Converting byte array to unsigned long in C++

Question

I am reading in binary data from a file:

char* buffIn = new char[8];
ifstream inFile(path, ifstream::binary);
inFile.read(buffIn, 8);

I then want to convert the char* read in (as binary) to an unsigned long but I am having problems - I am not quite sure what is going on, but for instance 0x00000000000ACD gets interpreted as 0xFFFFFFFFFFFFCD - I suspect all the 0x00 bytes are causing some sort of problem when converting from char* to unsigned long...

unsigned long number = *(buffIn);

How do I do this properly?

Answer 1

Since buffIn is of type char pointer, when you do *(buffIn) you are just grabbing one character. You have to reinterpret the memory address as an unsigned long pointer and then dereference it.

unsigned long number = *((unsigned long*)buffIn);

Answer 2

除了重铸char[8] （它将只读取第一个unsigned long -长度为32位）之外，您还可以使用一些简单的按位运算

unsigned long value = (((unsigned long)buffin[0]) << 24) | (((unsigned long)buffin[1]) << 16) | (((unsigned long)buffin[2]) << 8) | (unsigned long)buffin[3];

Answer 3

Try something like

unsigned long* buffInL = new unsigned long[2];
char* buffIn=(char*)buffInL;
ifstream inFile(path, ifstream::binary);
inFile.read(buffIn, 8);

Unlike other types, char* is allowed to alias.