MALLOC(3) Linux Programmer's Manual MALLOC(3)
calloc, malloc, free, realloc - Allocate and free dynamic memory
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 previ-
ous 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 con-
tents 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(), 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. If size
was equal to 0, either NULL or a pointer suitable to be passed to free() is returned. If
realloc() fails the original block is left untouched - it is not freed or moved.
The Unix98 standard requires malloc(), calloc(), and realloc() 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 double calls of 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 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.
Linux follows an optimistic memory allocation strategy. This means that when malloc()
returns non-NULL there is no guarantee that the memory really is available. In case it
turns out that the system is out of memory, one or more processes will be killed by the
infamous OOM killer.
GNU 1993-04-04 MALLOC(3)