In the picture, I have set two breakpoints, and the breakpoint marks are displayed as two small balls with backslashes, rather than just two small balls.
Why is it? and how to resolve this issue?
It should be similar to this annotation:
diagonal line through breakpoint: all breakpoints have been disabled (button skip all breakpoints in breakpoint view)
See "Skip All Breakpoints".
Also visible in PyDev (so not specific to CDT):