The situation varies worldwide, but in the United States before 1974, software and its source code was not copyrightable and therefore always public domain software.
In 1974, the US Commission on New Technological Uses of Copyrighted Works (CONTU) decided that "computer programs, to the extent that they embody an author's original creation, are proper subject matter of copyright".
In 1983 in the United States court case Apple v. Franklin it was ruled that the same applied to object code; and that the Copyright Act gave computer programs the copyright status of literary works.
In 1999, in the United States court case Bernstein v. United States it was further ruled that source code could be considered a constitutionally protected form of free speech. Proponents of free speech argued that because source code conveys information to programmers, is written in a language, and can be used to share humor and other artistic pursuits, it is a protected form of communication.
Copyright notice example:
Copyright [yyyy] [name of copyright owner]
Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at
Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
An author of a non-trivial work like software, has several exclusive rights, among them the copyright for the source code and object code. The author has the right and possibility to grant customers and users of his software some of his exclusive rights in form of software licensing. Software, and its accompanying source code, can be associated with several licensing paradigms; the most important distinction is open source vs proprietary software. This is done by including a copyright notice that declares licensing terms. If no notice is found, then the default of All rights reserved is implied.
Generally speaking, software is open source if the source code is free to use, distribute, modify and study, and proprietary if the source code is kept secret, or is privately owned and restricted. One of the first software licenses to be published and to explicitly grant these freedoms was the GNU General Public License in 1989; the BSD license is another early example from 1990.
For proprietary software, the provisions of the various copyright laws, trade secrecy and patents are used to keep the source code closed. Additionally, many pieces of retail software come with an end-user license agreement (EULA) which typically prohibits decompilation, reverse engineering, analysis, modification, or circumventing of copy protection. Types of source code protection—beyond traditional compilation to object code—include code encryption, code obfuscation or code morphing.