NAME
calloc, malloc, free, realloc - 动态分配和回收内存
SYNOPSIS
#include <stdlib.h>
void *malloc(size_t size);
void free(void *ptr);
void *calloc(size_t nmemb, size_t size);
void *realloc(void *ptr, size_t size);
DESCRIPTION
calloc() 为一个具有 nmemb 个元素，每个元素大小为 size 的数组（也可以为结构体类型分配）分配内存.并返回一个指向首元素的指针，同时将初始化该片内存为 0 . 如果 nmemb 或 size 为 0, calloc() 将会要么返回 NULL， 要么返回 一个稍后可以被成功传递给 free() 的唯一指针值。
malloc() 分配 size 字节 内存 ， 并且返回一个指向已分配内存的指针. 该内存并不会被初始化. 如果 size 为 0 ,则将会要么返回 NULL， 要么返回 一个稍后可以被成功传递给 free() 的唯一指针值。
free() 回收被指针 ptr 指向的内存空间, 并且 ptr 必须是在此之前调用 malloc(), calloc() 或 realloc() 返回所得到的. 此外 如果 free(ptr) 已经被回收过一次,则再次调用 free(ptr) 来进行回收，将会发生不可预料的后果，如果此时 ptr 为 NULL,则不会产生任何操作.
realloc() 修改 指针 ptr 指出的内存空间为 size 字节. 并且之前所指和之后所指的交集部分中的内容不会改变。且新分配的部分不会被初始化; . 若 ptr 为 NULL,该调用等同于调用 malloc(size); 若 size 为 zero,
该调用等同于调用 free(ptr). 除非 ptr 为 NULL, 否则 ptr 必须是在此之前调用 malloc(), calloc() 或 realloc() 返回所得到的.若 ptr 所指的内存空间已经被移动到其他地方，则此时的调用等同于调用 free(ptr).
RETURN VALUE
对于 calloc() 和 malloc(), 若请求分配成功，返回值是一个指向分配区域的指针,并且该指针可以合适的于、与任意类型的变量对齐, 若请求失败，则返回 NULL
free() 函数 无返回值.
realloc() 返回一个指向新分配内存区域的指针,的指针,并且该指针可以合适的于、与任意类型的变量对齐,
并且 该指针 可能与 ptr 不同, 或者在请求失败时候返回 NULL . 若 size 为 0, 要么 NULL 指针被返回
要么返回一个可以传递给 free（） 的适当的指针被返回. 如果 realloc（） 调用失败 ，则原来的内存块将不会被改变。
CONFORMING TO
C89, C99.
NOTES
通常, malloc() 从 heap 分配内存, 并且必须通过调节 heap 的大小来获得自由空间,与 using sbrk(2)不同. When allocating blocks of mem
ory larger than MMAP_THRESHOLD bytes, the glibc malloc() implementation
allocates the memory as a private anonymous mapping using mmap(2).
MMAP_THRESHOLD is 128 kB by default, but is adjustable using mal
lopt(3). Allocations performed using mmap(2) are unaffected by the
RLIMIT_DATA resource limit (see getrlimit(2)).
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(), calloc(), realloc(), or free() 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 glibc (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 over
runs of a single byte (off-by-one bugs). Not all such errors can be
protected against, however, and memory leaks can result. If MAL
LOC_CHECK_ is set to 0, any detected heap corruption is silently
ignored; if set to 1, a diagnostic message is printed on stderr; if set
to 2, abort(3) is called immediately; if set to 3, a diagnostic message
is printed on stderr and the program is aborted. Using a nonzero MAL
LOC_CHECK_ value 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 pro
cesses will be killed by the infamous OOM killer. In case Linux is
employed under circumstances where it would be less desirable to sud
denly lose some randomly picked processes, and moreover the kernel ver
sion is sufficiently 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.
SEE ALSO
brk(2), mmap(2), alloca(3), posix_memalign(3)
COLOPHON
This page is part of release 2.77 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can
be found at http://www.kernel.org/doc/man-pages/.
GNU 2007-09-15 MALLOC(3)