我想了解位掩码,我对某些结果有疑问。这是一些示例代码。
FILE * pFile;
long lSize;
char * buffer;
size_t result;
pFile = fopen ( "testFile.jpg" , "rb" );
if (pFile==NULL) {fputs ("File error",stderr); exit (1);}
// obtain file size:
fseek (pFile , 0 , SEEK_END);
lSize = ftell (pFile);
rewind (pFile);
// allocate memory to contain the whole file:
buffer = (char*) malloc (sizeof(char)*lSize);
if (buffer == NULL) {fputs ("Memory error",stderr); exit (2);}
// copy the file into the buffer:
result = fread (buffer,1,lSize,pFile);
if (result != lSize) {fputs ("Reading error",stderr); exit (3);}
/* the whole file is now loaded in the memory buffer. */
for (unsigned long long e = 0; e < lSize; e++){
unsigned short val1 = buffer[e] & 0x3;
cout << val1 << endl;
if (1 == val1){
}
if (1 == buffer[e] & 0x3){
//what does buffer[e] & 0x3 equal when I don't set it equal to an unsigned short.
}
}
所以如果我输出 val1
的值我总是得到一个介于 0 和 3 之间的值。但是当我进行比较时没有将类型分配给 buffer[e] & 0x3
我并不总是得到相同的结果。我试着输出 buffer[e] & 0x3
看看它等于什么,但我得到一个错误。所以我的问题是 buffer[e] & 0x3
的可能值是多少?当它在第二个 if 语句中使用时。谢谢。
最佳答案
那是因为运算符优先
7 == != For relational = and ≠ respectively
8 & Bitwise AND
所以 ==
优先于 &
(1 == buffer[e] & 0x3)
不是一回事
(1 == (buffer[e] & 0x3))
但是是
((1 == buffer[e]) & 0x3)
(相当于 (1 == buffer[e])
因为用 3 屏蔽 0 或 1 没有效果)
你想要的是(1 == (buffer[e] & 0x3))
关于c++ - 位屏蔽结果不一致,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/41537729/