A uniform is a global Shader variable declared with the “uniform” storage qualifier.
These act as parameters that the user of a shader program can pass to that program.
Their values are stored in a program object.
Uniforms are so named because they do not change from one shader invocation to the next within a particular rendering call thus their value is uniform among all invocations. This makes them unlike shader stage inputs and outputs, which are often different for each invocation of a shader stage.
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glGetUniformLocation returns an integer that represents the location of a specific uniform variable within a program object. name must be a null terminated string that contains no white space. name must be an active uniform variable name in program that is not a structure, an array of structures, or a subcomponent of a vector or a matrix. This function returns -1 if name does not correspond to an active uniform variable in program, if name starts with the reserved prefix "gl_", or if name is associated with an atomic counter or a named uniform block.
Uniform variables that are structures or arrays of structures may be queried by calling glGetUniformLocation for each field within the structure. The array element operator "[]" and the structure field operator "." may be used in name in order to select elements within an array or fields within a structure. The result of using these operators is not allowed to be another structure, an array of structures, or a subcomponent of a vector or a matrix. Except if the last part of name indicates a uniform variable array, the location of the first element of an array can be retrieved by using the name of the array, or by using the name appended by "[0]".
The actual locations assigned to uniform variables are not known until the program object is linked successfully. After linking has occurred, the command glGetUniformLocation can be used to obtain the location of a uniform variable. This location value can then be passed to glUniform to set the value of the uniform variable or to glGetUniform in order to query the current value of the uniform variable. After a program object has been linked successfully, the index values for uniform variables remain fixed until the next link command occurs. Uniform variable locations and values can only be queried after a link if the link was successful.
这一这里的说明：The actual locations assigned to uniform variables are not known until the program object is linked successfully.