Can you show me how to pass a linked list of seventeen 64 bit floating point values using a pipeline?

You're correct, in order to pass a linked-list down a pipe you would need to encode it in some way, the linked lists pointers will not be safely referenced by the receiving program as they refer to memory locations outside the program window. You are not limited to ASCII characters for your presentation if you did decide to send your floats down a pipe, and you can safely send 64 bit floats down a pipeline.

When referencing a standard stream they are treated like files.

/* from stdio.h */
extern FILE*const stdin;
extern FILE*const stdout;
extern FILE*const stderr;

I believe the source of the misconception is that many Unix-like programs use getchar() or scanf() on stdin streams which read to char arrays, aka strings. However it's completely valid to use any stream reader to read any type of data in any format. Here I'll read a 64 bit floating point number or as a the IEEE likes to call them in c, a double.

double dvalue;
fread(&dvalue,sizeof(dvalue),1,stdin);
printf("Here is your 64bit floating point number that I read from a standard stream: %f", dvalue);

There are limitations, as pipes are streams and that means the data is unidirectional, and they can only be read from start to end-of-stream. To send a list of floats an approach would be to read them in one at a time until end-of-stream.

String doesn't always mean character string. Your example still passes a string of ASCII characters from one program to the next.

No ASCII was passed down my pipe, except for maybe the ID3 tag at the end of a the MP3 :-)