calloc, malloc, free, realloc - Allocate and free dynamic
void *calloc(size_t nmemb, size_t size));
void *malloc(size_t size));
void free(void *ptr));
void *realloc(void *ptr, size_t size));
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.
malloc() allocates size bytes and returns a pointer to the
allocated memory. The memory is not cleared.
free() frees the memory space pointed to by ptr, which must
have been returned by a previous call to malloc(), calloc()
or realloc(). Otherwise, or if free(ptr)) has already been
called before, undefined behaviour occurs. If ptr is NULL,
no operation is performed.
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 malloc(size); if size is equal to zero, the
call is equivalent to free(ptr)). Unless ptr is NULL, it
must have been returned by an earlier call to malloc(), cal-
loc() or realloc().
For calloc() and 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.
free() returns no value.
realloc() returns a pointer to the newly allocated memory,
which is suitably aligned for any kind of variable and may
be different from ptr, or NULL if the request fails or if
size was equal to 0. If realloc() fails the original block
is left untouched - it is not freed or moved.
The Unix98 standard requires malloc(), calloc(), and real-
loc() to set errno to 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 errno, then certain library routines may fail
without having a reason in errno.
Crashes in malloc(), free() or realloc() 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 MALLOC_CHECK_ is set, a
special (less efficient) implementation is used which is
designed to be tolerant against simple errors, such as dou-
ble calls of free() with the same argument, or overruns of a
single byte (off-by-one bugs). Not all such errors can be
proteced against, however, and memory leaks can result. If
MALLOC_CHECK_ is set to 0, any detected heap corruption is
silently ignored; if set to 1, a diagnostic is printed on
stderr; if set to 2, abort() is called immediately. This
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.