GNU C Library
GNU Readline is notable for being a free software library which is licensed under the GNU General Public License (GPL). Free software libraries are far more often licensed under the GNU Lesser General Public License (LGPL), for example, the GNU C Library, GNU gettext and FLTK. A developer of an application who chooses to link to an LGPL licensed library can use any license for the application. But linking to a "GPL" licensed library such as Readline requires the "entire" combined resulting application to be licensed under the GPL when distributed, to comply with section 5 of the GPL.