Why don't these mismatched shader variables produce a linker error? - opengl

Question

I am trying to throw an exception and abort my program when something fails during shader compile and link. I have introduced a typo: naming the output variable from my vertex shader something different from the expected input variable in my fragment shader. I would expect this to fail on link, but everything works fine (and the value of the variable in the fragment shader just ends up being 0). In particular, glGetProgramiv(id, GL_LINK_STATUS, &success); is setting success to true. What is going on?
Vertex shader:
#version 330 core
layout (location = 0) in vec3 aPos;
out vec4 vertexColo; // NOTE THE MISSPELLING HERE
void main()
{
gl_Position = vec4(aPos.x, aPos.y, aPos.z, 1.0);
vertexColo = vec4(1.0 - aPos.y, 0.8 - aPos.y, aPos.x, 1.0);
}
Fragment shader:
#version 330 core
out vec4 FragColor;
in vec4 vertexColor;
void main()
{
FragColor = vertexColor;
}
Test code:
namespace btv {
namespace {
static void test_compile(unsigned id, const char* path)
{
int success;
glGetShaderiv(id, GL_COMPILE_STATUS, &success);
if (!success) {
constexpr size_t BUF_SIZE = 1024;
char buf[BUF_SIZE];
glGetShaderInfoLog(id, BUF_SIZE, NULL, buf);
std::stringstream ss;
ss << "Shader at path: " << path << " failed with message: "
<< buf;
throw std::runtime_error(ss.str());
}
}
static void test_link(unsigned id, const char* v_path, const char* f_path)
{
int success;
glGetProgramiv(id, GL_LINK_STATUS, &success);
if (!success) {
constexpr size_t BUF_SIZE = 1024;
char buf[BUF_SIZE];
glGetProgramInfoLog(id, BUF_SIZE, NULL, buf);
std::stringstream ss;
ss << "Program with v shader: " << v_path << ", f shader: " << f_path
<< " failed with message: " << buf;
throw std::runtime_error(ss.str());
}
}
}
Program::Program(const char* v_path, const char* f_path)
{
std::string v_src = slurp(v_path);
std::string f_src = slurp(f_path);
const char *v_c = v_src.data();
const char *f_c = f_src.data();
unsigned v_shdr = glCreateShader(GL_VERTEX_SHADER);
glShaderSource(v_shdr, 1, &v_c, nullptr);
glCompileShader(v_shdr);
test_compile(v_shdr, v_path);
unsigned f_shdr = glCreateShader(GL_FRAGMENT_SHADER);
glShaderSource(f_shdr, 1, &f_c, nullptr);
glCompileShader(f_shdr);
test_compile(f_shdr, f_path);
id = glCreateProgram();
glAttachShader(id, v_shdr);
glAttachShader(id, f_shdr);
glLinkProgram(id);
test_link(id, v_path, f_path);
}
} // namespace btv

Answer 1

The issue is related to GLSL version. In the OpenGL Shading Language 4.60 Specification (4.3.4 Input Variables) it is clearly specified, that it is a link-time error, if an input variable is static used, but no corresponding output variable is declared in the previous stage.
This part of the specification is missing in the OpenGL Shading Language 3.30 Specification.
Change the GLSL version to (e.g.) 4.60 (#version 460 core) and you'll get the expected error.
e.g.
"error: "vertexColor" not declared as an output from the previous stage"