You aren't doing anything wrong. Asciidoctor currently removes all line comments before processing the content of the table. I recognize this behavior is not consistent with AsciiDoc Python. However, knowing when to remove the line comments and when not is tricky. We'll need to open an issue to explore possible solutions.
Until then, you have a few workarounds:
1) Put at least one space at the start of the comment line (since an AsciiDoc comment line must start at the left column)
2) Use three forward slashes (AsciiDoc does not allow a comment line to start with exactly three forward slashes)
3) Use an attribute reference instead of a forward slash and add the subs=attributes+ to the source listing to expand
I think (2) is the most elegant:
[source,c]
----
int fun1()
{
/// ... comment 1
return 1;
/// ...
}
----
Cheers,
-Dan