Error when compiling shaders GLSL 3.30 - c++

Question

I create context by:
new sf::Window(sf::VideoMode(800, 600), "OpenGL",
sf::Style::Default,
sf::ContextSettings(24, 8, 0, 3, 3, sf::ContextSettings::Core)));
I'm loading extensions by glLoadGen for OpenGL 3.3 Core Profile with one extension EXT_texture_compression_s3tc. When I'm compiling shader:
#version 330 core
layout (location = 0) in vec3 vertPos;
layout (location = 5) uniform mat4 modelMat;
layout (location = 6) uniform mat4 viewMat;
layout (location = 7) uniform mat4 projectionMat;
out vec4 fragColor;
void main()
{
gl_Position = projectionMat * viewMat * modelMat * vec4(vertPos, 1.0);
fragColor = vec4(0.5, 0.5, 0.5, 1.0);
}
``
#version 330 core
in vec4 fragColor;
out vec4 outColor;
void main()
{
outColor = fragColor;
}
I get error string:
ERROR: Shader compilation error at shader: "media/shaders/shader.vs.glsl"
0:7(1): error: uniform explicit location requires GL_ARB_explicit_uniform_location and either GL_ARB_explicit_attrib_location or GLSL 3.30.
0:8(1): error: uniform explicit location requires GL_ARB_explicit_uniform_location and either GL_ARB_explicit_attrib_location or GLSL 3.30.
0:9(1): error: uniform explicit location requires GL_ARB_explicit_uniform_location and either GL_ARB_explicit_attrib_location or GLSL 3.30.
but I have OpenGL 3.3 (so GLSL 3.30).
glxinfo prints:
Extended renderer info (GLX_MESA_query_renderer):
Vendor: X.Org (0x1002)
Device: AMD JUNIPER (DRM 2.43.0, LLVM 3.8.0) (0x68be)
Version: 11.2.0
Accelerated: yes
Video memory: 512MB
Unified memory: no
Preferred profile: core (0x1)
Max core profile version: 3.3
Max compat profile version: 3.0
Max GLES1 profile version: 1.1
Max GLES[23] profile version: 3.0
OpenGL vendor string: X.Org
OpenGL renderer string: Gallium 0.4 on AMD JUNIPER (DRM 2.43.0, LLVM 3.8.0)
OpenGL core profile version string: 3.3 (Core Profile) Mesa 11.2.0
OpenGL core profile shading language version string: 3.30
OpenGL core profile context flags: (none)
OpenGL core profile profile mask: core profile
OpenGL core profile extensions:
So I should be able use GLSL 3.30.

Answer 1

The ability to specify uniform locations in a shader is not part of OpenGL version 3.3 or GLSL version 3.30. It's only a core feature of GL 4.3 or GLSL 4.30. The ability to specify vertex shader input and fragment shader output locations is 3.30, but uniform locations are not.
Explicit uniform location specification doesn't actually require special hardware; it's purely an interface thing. As such, pre-4.x hardware could implement it. However, if your hardware is limited to GL 3.3, then there's a very good chance that the hardware is so old that it stopped being updated by the IHV with new OpenGL features. So even though it could support it, the feature appeared after the IHV stopped updating the hardware.
While NVIDIA has kept some of their 3.3-only hardware up-to-date on recent, non-hardware features, the same cannot be said for Intel or AMD. So even if you have an NVIDIA 3.x GPU that this works on, it's likely that Intel or AMD's 3.x GPUs won't work.
In your case, "Juniper" refers to the Radeon 67xx line. These are GL 4.x parts. However, you're using the open source driver rather than AMD's actual Linux driver, so you're only able to get 3.3 from it.
It would be better to bump your required OpenGL version to match your shader. However, if you wish to keep it as a 3.30 shader and use it as an extension (since you're using the open source driver instead of AMD's driver), you will need an extension declaration below the #version declaration:
#extension GL_ARB_explicit_uniform_location : require

Answer 2

You could try adding the following line that enables the extension below #version 330 core:
#extension GL_ARB_explicit_uniform_location : require