Parsing a binary file. What is a modern way?

Question

I have a binary file with some layout I know. For example let format be like this:

• 2 bytes (unsigned short) - length of a string
• 5 bytes (5 x chars) - the

Answer 1

I had to solve this problem once. The data files were packed FORTRAN output. Alignments were all wrong. I succeeded with preprocessor tricks that did automatically what you are doing manually: unpack the raw data from a byte buffer to a struct. The idea is to describe the data in an include file:

BEGIN_STRUCT(foo)
    UNSIGNED_SHORT(length)
    STRING_FIELD(length, label)
    UNSIGNED_INT(stride)
    FLOAT_ARRAY(3 * stride)
END_STRUCT(foo)

Now you can define these macros to generate the code you need, say the struct declaration, include the above, undef and define the macros again to generate unpacking functions, followed by another include, etc.

NB I first saw this technique used in gcc for abstract syntax tree-related code generation.

If CPP is not powerful enough (or such preprocessor abuse is not for you), substitute a small lex/yacc program (or pick your favorite tool).

It's amazing to me how often it pays to think in terms of generating code rather than writing it by hand, at least in low level foundation code like this.