glLinkProgram

glLinkProgram: man2/glLinkProgram.xml

glLinkProgram links the program object specified by program. If any shader objects of type GL_VERTEX_SHADER are attached to program, they will be used to create an executable that will run on the programmable vertex processor. If any shader objects of type GL_FRAGMENT_SHADER are attached to program, they will be used to create an executable that will run on the programmable fragment processor. The status of the link operation will be stored as part of the program object's state. This value will be set to GL_TRUE if the program object was linked without errors and is ready for use, and GL_FALSE otherwise. It can be queried by calling glGetProgram with arguments program and GL_LINK_STATUS. As a result of a successful link operation, all active user-defined uniform variables belonging to program will be initialized to 0, and each of the program object's active uniform variables will be assigned a location that can be queried by calling glGetUniformLocation. Also, any active user-defined attribute variables that have not been bound to a generic vertex attribute index will be bound to one at this time. Linking of a program object can fail for a number of reasons as specified in the. The following lists some of the conditions that will cause a link error.

1. The number of active attribute variables supported by the implementation has been exceeded.
2. The storage limit for uniform variables has been exceeded.
3. The number of active uniform variables supported by the implementation has been exceeded.
4. The main function is missing for the vertex shader or the fragment shader.
5. A varying variable actually used in the fragment shader is not declared in the same way (or is not declared at all) in the vertex shader.
6. A reference to a function or variable name is unresolved.
7. A shared global is declared with two different types or two different initial values.
8. One or more of the attached shader objects has not been successfully compiled.
9. Binding a generic attribute matrix caused some rows of the matrix to fall outside the allowed maximum of GL_MAX_VERTEX_ATTRIBS.
10. Not enough contiguous vertex attribute slots could be found to bind attribute matrices.

When a program object has been successfully linked, the program object can be made part of current state by calling glUseProgram. Whether or not the link operation was successful, the program object's information log will be overwritten. The information log can be retrieved by calling glGetProgramInfoLog. glLinkProgram will also install the generated executables as part of the current rendering state if the link operation was successful and the specified program object is already currently in use as a result of a previous call to glUseProgram. If the program object currently in use is relinked unsuccessfully, its link status will be set to GL_FALSE, but the executables and associated state will remain part of the current state until a subsequent call to glUseProgram removes it from use. After it is removed from use, it cannot be made part of current state until it has been successfully relinked. If program contains shader objects of type GL_VERTEX_SHADER but does not contain shader objects of type GL_FRAGMENT_SHADER, the vertex shader will be linked against the implicit interface for fixed functionality fragment processing. Similarly, if program contains shader objects of type GL_FRAGMENT_SHADER but it does not contain shader objects of type GL_VERTEX_SHADER, the fragment shader will be linked against the implicit interface for fixed functionality vertex processing. The program object's information log is updated and the program is generated at the time of the link operation. After the link operation, applications are free to modify attached shader objects, compile attached shader objects, detach shader objects, delete shader objects, and attach additional shader objects. None of these operations affects the information log or the program that is part of the program object.

glLinkProgram is available only if the GL version is 2.0 or greater. If the link operation is unsuccessful, any information about a previous link operation on program is lost (i.e., a failed link does not restore the old state of program ). Certain information can still be retrieved from program even after an unsuccessful link operation. See for instance glGetActiveAttrib and glGetActiveUniform.