@hustbaer sagte in Using GSL for solution of a differntial equation: AFAIK this goes back to K&R C, which did not even allow to specify the parameter list in a function declaration. That's not correct. K&R C declares the type of parameters differently. do_something(s) char[] s; { double r; /* some code */ return r; } main() { double do_something(); double s; static char *str[] = "123456789"; s = do_something(str) } If you want, you can download a copy of the 1978 edition of "The C Programming language" from the internet archive. Robert Nordier released a UNIX 7 Version which can be run on a virtual machine, which includes an ancient K&R C compiler.