallocarray() to return NULL.

     "malloc init mmap failed"
             This is a rather weird condition that is most likely to indicate
             a seriously overloaded system or a ulimit restriction.

     "bogus pointer (double free?)"
             An attempt to free(), realloc(), or reallocarray() an unallocated
             pointer was made.

     "chunk is already free"
             There was an attempt to free a chunk that had already been freed.

     "modified chunk-pointer"
             The pointer passed to free(), realloc(), or reallocarray() has
             been modified.

     "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 utilise any other functions which may call malloc()
             (e.g., stdio(3) routines).

     "unknown char in MALLOC_OPTIONS"
             We found something we didn't understand.

     "malloc cache overflow/underflow"
             The internal malloc page cache has been corrupted.

     "malloc free slot lost"
             The internal malloc page cache has been corrupted.

     "guard size"
             An inconsistent guard size was detected.

     any other error
             malloc() detected an internal error; consult sources and/or


     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 malloc_options and scan them for flags in
     that order. Flags are single letters, uppercase means on, lowercase means

     A       "Abort". malloc() will coredump the process, rather than tolerate
             internal inconsistencies or incorrect usage. This is the default
             and a very handy debugging aid, since the core file represents
             the time of failure, rather than when the bogus pointer was used.

     D       "Dump". malloc() will dump statistics to the file ./malloc.out,
             if it already exists, at exit. This option requires the library
             to have been compiled with -DMALLOC_STATS in order to have any

     F       "Freeguard". Enable use after free detection. Unused pages on the
             freelist are read and write protected to cause a segmentation
             fault upon access. This will also switch off the delayed freeing
             of chunks, reducing random behaviour but detecting double free()
             calls as early as possible. This option is intended for debugging
             rather than improved security (use the U option for security).

     G       "Guard". Enable guard pages. Each page size or larger allocation
             is followed by a guard page that will cause a segmentation fault
             upon any access.

     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.

     J       "Junk". Fill some junk into the area allocated. Currently junk is
             bytes of 0xd0 when allocating; this is pronounced "Duh". :-)
             Freed chunks are filled with 0xdf.

     j       "Don't Junk". By default, small chunks are always junked, and the
             first part of pages is junked after free. This option ensures
             that no junking is performed.

     P       "Move allocations within a page." Allocations larger than half a
             page but smaller than a page are aligned to the end of a page to
             catch buffer overruns in more cases. This is the default.

     R       "realloc". Always reallocate when realloc() is called, even if
             the initial allocation was big enough. This can substantially aid
             in compacting memory.

     S       Enable all options suitable for security auditing.

     U       "Free unmap". Enable use after free protection for larger alloca-
             tions. Unused pages on the freelist are read and write protected
             to cause a segmentation fault upon access.

     X       "xmalloc". Rather than return failure, abort(3) the program with
             a diagnostic message on stderr. It is the intention that this op-
             tion be set at compile time by including in the source:

                   extern char *malloc_options;
                   malloc_options = "X";

             Note that this will cause code that is supposed to handle out-
             of-memory conditions gracefully to abort instead.

     <       "Half the cache size". Decrease the size of the free page cache
             by a factor of two.

     >       "Double the cache size". Increase the size of the free page cache
             by a factor of two.

     So to set a systemwide reduction of the cache to a quarter of the default
     size and use guard pages:
           # ln -s 'G<<' /etc/malloc.conf

     The flags are mostly for testing and debugging. If a program changes
     behavior if any of these options (except X) are used, it is buggy.

     The default number of free pages cached is 64.


     brk(2), mmap(2), munmap(2), alloca(3), getpagesize(3), posix_memalign(3),


     The malloc(), calloc(), realloc(), and free() functions conform to ANSI
     X3.159-1989 ("ANSI C").

     If size or nmemb are 0, the return value is implementation defined; other
     conforming implementations may return NULL in this case.

     The standard does not require calloc() to check for integer overflow, but
     most modern implementations provide this check.

     The MALLOC_OPTIONS environment variable, the file /etc/malloc.conf, and
     the DIAGNOSTICS output are extensions to the standard.


     A free() internal kernel function and a antecessor to malloc(), alloc(),
     first appeared in Version 1 AT&T UNIX. C library functions alloc() and
     free() appeared in Version 6 AT&T UNIX. The functions malloc(), calloc(),
     and realloc() first appeared in Version 7 AT&T UNIX.

     A new implementation by Chris Kingsley was introduced in 4.2BSD, followed
     by a complete rewrite by Poul-Henning Kamp which appeared in FreeBSD 2.2
     and was included in OpenBSD 2.0. These implementations were all sbrk(2)
     based. In OpenBSD 3.8, Thierry Deval rewrote malloc to use the mmap(2)
     system call, making the page addresses returned by malloc random. A
     rewrite by Otto Moerbeek introducing a new central data structure and
     more randomisation appeared in OpenBSD 4.4.

     The reallocarray() function appeared in OpenBSD 5.6.

     The cfree() function appeared in SunOS 4.x.


     When using malloc(), be wary of signed integer and size_t overflow espe-
     cially when there is multiplication in the size argument.

     Signed integer overflow will cause undefined behavior which compilers
     typically handle by wrapping back around to negative numbers. Depending
     on the input, this can result in allocating more or less memory than in-

     An unsigned overflow has defined behavior which will wrap back around and
     return less memory than intended.

     A signed or unsigned integer overflow is a security risk if less memory
     is returned than intended. Subsequent code may corrupt the heap by writ-
     ing beyond the memory that was allocated. An attacker may be able to lev-
     erage this heap corruption to execute arbitrary code.

     Consider using calloc() or reallocarray() instead of using multiplication
     in malloc() and realloc() to avoid these problems on OpenBSD.

MirOS BSD #10-current            July 7, 2015                                5

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