Version:           3.0.x (using KDE KDE 3.2.2)
Installed from:    RedHat RPMs
OS:                Linux

It looks like the issue has been brought up before for C++.
However this bug is specific for C projects right now. it can be reproduced easily

this is the code ( important: C only project)

typedef struct mystruct_
{
  int a;
  int b;
}my_type_t;

int func()
{
  my_type_t var;
  my_type_t *ptr;
  
  ptr=malloc(sizeof(my_type_t));

  var.[CTRL-SPACE]
  ptr->[CTRL-SPACE]
}

in both cases code completion fails;
if you remove typedef and just use
struct mystruct_ var;
then code completion works perfectly.

So this is an issue related to typedefs

FYI C++ specific project options have typedefs are enabled for code completions. I am not sure if it supposed to affect C projects.  In any case enabling it there doesn't make code completion work, unfortunately.