What is a linking exception?

Definition

A linking exception is an explicit permission that a copyright holder attaches to a copyleft license, granting the right to link to the covered code from a separate program without that program inheriting the copyleft. To see why it exists, start with the default. Under strong copyleft, combining the covered code with your own can create a single work governed by the copyleft, which would require you to make your code available under the same terms. There has long been debate about whether linking, as opposed to copying source, creates that combined work. A linking exception removes the debate for the covered project. It states plainly that you may link to the library and keep your own program under whatever license you choose, subject to whatever conditions the exception sets.

Why it matters to enterprises

For a business shipping proprietary software, a linking exception can be the difference between a component being usable and being off limits. Without one, linking your application to a strong copyleft library invites the argument that your application has become subject to the copyleft and must be released as source. That is rarely a risk a commercial product can accept. With a linking exception, the library can be used as intended, linked into a proprietary program, without that consequence. The classic example is the runtime libraries that ship with the GCC compiler, which carry an exception so that programs compiled with GCC are not themselves forced open. The practical lesson is that the same underlying license can carry very different risk depending on whether an exception applies, so a license name alone does not tell you the answer.

This is why a dependency review records not just the license of each component but any exception attached to it. A copyleft library with a linking exception and the same library without one belong in different risk categories.

How it relates to the LGPL and copyleft

A linking exception achieves for a specific project what the GNU Lesser General Public License achieves as a whole license. The LGPL is designed so that linking to the covered library does not extend its copyleft to your program, subject to conditions, and is covered in the glossary entry on what is LGPL. A linking exception is the narrower instrument: an add on to an otherwise stronger copyleft license, such as the GPL, that grants the same linking freedom for one project. Understanding both depends on understanding the default rule they modify, which is set out in the glossary entry on copyleft. For the surrounding vocabulary, browse the full open source license risk glossary.