Shaders and Programs - Praise for OpenGL ES 30 Programming Guide - page 77

To check whether a link was successful, you can query for GL_LINK_
STATUS . A large number of other queries can also be executed on
program objects. Querying for GL_ACTIVE_ATTRIBUTES returns a count
of the number of active attributes in the vertex shader. Querying for
GL_ACTIVE_ATTRIBUTE_MAX_LENGTH returns the maximum length (in
characters) of the largest attribute name; this information can be used to
determine how much memory to allocate to store attribute name strings.
Likewise, GL_ACTIVE_UNIFORMS and GL_ACTIVE_UNIFORM_MAX_LENGTH
return the number of active uniforms and the maximum length of the
largest uniform name, respectively. The number of shaders attached to
the program object can be queried using GL_ATTACHED_SHADERS . The
GL_DELETE_STATUS query returns whether a program object has been
marked for deletion. As with shader objects, program objects store an info
log, the length of which can be queried for using GL_INFO_LOG_LENGTH .
Querying for GL_TRANSFORM_FEEDBACK_BUFFER_MODE returns either
GL_SEPARATE_ATTRIBS or GL_INTERLEAVED_ATTRIBS , which is the buffer
mode when transform feedback is active. Queries for GL_TRANSFORM_
FEEDBACK_VARYINGS and GL_TRANSFORM_FEEDBACK_VARYING_MAX_LENGTH
return the number of output variables to capture in transform feedback
mode for the program and the maximum length of the output variable
names, respectively. The transform feedback is described in Chapter 8,
“Vertex Shaders.” The number of uniform blocks for programs containing
active uniforms and the maximum length of the uniform block names can
be queried using GL_ACTIVE_UNIFORM_BLOCKS and GL_ACTIVE_UNIFORM_
BLOCK_MAX_LENGTH , respectively. Uniform blocks are described in a later
section. Querying for GL_PROGRAM_BINARY_RETRIEVABLE_HINT returns a
value indicating whether the binary retrieval hint is currently enabled for
program. Finally, the status of the last validation operation can be queried
for using GL_VALIDATE_STATUS . The validation of program objects is
described later in this section.
After linking the program, we next want to get information from the
program info log (particularly if a link failure occurred). Doing so is
similar to getting the info log for shader objects.
void glGetProgramInfoLog (GLuint program , GLsizei maxLength ,
GLsizei * length ,
GLchar * infoLog )
program handle to the program object for which to get information
maxLength the size of the buffer in which to store the info log
(continues)