write a binary file

Hi, I'd like to write a binary file from a set of text & numeric data. I know the byte "structure" or "frame" of the output binary file (first 3200 byte of that and then 400 bytes of this ...). I try it using fwrite but I can't figure out how to sort everything out. If someone can help I apreciate.

I'm on a lead with 'fwrite' & the 'skip' option and also do the maths. Knowing the number of byte of each data I want to write (some in int8 or float), I can find out how many byte I have to skip to fill properly the structure. I really enjoy when posting a problem gives me an idea to solve it...

Thanks

[EDITED, Jan Simon, answer appended to question]

[Jan]

Please post what you have tried already and ask as specific as possible. I cannot recommend a command to implement "400 bytes of this".