malloc

calloc, malloc, free, realloc - Allocate and free dynamic memory

#include <stdlib.h>
 I void *calloc(size_t  nmemb , size_t  size );

I void *malloc(size_t  size );

I void free(void  *ptr );

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

R calloc () allocates memory for an array of nmemb elements of size bytes each and returns a pointer to the allocated memory. The memory is set to zero.

R malloc () allocates size bytes and returns a pointer to the allocated memory. The memory is not cleared.

R free () frees the memory space pointed to by R ptr , which must have been returned by a previous call to R malloc (), R calloc () or R realloc (). Otherwise, or if I free( ptr ) has already been called before, undefined behavior occurs. If ptr is NULL, no operation is performed.

R realloc () changes the size of the memory block pointed to by ptr to size bytes. The contents will be unchanged to the minimum of the old and new sizes; newly allocated memory will be uninitialized. If ptr is NULL, the call is equivalent to R malloc(size) ; if size is equal to zero, the call is equivalent to I free( ptr ) . Unless ptr is NULL, it must have been returned by an earlier call to R malloc (), R calloc () or R realloc (). If the area pointed to was moved, a I free( ptr ) is done.

For R calloc () and R malloc (), the value returned is a pointer to the allocated memory, which is suitably aligned for any kind of variable, or NULL if the request fails.

R free () returns no value.

R realloc () returns a pointer to the newly allocated memory, which is suitably aligned for any kind of variable and may be different from R ptr , or NULL if the request fails. If size was equal to 0, either NULL or a pointer suitable to be passed to R free () is returned. If R realloc () fails the original block is left untouched; it is not freed or moved.

C89, C99.

The Unix98 standard requires R malloc (), R calloc (), and R realloc () to set errno to R ENOMEM upon failure. Glibc assumes that this is done (and the glibc versions of these routines do this); if you use a private malloc implementation that does not set R errno , then certain library routines may fail without having a reason in R errno .

Crashes in R malloc (), R calloc (), R realloc (), or R free () are almost always related to heap corruption, such as overflowing an allocated chunk or freeing the same pointer twice.

Recent versions of Linux libc (later than 5.4.23) and GNU libc (2.x) include a malloc implementation which is tunable via environment variables. When R MALLOC_CHECK_ is set, a special (less efficient) implementation is used which is designed to be tolerant against simple errors, such as double calls of R free () with the same argument, or overruns of a single byte (off-by-one bugs). Not all such errors can be protected against, however, and memory leaks can result. If R MALLOC_CHECK_ is set to 0, any detected heap corruption is silently ignored; if set to 1, a diagnostic message is printed on stderr; if set to 2, abort(3) is called immediately; if set to 3, a diagnostic message is printed on stderr and the program is aborted. Using a non-zero MALLOC_CHECK_ value can be useful because otherwise a crash may happen much later, and the true cause for the problem is then very hard to track down.

By default, Linux follows an optimistic memory allocation strategy. This means that when R malloc () returns non-NULL there is no guarantee that the memory really is available. This is a really bad bug. In case it turns out that the system is out of memory, one or more processes will be killed by the infamous OOM killer. In case Linux is employed under circumstances where it would be less desirable to suddenly lose some randomly picked processes, and moreover the kernel version is sufficiently recent, one can switch off this overcommitting behavior using a command like:

# echo 2 > /proc/sys/vm/overcommit_memory

See also the kernel Documentation directory, files vm/overcommit-accounting and R sysctl/vm.txt .

brk(2), posix_memalign(3)