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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
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(), 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
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 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.