（C） 如何写入/读取mmap返回的内存地址？

我已经阅读了一些关于如何提问的页面，所以我希望这符合标准

我们的教授希望我们建立一个自定义的malloc和free，一个使用伙伴分配的。他希望我们只使用mmap从操作系统请求1Gib的空间，而不是搅乱堆：

MAX_MEM = 1 << 30.
void * base = mmap(NULL, MAX_MEM, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANON, 0, 0);

因此，假设整个1 GiB是免费的。伪代码：

Occupancy = 0; // 0 if empty, 1 if allocated
Size = 0011110; // where size in bytes = 2^Size
next = NULL;
prev = NULL; //note that these are part of a struct called mallocList

如何在我想要的地址创建这些变量

我试过这个

int MAX_MEM = 1 << 30;
base = mmap(NULL, MAX_MEM, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANON, 0, 0);

*((unsigned char*) base) = 0x1E;
struct mallocList* temp;
temp->prev = NULL;
temp->next = NULL;
void* tempaddr = base + 1;

*((struct mallocList*) tempaddr) = *temp;

munmap(base, 1 <<30);

编辑说

3.c:37: warning: dereferencing ‘void *’ pointer
3.c:37: error: invalid use of void expression
3.c:41: warning: dereferencing ‘void *’ pointer
3.c:41: error: request for member ‘next’ in something not a structure or union

因此，我认为我存储数据或检索数据的方法有问题，我非常感谢能够提供的任何帮助

下面是一个头文件mymalloc.h：

void *my_buddy_malloc(int size);
void my_free(void *ptr);

struct mallocList
{
  struct mallocList *prev;
  struct mallocList *next;

} mallocList;

---

编译器错误解释了主要问题：无法取消对

void*

的引用。将指针强制转换为

char*

并存储所需的任何字节，或将其强制转换为

struct yourstruct*

并使用

p->field

存储到struct字段

/* You need to tell gcc to pack the struct without padding,
 * because you want the pointers stored starting with the second byte,     i.e. unaligned.
 * That's actually fine in *this* case, since they won't cross a cache-line boundary.
 * They'll be at the beginning of a page, from mmap, and thus the beginning of a cache line.  
 * Modern CPUs are fast at handling misaligned loads within a cache line.
 */

struct __attribute__ ((__packed__)) mem_block {
    unsigned int occupied:1;
    unsigned int size:7;   // bitfields.  Not necessarily a good idea.  Just using a signed int yourself might be better.  positive for allocated, negative for free.
    struct mallocList { // nested definition.  You can do this differently
        struct mallocList *prev, *next;
    } pointers;
};  // don't need the type-name here.  That would declare a variable of the struct type.


int MAX_MEM = 1 << 30;
void *base = mmap(NULL, MAX_MEM, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANON, 0, 0);

char *cp = base;
cp[0] = size << 1 | 1;  // pack the size and occupied bits into a byte
struct mallocList *mlp = (struct mallocList*)(cp+1);  // This avoids needing a compiler-specific way to pack your struct.

// or
struct mem_block *mbp = base;
mbp->occupied = 1;
mbp->size=whatever;
mbp->pointers.prev = NULL;
mbp->pointers.next = NULL;

/*您需要告诉gcc在不填充的情况下打包结构，
*因为您希望指针以第二个字节开始存储，即未对齐。
*在这种情况下，这实际上很好，因为它们不会跨越缓存线边界。
*它们将位于页面的开头，来自mmap，因此是缓存线的开头。
*现代CPU能够快速处理缓存线中未对齐的负载。
*/
结构属性内存块{
无符号整数：1；
unsigned int size:7；//位域。不一定是个好主意。自己使用带符号的int可能更好。分配为正，免费为负。
结构mallocList{//嵌套定义。您可以用不同的方法执行此操作
结构mallocList*prev，*next；
}指针；
};  // 这里不需要类型名称。这将声明一个结构类型的变量。
int MAX_MEM=1大小=任意值；
mbp->pointers.prev=NULL；
mbp->pointers.next=NULL；

---

抱歉，这可能无法编译，但转换指针的基本思想是可靠的。

您需要为指针

two

分配内存，因为它目前尚未初始化，还请显示您的结构定义。请注意，

mymalloc.h

的

struct mallocList

定义也声明了该类型的全局变量。如果定义位于

typedef

中，则在结构定义之后使用标识符可以执行类似

typedef结构名称{int foo；}name\t

所以你不必到处写

struct name

，只要

name\u t

。

void *my_buddy_malloc(int size);
void my_free(void *ptr);

struct mallocList
{
  struct mallocList *prev;
  struct mallocList *next;

} mallocList;

/* You need to tell gcc to pack the struct without padding,
 * because you want the pointers stored starting with the second byte,     i.e. unaligned.
 * That's actually fine in *this* case, since they won't cross a cache-line boundary.
 * They'll be at the beginning of a page, from mmap, and thus the beginning of a cache line.  
 * Modern CPUs are fast at handling misaligned loads within a cache line.
 */

struct __attribute__ ((__packed__)) mem_block {
    unsigned int occupied:1;
    unsigned int size:7;   // bitfields.  Not necessarily a good idea.  Just using a signed int yourself might be better.  positive for allocated, negative for free.
    struct mallocList { // nested definition.  You can do this differently
        struct mallocList *prev, *next;
    } pointers;
};  // don't need the type-name here.  That would declare a variable of the struct type.


int MAX_MEM = 1 << 30;
void *base = mmap(NULL, MAX_MEM, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANON, 0, 0);

char *cp = base;
cp[0] = size << 1 | 1;  // pack the size and occupied bits into a byte
struct mallocList *mlp = (struct mallocList*)(cp+1);  // This avoids needing a compiler-specific way to pack your struct.

// or
struct mem_block *mbp = base;
mbp->occupied = 1;
mbp->size=whatever;
mbp->pointers.prev = NULL;
mbp->pointers.next = NULL;