MALLOC (3)
Allocate and free dynamic memory
SYNOPSIS
#include <stdlib.h>
void *calloc(size_t nmemb , size_t size );
void *malloc(size_t size );
void free(void *ptr );
void *realloc(void *ptr , size_t size );
DESCRIPTION
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() ,
calloc()
or
realloc() .
RETURN VALUES
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.
CONFORMING TO
SEE ALSO
NOTES
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.
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