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A general tour through the reprepro source tree:

See also the "naming conventions" section in README.

Most code returns a "retvalue". That is just a int, but a int with semantics.
Errors are negative number (RET_WAS_ERROR is true), RET_OK means it returned
successfully. RET_NOTHING can mean nothing has happened or nothing to do, or
outgoing arguments were not initialized or something like that.

Errors are to be checked always and immediately.

Most of the code is written in some POOP (Pseudo Object Orientated
Programming), i.e. for a struct foo there is a foo_something allocating and
initiating it, several foo_something having a struct foo as first argument
(think methods) and some destructor (usually foo_free if it also deallocates it
and foo_done if you have to do the deallocation).

A special case are binary and source packages. Those are (except in some
special cases when preparing to add packages) not found as some struct but
instead in serialized form as "char *" as 'controlchunk' (older code often
calls it 'chunk', newer 'control'). The serialisation format is the same as
the part of the package in the generated Packages or Sources file.

Extracing data from this format is usually done via routines in chunk.c.
For source and binary packages those chunk_* routines are usually called from
functions in sources.c or binaries.c, which are usually called via function
pointers in struct target (think of the function pointers in struct target as
some externalized virtual method table for packages).

Implementation (including the layout of structs) is hidden in the specific
module (i.e. foo.c and not foo.h), unless doing so would require writing glue
code or significant getter/setter code or error messages would not have enough
information to be helpful.

Some coding style guidelines I try to adhere to:

Function prototypes in a single line.
No other line longer than 80 characters.
Indentation by tabs (which count as 8 spaces for the 80 characters rule).
No appreciations unless there is really no other way.
One or two letter names only for core variables.
Spaces before and after binary operators ('=',...) and after ',' and
before the '(' of flow controls (if, for, while, assert).