Binary file I/O

Question

How to read and write to binary files in D language? In C would be:

    FILE *fp = fopen("/home/peu/Desktop/bla.bin", "wb");
    char x[4] = "RIFF";

    fwrite(x, sizeof(char), 4, fp);

I found rawWrite at D docs, but I don't know the usage, nor if does what I think. fread is from C:

T[] rawRead(T)(T[] buffer);

If the file is not opened, throws an exception. Otherwise, calls fread for the file handle and throws on error.

rawRead always read in binary mode on Windows.

Answer 1

rawRead and rawWrite should behave exactly like fread, fwrite, only they are templates to take care of argument sizes and lengths.

e.g.

 auto stream = File("filename","r+");
 auto outstring = "abcd";
 stream.rawWrite(outstring);
 stream.rewind();
 auto inbytes = new char[4];
 stream.rawRead(inbytes);
 assert(inbytes[3] == outstring[3]);

rawRead is implemented in terms of fread as

 T[] rawRead(T)(T[] buffer)
    {
        enforce(buffer.length, "rawRead must take a non-empty buffer");
        immutable result =
            .fread(buffer.ptr, T.sizeof, buffer.length, p.handle);
        errnoEnforce(!error);
        return result ? buffer[0 .. result] : null;
    }

Answer 2

If you just want to read in a big buffer of values (say, ints), you can simply do:

int[] ints = cast(int[]) std.file.read("ints.bin", numInts * int.sizeof);

and

std.file.write("ints.bin", ints);

Of course, if you have more structured data then Scott Wales' answer is more appropriate.