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 mem
ory 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(),
calloc() 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 rou
tines 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 allo
cated 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 tun
able 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 double 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 diag
nostic 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.