glslang Error: Overlapping Binding Numbers in GLSL

When compiling GLSL shaders with glslang, assigning the same binding location to multiple interface blocks within the same scope results in a compilation failure. This article covers the specific diagnostic error produced by glslang, explains why the collision occurs under OpenGL and Vulkan specifications, provides a reproducing code example, and details how to resolve the issue.

The Diagnostic Error

When two interface blocks (such as uniform blocks or buffer storage blocks) share an identical binding index within the same descriptor set, glslang terminates compilation and raises the following diagnostic error:

ERROR: <line_number>: 'binding' : overlapping binding numbers

In typical output from glslangValidator, the error message specifies the exact source file or ID, line number, the layout qualifier key ('binding'), and the explanation 'overlapping binding numbers'.

Why the Error Occurs

In both OpenGL (via GL_ARB_shading_language_420pack / core GLSL 4.20+) and Vulkan SPIR-V generation, binding indices identify the specific slot where resource descriptors connect to pipeline stages.

The OpenGL and Vulkan GLSL specifications enforce that binding qualifiers must not alias:

• Under Vulkan (-V mode), each resource must have a unique (set, binding) tuple across the shader stage. If no set qualifier is provided, resources default to set = 0.
• Under OpenGL, binding points for uniform blocks or shader storage buffer objects (SSBOs) must be unique within the active stage.

If two interface blocks occupy the same set and binding, the compiler cannot generate unambiguous descriptor decorations in the resulting SPIR-V binary or GLSL AST, triggering the diagnostic.

Reproducing Example

The following code illustrates a collision between two uniform interface blocks:

#version 450

layout(set = 0, binding = 0) uniform CameraData {
    mat4 viewProjection;
};

// Error: binding 0 is already occupied in set 0
layout(set = 0, binding = 0) uniform ObjectData {
    mat4 modelMatrix;
};

void main() {
    // Shader logic
}

Compiling this shader via glslangValidator -V shader.vert generates:

ERROR: 0:8: 'binding' : overlapping binding numbers 
ERROR: 1 compilation errors.  No code generated.

How to Fix the Error

To fix the issue, assign each interface block a distinct binding index, or isolate the blocks into separate descriptor sets if targeting Vulkan:

Solution 1: Use Unique Binding Indices

Assign incremented or unique binding numbers within the same set:

layout(set = 0, binding = 0) uniform CameraData {
    mat4 viewProjection;
};

layout(set = 0, binding = 1) uniform ObjectData {
    mat4 modelMatrix;
};

Solution 2: Assign Different Descriptor Sets (Vulkan)

If the resources represent different update frequencies (for example, per-frame data versus per-draw data), place them in separate sets:

layout(set = 0, binding = 0) uniform CameraData {
    mat4 viewProjection;
};

layout(set = 1, binding = 0) uniform ObjectData {
    mat4 modelMatrix;
};

Using unique (set, binding) coordinates satisfies glslang layout validation rules and allows successful compilation to SPIR-V.