关于malloc(0)的返回值问题--这两天的总结与实践篇

malloc() allocates size bytes and returns a pointer to the allocated memory. The memory is not cleared. 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。那是不是这篇文章中说的，通过这句话“

复制代码if(int pp = (strlen(ptr=(char *)malloc(0))) == 0)

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if(int pp = (strlen(ptr=(char *)malloc(0))) == 0)

”来判断是不是NULL指针呢？当然，实际情况到底如何，还得看代码。

刚看到@garbageMan一篇文章 http://www.cnblogs.com/pmer/p/3222648.html 这样写道：“malloc(0)唯一不同的地方就是，就算你申请内存成功，即malloc(0)返回值不为NULL，你也没法使用这块内存。”那到底是不是就没法使用呢？

我的测试代码：

复制代码#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 error n"); return -1; } //len of malloc(0) printf("len of malloc(%d) is %d ,the ture is %dn",size,strlen(p),malloc_usable_size(p)); //the first member printf("the first member of malloc(%d) is %p:%d n",size,p,*p); //set the first member *p = 10; printf("set the first member of malloc(%d) is %p:%d n",size,p,*p); //memcpy memset(p,'',12); memcpy(p,"01234567890123456789",12); printf("after memcpy , the content is %s len is %d , the ture is %d n",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; //malloc(0) size = 0; alloc_memory(p,size); //malloc(5) size = 5; alloc_memory(p,size); //malloc(20) size = 20; alloc_memory(p,size); return 0; }

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#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 error  n");
        return -1;
    }

    //len of malloc(0)
    printf("len of malloc(%d)  is  %d  ,the ture is %dn",size,strlen(p),malloc_usable_size(p));

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

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

    //memcpy
    memset(p,'',12);
    memcpy(p,"01234567890123456789",12);
    printf("after memcpy , the content is %s   len is %d  , the ture is %d n",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;

    //malloc(0)
    size = 0;
    alloc_memory(p,size);
    
    //malloc(5)
    size = 5;
    alloc_memory(p,size);

    //malloc(20)
    size = 20;
    alloc_memory(p,size);
    return 0;
}

测试结果如下：

复制代码tiger@ubuntu:/mnt/hgfs/e/Lessons/MyExercise/UtilLibs/EXERCISE$ gcc -o malloc malloc.c tiger@ubuntu:/mnt/hgfs/e/Lessons/MyExercise/UtilLibs/EXERCISE$ ./malloc before malloc (nil) len of malloc(0) is 0 ,the ture is 12 the first member of malloc(0) is 0x9e78008:0 set the first member of malloc(0) is 0x9e78008:10 after memcpy , the content is 012345678901len is 15 , the ture is 12 before malloc (nil) len of malloc(5) is 0 ,the ture is 12 the first member of malloc(5) is 0x9e78008:0 set the first member of malloc(5) is 0x9e78008:10 after memcpy , the content is 012345678901len is 15 , the ture is 12 before malloc (nil) len of malloc(20) is 0 ,the ture is 20 the first member of malloc(20) is 0x9e78018:0 set the first member of malloc(20) is 0x9e78018:10 after memcpy , the content is 012345678901 len is 12 , the ture is 20 tiger@ubuntu:/mnt/hgfs/e/Lessons/MyExercise/UtilLibs/EXERCISE$

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tiger@ubuntu:/mnt/hgfs/e/Lessons/MyExercise/UtilLibs/EXERCISE$ gcc -o malloc malloc.c 
tiger@ubuntu:/mnt/hgfs/e/Lessons/MyExercise/UtilLibs/EXERCISE$ ./malloc               

before malloc (nil)
len of malloc(0)  is  0  ,the ture is 12
the first member of malloc(0) is 0x9e78008:0 
set the first member of malloc(0) is 0x9e78008:10 
after memcpy , the content is 012345678901len is 15  , the ture is 12 


before malloc (nil)
len of malloc(5)  is  0  ,the ture is 12
the first member of malloc(5) is 0x9e78008:0 
set the first member of malloc(5) is 0x9e78008:10 
after memcpy , the content is 012345678901len is 15  , the ture is 12 


before malloc (nil)
len of malloc(20)  is  0  ,the ture is 20
the first member of malloc(20) is 0x9e78018:0 
set the first member of malloc(20) is 0x9e78018:10 
after memcpy , the content is 012345678901   len is 12  , the ture is 20 

tiger@ubuntu:/mnt/hgfs/e/Lessons/MyExercise/UtilLibs/EXERCISE$

从测试结果来看，可以得出以下几个结论：

1. 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0x9e78008可以看出来，返回了一个正常的地址。

2. 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。

3. malloc(0)申请的空间长度不是0，在我的系统里它是12，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12B的空间。这个可以从malloc(0)和malloc(5)的返回值都是12，而malloc(20)的返回值是20得到。---其实，如果你真的调用了这个程序的话，会发现，这个12确实是”至少12“的。

4. malloc(0)申请的空间是可以被使用的。这个可以从*p = 10;及memcpy(p,"01234567890123456789",12);可以得出。

 

虽然malloc(0)没有发现在现实中有什么意义，但是既然有些人非要我们回答，那我们还是有必要探究一下的，否则你只有被pass掉了。关于这个问题的讨论很值得，因为它让我对技术更加感兴趣，不经意间学到了其他的知识。

如果大家有什么不同意见，欢迎跟帖讨论，谢谢！

注：---后为新增内容。

---

总结：为了安全起见，malloc(0)的非NULL返回值，最好不要进行除了free()之外的任何操作！