calloc, malloc, free, realloc - Allocate and free dynamic memory
1. calloc: void *calloc(size_t nmemb, 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 se to zero.
函数：calloc(nmemb, size) 分配连续的大小为nmemb*size个字节（byte）内存空间， 返回一个指向所分配的内存空间的首地址的指针，并且所有的内存单元都被设置为0.
分析：由于要把所有分配的内存单元都要初始化为0，其速度相比malloc()，要慢，由于malloc分配的内存并不初始化。
2. malloc： void *malloc(size_t size);
malloc() allocates size bytes and returns a pointer to the allocated memory. The memory is not cleared.
函数：malloc(size) 从堆（heap）上动态分配 size个字节（byte）的内存空间，并返回一个指向所分配的内存空间的首地址的指针。所分配的内存空间没有被初始化和清理。
3. free： void free(void *ptr);
free() frees the memory space pointed to by ptr, which must have been returned by a previous call to malloc(), calloc() or realloc().
Other- wise, or if free(ptr) has already been called before, undefined behaviour occurs. If ptr is NULL, no operation is performed.
函数： free() 函数将 指针ptr指向的内存空间释放，但ptr指向的内存空间必须是由malloc()或者calloc()或者realloc() 从堆上的动态内存，已分配的。
否则， 如果free(ptr) 在此之前 已经被调用， 将产生一个未定义的行为。
如果ptr==NULL， free(ptr), 不执行任何操作。
可见，释放动态内存的一个好习惯：
if(NULL != ptr) {
free(ptr);
ptr = NULL;
}
4. realloc: void *realloc(void *ptr, size_t size);
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().
If the area pointed to was moved, a free(ptr) is done.
函数： realloc(ptr, size) 将ptr指向的内存块的大小重分配为size个字节（byte）。 （可见，可以使ptr指向的内存块增大，也可以是ptr指向的内存块减小）。
内存块中的内容在size比原来的size小时，内容保持不变； 当新size比原来的size大时，原来size大小的内存的内容保持不变，并且新分配的内存没有初始化。
如果ptr==NULL， realloc(ptr, size)功能 等价于malloc(size); 若果size ==0 ， realloc(ptr, size) 功能等价于 free(ptr).
若ptr != NULL, ptr 必须是前一个malloc() 或者 calloc()或者 realloc() 函数返回的指针。
如果内存空间在重分配的过程中发生了移动， 则期间，free(ptr) 被调用，即ptr指向的内存空间清理掉了。
5. calloc(), malloc(), free(), realloc() 的返回值：
RETURN VALUE
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.
对于函数calloc() 和malloc()， 其返回值是一个指向所分配内存空间的首地址的指针，该指针对于任何类型的变量进行适当的对齐。
否则，分配失败时， NULL 作为返回值。
free() returns no value.
free()函数不返回任何值。
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.
函数realloc() , 在动态内存成功分配时，返回一个指向新分配的内存块的首地址的指针，该指针适用于任何类型变量的对齐，而且其可能与ptr不同；否则，失败时，返回一个 NULL.
若果 size==0, 或者返回NULL， 或者返回一个适合传递给free()函数的指针被返回。
若果realloc(ptr， size)函数动态内存分配失败，ptr指向的原有的内存块保持不变，即原来的内存空间，既没有被释放也没有被移动。
6. Linux manual 中的解释如下：
[root@wzb test]# man realloc | cat
MALLOC(3) Linux Programmer’s Manual MALLOC(3)
NAME
calloc, malloc, free, realloc - 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(). Other-
wise, 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().
If the area pointed to was moved, a free(ptr) is done.
RETURN VALUE
For calloc() and malloc(), the value returned is a pointer to the allo-
cated 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.
CONFORMING TO
C89, C99.
SEE ALSO
brk(2), posix_memalign(3)
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 vari-
ables. When MALLOC_CHECK_ is set, a special (less efficient) implemen-
tation 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 and an error
message is not generated; if set to 1, the error message is printed on
stderr, but the program is not aborted; if set to 2, abort() is called
immediately, but the error message is not generated; if set to 3, the
error message is printed on stderr and program is aborted. 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.
BUGS
By default, 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. This is a really bad bug. In case
it turns out that the system is out of memory, one or more processes
will be killed by the infamous OOM killer. In case Linux is employed
under circumstances where it would be less desirable to suddenly lose
some randomly picked processes, and moreover the kernel version is suf-
ficiently recent, one can switch off this overcommitting behavior using
a command like
# echo 2 > /proc/sys/vm/overcommit_memory
See also the kernel Documentation directory, files vm/overcommit-
accounting and sysctl/vm.txt.
GNU 1993-04-04 MALLOC(3)
[root@wzb test]#