glGetUniformBlockIndex

glGetUniformBlockIndex: man4/glGetUniformBlockIndex.xml

glGetUniformBlockIndex retrieves the index of a uniform block within program. program must be the name of a program object for which the command glLinkProgram must have been called in the past, although it is not required that glLinkProgram must have succeeded. The link could have failed because the number of active uniforms exceeded the limit. uniformBlockName must contain a nul-terminated string specifying the name of the uniform block. glGetUniformBlockIndex returns the uniform block index for the uniform block named uniformBlockName of program. If uniformBlockName does not identify an active uniform block of program, glGetUniformBlockIndex returns the special identifier, GL_INVALID_INDEX. Indices of the active uniform blocks of a program are assigned in consecutive order, beginning with zero.

glGetUniformBlockIndex is available only if the GL version is 3.1 or greater.