将二进制文件中的小端读取到 C++ 中的整数

Question

I am parsing a wav file and slowing it down or speeding it up by a given factor. So I want to read the sample rate and the byte rate from the wave file header and modify it according to the factor. The problem is that they are in little endian.

In C, I can do this

inline void endian_swap(unsigned int& x)
    {
        x = (x>>24) | 
            ((x<<8) & 0x00FF0000) |
            ((x>>8) & 0x0000FF00) |
            (x<<24);
    } 

and then

unsigned int num;
while ( fread(&num, 1, 4,fp) != 0) 
endian_swap(num); 

but how can I do the same in C++? I have tried

    int length = 4; 
    my_input_file.read(&num, length);

but it didn't work because the compiler was expecting a char pointer in the first argument. I am using filestreams everywhere in the code and that's why I cannot just use a file pointer and do it the C way. Any idea how can I do this?

Also, Can I use sscanf to read little-endian formatted data from the binary file into an integer directly?

Answer 1

In C, I can do this

Problems with your approach are:

1. It only works if bytes have 8 bits. This is not guaranteed by the C (nor C++).
2. It only works if usigned int has 4 bytes. This is not guaranteed by the C (nor C++) standard.
3. Swapping should only be done for little endian input if the CPU is natively big endian. This is not guaranteed by the C (nor C++) standard. _{Note that x86 is little endian.}
4. C doesn't have references.

but how can I do the same in C++?

Besides the issues above, endian_swap happens to be valid C++.

In C++, you can use for example this to read a binary file into memory:

std::ifstream is(filename, std::ios::in | std::ios::binary);
std::istreambuf_iterator<char> begin = is;
std::istreambuf_iterator<char> end = {};
std::vector<unsigned char> data(begin, end);