pack/unpack functions for C++

Question

NOTE: I know that this has been asked many times before, but none of the questions have had a link to a concrete, portable, maintained library for this.

I need a C or C++ library that implements Python/Ruby/Perl like pack/unpack functions. Does such a library exist?

EDIT: Because the data I am sending is simple, I have decided to just use memcpy, pointers, and the hton* functions. Do I need to manipulate a char in any way to send it over the network in a platform agnostic manner? (the char is only used as a byte, not as a character).

Answer 1

In C/C++ usually you would just write a struct with the various members in the correct order (correct packing may require compiler-specific pragmas) and dump/read it to/from file with a raw fwrite/fread (or read/write when dealing with C++ streams). Actually, pack and unpack were born to read stuff generated with this method.

If you instead need the result in a buffer instead of a file it's even easier, just copy the structure to your buffer with a memcpy.

If the representation must be portable, your main concerns are is byte ordering and fields packing; the first problem can be solved with the various hton* functions, while the second one with compiler-specific directives.

In particular, many compilers support the #pragma pack directive (see here for VC++, here for gcc), that allows you to manage the (unwanted) padding that the compiler may insert in the struct to have its fields aligned on convenient boundaries.

Keep in mind, however, that on some architectures it's not allowed to access fields of particular types if they are not aligned on their natural boundaries, so in these cases you would probably need to do some manual memcpys to copy the raw bytes to variables that are properly aligned.