Question

According to here, the C compiler will pad out values when writing a structure to a binary file. As the example in the link says, when writing a struct like this:

struct {
 char c;
 int i;
} a;

to a binary file, the compiler will usually leave an unnamed, unused hole between the char and int fields, to ensure that the int field is properly aligned.

How could I to create an exact replica of the binary output file (generated in C), using a different language (in my case, Java)?

Is there an automatic way to apply C padding in Java output? Or do I have to go through compiler documentation to see how it works (the compiler is g++ by the way).

Answer 1

This is true not only when writing to files, but also in memory. It is the fact that the struct is padded in memory, that leads to the padding showing up in the file, if the struct is written out byte-by-byte.

It is in general very hard to replicate with certainty the exact padding scheme, although I guess some heuristics would get you quite far. It helps if you have the struct declaration, for analysis.

Typically, fields larger than one char will be aligned so that their starting offset inside the structure is a multiple of their size. This means shorts will generally be on even offsets (divisible by 2, assuming sizeof (short) == 2), while doubles will be on offsets divisible by 8, and so on.

UPDATE: It is for reasons like this (and also reasons having to do with endianness) that it is generally a bad idea to dump whole structs out to files. It’s better to do it field-by-field, like so:

put_char(out, a.c);
put_int(out, a.i);

Assuming the put-functions only write the bytes needed for the value, this will emit a padding-less version of the struct to the file, solving the problem. It is also possible to ensure a proper, known, byte-ordering by writing these functions accordingly.