malloc(0)

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我是靠谱客的博主明亮月饼，最近开发中收集的这篇文章主要介绍malloc(0)，觉得挺不错的，现在分享给大家，希望可以做个参考。

概述

最近，看了有关malloc(0)的返回值以及其他一些问题的讨论，我把自己的感受和看法记录如下：

问题：

char* ptr = malloc(0*sizeof(char));

if(NULL == ptr) {
    printf("got a NULL pointer");
} else {
    printf("got a Valid pointer");
}

　　通过查看malloc的man手册：The malloc() function allocates size bytes and returns a pointer to the allo‐cated memory. The memory is not initialized. If size is 0, then malloc() returns either NULL, or a unique pointer value that can later be successfully passed to free().这句话翻译起来，就是传个0的话，返回值要么是NULL，要么是一个可以被free调用的唯一的指针。
　　通过下面代码来测试：

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <malloc.h>

int alloc_memory(char *p , int size)
{
    printf("nbefore malloc %pn",p);

    p = (char *)malloc(size);
    if (!p) {
        printf("malloc errorn");
        return -1;
    }

    printf("len of malloc(%d) is %lu,the ture is %lun", size, strlen(p), malloc_usable_size(p));

    printf("the first member of malloc(%d) is %p : %d n", size, p, *p);

    *p = 10;
    printf("set the first member of malloc(%d) is %p : %d n", size, p, *p);

    memset(p, '', 24);
    memcpy(p, "012345678901234567891235", 23);
    printf("after memcpy, the content is %s len is %lu, the ture is %lun", p, strlen(p), malloc_usable_size(p));

    free(p);
    p = NULL;

    printf("n");
}


int main(int argc ,char **argv)
{
    int size = -1;

    char *p = NULL;

    size = 0;
    alloc_memory(p,size);

    size = 5;
    alloc_memory(p,size);

    size = 40;
    alloc_memory(p,size);

    return 0;
}

上述代码用gcc 4.8.4编译运行结果如下：

zxd@xlg:~/c++$ gcc test.c
zxd@xlg:~/c++$ ./a.out 

before malloc (nil)
len of malloc(0) is 0,the ture is 24
the first member of malloc(0) is 0x22fc010 : 0 
set the first member of malloc(0) is 0x22fc010 : 10 
after memcpy, the content is 01234567890123456789123 len is 23, the ture is 24


before malloc (nil)
len of malloc(5) is 0,the ture is 24
the first member of malloc(5) is 0x22fc010 : 0 
set the first member of malloc(5) is 0x22fc010 : 10 
after memcpy, the content is 01234567890123456789123 len is 23, the ture is 24


before malloc (nil)
len of malloc(40) is 0,the ture is 40
the first member of malloc(30) is 0x22fc030 : 0 
set the first member of malloc(30) is 0x22fc030 : 10 
after memcpy, the content is 01234567890123456789123 len is 23, the ture is 40

从测试结果来看，可以得出以下几个结论：

malloc(0)在我的系统里是可以正常返回一个非NULL值的。这个从申请前打印的before malloc (nil)和申请后的地址0x22fc010可以看出来，返回了一个正常的地址。
malloc(0)申请的空间到底有多大不是用strlen或者sizeof来看的，而是通过malloc_usable_size这个函数来看的。—当然这个函数并不能完全正确的反映出申请内存的范围。
malloc(0)申请的空间长度不是0，在我的系统里它是24，也就是你使用malloc申请内存空间的话，正常情况下系统会返回给你一个至少24Byte的空间。这个可以从malloc(0)和malloc(5)的返回值都是24，而malloc(40)的返回值是40得到。—其实，如果你真的调用了这个程序的话，会发现，这个24确实是”至少24“的。
malloc(0)申请的空间是可以被使用的。这个可以从*p = 10;及memcpy(p, "012345678901234567891235", 23);可以得出。