Vim match containedin not working as expected

Question

I have tried to extend the C syntax in Vim with the example in the manual.

First I launch vim without any config:

vim -u NONE test.c

Then I load the c-syntax and create my extension

syntax on
syntax keyword myword HELP containedin=cComment contained
highlight myword guibg=red

code:

1: /* HELP */
2: int main(int argc, char **argv) {
3:     int HELP = 0;
4:     if(HELP);
5: }

HELP is highlighted on line 1 as expected. It is also highlighted on line 4, is there some explanation for this?

Answer 1

You need to find out which syntax group "occupies" the HELP in the third line that is not covered by your syntax rule. :syn list shows all active groups, but it's easier when you install the SyntaxAttr.vim - Show syntax highlighting attributes of character under cursor plugin.

SyntaxAttr tells me the cBlock syntax matches there; this needs to be included in your extension, too:

syntax keyword myword HELP containedin=cComment,cBlock contained