C++ 如何从 Core Foundation CFDataRef 中获取最左边的位

发布于06月09日

我试图从核心基金会CFDataRef获取N个最左边的位.CF和C的新手，以下是我目前掌握的内容.它正在打印0，所以有问题.

int get_N_leftmost_bits_from_data(int N)
{
    // Create a CFDataRef from a 12-byte buffer
    const unsigned char * _Nullable buffer = malloc(12);
    const CFDataRef data = CFDataCreate(buffer, 12);

    const unsigned char *_Nullable bytesPtr = CFDataGetBytePtr(data);

    // Create a buffer for the underlying bytes
    void *underlyingBytes = malloc(CFDataGetLength(data));

    // Copy the first 4 bytes to underlyingBytes
    memcpy(underlyingBytes, bytesPtr, 4);

    // Convert underlying bytes to an int
    int num = atoi(underlyingBytes);

    // Shift all the needed bits right
    const int numBitsInByte = 8;
    const int shiftedNum = num >> ((4 * numBitsInByte) - N);

    return shiftedNum;
}

谢谢你的帮助！

推荐答案

因为您只关心前四个字节中的位，所以您可以将字节复制到一个整数，然后对该整数执行位移位.


#include <string.h>
#include <stdint.h>
int main(){
    
    uint8_t bytes[12] = {0};
    for(int n = 0; n < sizeof(bytes) ; ++n){
        bytes[n] = n+0xF;
        //printf("%02x\n",bytes[n]);
    }
    uint32_t firstfour = 0;
    //copy the first four bytes
    memcpy(&firstfour,bytes,sizeof(uint32_t));
    //get the left most bit
    uint32_t bit = firstfour>>31&1;
    
    return 0;
}

您仍然可以在CF中执行memcpy