（C）如何寫入/讀取mmap返回的內存地址？

Question

我已經閱讀了一些關於如何提問的頁面，所以我希望這符合標准。

我們的教授希望我們建立一個自定義的malloc和free，一個使用伙伴分配。 他沒有亂用堆，而是希望我們只使用mmap從操作系統請求1 GiB的空間：

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

每個內存塊應該有一個標頭，如果內存為空，則通過鏈表指向下一個和前一個空閑塊。

我不知道怎么說“我想把這些具體數據放在這個特定的地方。” 我想象一個免費的塊在內存中看起來像這樣：

[Occupancy (1 bit)][Size (7 bits)][prev pointer (8 bytes)][next pointer (8bytes)][junk]

所以，讓我們說整個1 GiB是免費的。 偽代碼：

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

我如何在我想要的地址創建這些變量？

我試過這個，

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);

編譯和運行沒有問題，但我意識到嘗試訪問值，

printf("%c", *base); //line 37
struct mallocList* two;
two->prev = NULL;
two->next = NULL;
tempaddr->next = *two; //line 41

編譯說，

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，我認為存儲數據或檢索數據的方法有問題，我非常感謝可以提供的任何幫助。

這是一個頭文件mymalloc.h：

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

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

} mallocList;

Answer 1

您的編譯器錯誤解釋了主要問題：您無法取消引用void* 。 將指針轉換為char*並存儲您想要的任何字節，或將其轉換為struct yourstruct *並使用p->field存儲到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;

抱歉，這可能無法編譯，但關於轉換指針的基本想法是可靠的。