unknown type name ''size_t''

[YaoQ1580]

When I index my project, a lot of errors occur with message "unknown type name ''size_t" ", I googled that "size_t" is defined in <stddef.h> but the <stddef.h> file included by sourcetrail has empty content. It just has the following 3 lines:

#ifndef __always_inline
#define __always_inline inline
#endif

No "size_t" is defined. I don't know why? The global include paths of my project are auto detected from gcc