彙編+ WinApi MapViewOfFile

Question

我使用MapViewOfFile遇到了一些問題。這個函數返回映射視圖的起始地址，所以我認爲它是一個字節序列。這是我堆疊的地方：

INVOKE MapViewOfFile, hMapFile, FILE_MAP_READ, 0, 0, 0 
mov pMemory, eax 
mov edx, DWORD PTR [pMemory] 

指針是正確的原因在保存整個內存塊到文件，一切都很好。所以我的問題是：如何引用每一個元素（字節）。

在此先感謝

Answer 1

演員pMemory到正確的類型，然後從pMemory移動它被映射的內存的pMemory +大小 - 你提到的類型的大小...

換句話說，您已經有效地分配了內存，並將這個菜單與一個隨着您更改內存而更改的文件關聯起來。

在C假設pMemory是由MapViewOfFile返回的指針：

int x = (*(int *)pMemory); // Read the first int 
char c = (*(char *)pMemory); // Read the first char 
typedef struct oddball { int x, int y, int z, char str[256] } oddball; // assume the typedef syntax is right... 
oddball *p = (oddball *)pMemory; // point to the base of the mapped memory 
p += 14; // p now points to the 15th instance of oddball in the file. 
// Or... p = &p[14]; 
p->x = 0; 
p->y = 0; 
p->z = 0; 
strcpy(p->str("This is the 0, 0, 0 position")); 
// You've now changed the memory to which p[14] refers. 

// To read every byte... (Again in C, use the compiler to generate asm 
// Assumes: 
//  fileSize is the size of the mapped memory in bytes 
//  pMemory is the pointer returned by MapViewOfFile 
//  buffer is a block of memory that will hold n bytes 
//  pos is the position from which you want to read 
//  n is the number of bytes to read from position pos and the smallest size in bytes to which buffer can point 
void readBytes(unsigned int fileSize, char *pMemory, char *buffer, unsigned int n, unsigned int pos) 
{ 
    char *endP = pMemory + fileSize; 
    char *start = pMemory + pos; 
    char *end = start + n; 
    int i = 0; 

    // Code to stay within your memory boundaries 
    if(end > endP) 
    { 
     n -= (end - endP); // This might be wrong... 
     end = endP; 
    } 

    if(end < start) 
     return; 
    // end boundary check 

    for(; start < end; start++, i++) 
    { 
     buffer[i] = *start; 
    } 
} 

下面是從由編譯器上面的代碼生成的彙編代碼與-O2

 .686P 
     .XMM 
     .model flat 

PUBLIC _readBytes 
_TEXT SEGMENT 
_fileSize$ = 8     ; size = 4 
_pMemory$ = 12     ; size = 4 
_buffer$ = 16     ; size = 4 
_n$ = 20      ; size = 4 
_pos$ = 24      ; size = 4 
_readBytes PROC     ; COMDAT 
    mov eax, DWORD PTR _pMemory$[esp-4] 
    mov edx, DWORD PTR _fileSize$[esp-4] 

    mov ecx, DWORD PTR _n$[esp-4] 
    add edx, eax 
    add eax, DWORD PTR _pos$[esp-4] 
    add ecx, eax 

    cmp ecx, edx 
    jbe SHORT $LN5@readBytes 

    mov ecx, edx 
$LN5@readBytes: 
    cmp eax, ecx 
    jae SHORT $LN1@readBytes 
    push esi 
    mov esi, DWORD PTR _buffer$[esp] 
    sub esi, eax 
$LL3@readBytes: 
    mov dl, BYTE PTR [eax] 
    mov BYTE PTR [esi+eax], dl 
    inc eax 
    cmp eax, ecx 
    jb SHORT $LL3@readBytes 
    pop esi 
$LN1@readBytes: 
    ret 0 
_readBytes ENDP 
_TEXT ENDS 
END