The current manual page reads:

.I "PKG_CONFIG_PATH"
A colon-separated (on Windows, semicolon-separated) list of
directories to search for .pc files.  The default directory will
always be searched after searching the path; the default is
.I \%libdir/\fPpkgconfig:\fIdatadir\fP/pkgconfig where \fIlibdir\fP is
the libdir for \fIpkg-config\fP and \fIdatadir\fP is the datadir
for \fIpkg-config\fP when it was installed.

.I "PKG_CONFIG_LIBDIR"
Replaces the default \fIpkg-config\fP search directory, usually \fI/usr/lib/pkgconfig\fP


On the other hand, the source code for this logic is:

 search_path = getenv ("PKG_CONFIG_PATH");
 if (search_path) 
   {
     add_search_dirs(search_path, G_SEARCHPATH_SEPARATOR_S);
   }
 if (getenv("PKG_CONFIG_LIBDIR") != NULL) 
   {
     add_search_dirs(getenv("PKG_CONFIG_LIBDIR"), G_SEARCHPATH_SEPARATOR_S);
   }
 else
   {
     add_search_dirs(pkg_config_pc_path, G_SEARCHPATH_SEPARATOR_S);
   }

My understanding of the above is the following:

PKG_CONFIG_PATH

A colon-separated (on Windows, semicolon-separated) list of directories to be searched for .pc files before the default locations.

PKG_CONFIG_LIBDIR

A colon-separated (on Windows, semicolon-separated) list of directories to replace the default pkg-config search path, usually defined at build time.


I saw some other comments related to PKG_CONFIG_PATH being overwritten by PKG_CONFIG_LIBDIR; the source code does not reflect this, the two environment variables seem independent; when defined, PKG_CONFIG_PATH is always searched before anything else; similarly, when defined, PKG_CONFIG_LIBDIR will always replace the system locations.

A clarification in the man page would be highly appreciated.